AR-15 vs. M-16 bolt carrier

[1911 guy]

I was under the assumption that any M-16 specific part in an AR made it illegal. Apperently, by recent posts I've read, this is incorrect. Whick leads me to my question. I use an AR carrier with a weight. Is there any advantage worth mentioning to using the "milspec" carrier? And if so, do parts retailers have a problem selling a non NFA person a carrier? Thanks.

 

[TexasRifleman]

If you search around, you'll find posted scans of the ATF letter from their Tech Branch talking about the carriers, so from the legal side we're fine.

Some of the paranoid types are saying that older letters implied that ANY M16 part made a machine gun, but more recent letters say otherwise regarding the B/C specifically. If you have ANY other M16 part you're in deep doo doo.

My opinion is that if Colt isn't worried about shipping AR's with M16 bolt carriers to the civilian market, then I am sure not worried about using one.

Now as to the advantages, that's not so clear.
That's the 9mm vs 45 argument in a lot of ways
I use them because I found some cheaper at one place, and another time that's all my favorite vendor had in stock, so now I have them everywhere.

As for selling them, most online vendors know the law and don't have a problem. I have purchased several from a few online vendors.

Most have a disclaimer like "this COULD be an NFA part if combined with other things, so it's your butt if you use it wrong" or words to that effect.

A few unknowledgeable vendors still think they are an NFA item. If your seller thinks an M16 B/C is an NFA item, and that is clearly very old information, do you trust anything else that seller does?

My opinion only, but copies of that letter are around if you want to search for them.

As for the paranoid, the only actual court case I have ever heard anyone reference regarding this
is the Staples case. This guy DID have an M16 Bolt Carrier in his pocket, but he was also in posession of an AR15 with a selector, M16 hammer, trigger, disconnector, and the selector stop had been filed down.
He did NOT have an auto sear, but the ATF was able to make it full auto using those parts.
ATF could only make it work with "soft-primer" ammunition, whatever the heck that is. So, the hammer was simply following the firing pin and it would shoot bursts here and there. He pled that down to 5 years probation and $5000 fine.

It's the only case anyone has shown that actually mentions the Bolt Carrier by name, but it was not the only
M16 part in the rifle.

http://www.cs.cmu.edu/afs/cs.cmu.edu/user/wbardwel/public/nfalist/staples_10cir.txt

 

[1911 guy]

Just so everybody knows,

I'm not looking to get into trouble by asking the question. I'm more interested if the heavier M-16 carrier would give the same effect as the weight I use. As for the weight, I put it in because I was told it had some sort of magical benefit and I work in a machine shop so I made my own. Something about delaying bolt unlock. Any education to offer on that subject, too?

 

[TexasRifleman]

That's the argument posed by some competitive shooters, that the weight gives them a better cyclic rate.
Again, that just seems to be the "mine vs yours" argument. Personally I see no difference in the performance of my AR's using the thing. I read a writeup on this at another forum once that was interesting. If you put an M16 and an AR15 b/c on a scale the weight difference is very slight. Seems like the competetive guys that really believe in it were still adding weight, even to the M16 carriers. Seems like most of them were getting them chromed too, so maybe that adds some weight?

I don't think you're going to get a definite answer as to whether or not it helps, but maybe someone will post some proof one way or another. It's endlessly debated at the ar15 forum.


I also post all the legal stuff not just for you, but for those that are reading this are not sure about all of this.
I've gotten a couple of PMs calling me a criminal since I posted yesterday that M16 b/c were OK
Colt has used this letter to justify installing M16 bolt carriers in their ARs now, to save on manufacturing costs.

Here is the text of the ATF letter to Colt:

U.S. Department of Justice

Bureau of Alcohol, Tobacco
Firearms and Explosives
[stamp] FEB 10 2005

903050:RV
3311/2005-167
www.atf.gov

[stamp] Received FEB 14 2005
Legal Department

Mr. Carlton S. Chen
Colt Defense LLC
547 New Park Avenue
West Hartford, CT 06110

Dear Mr. Chen:

This is in reference to your most recent facsimile transmitted to the Firearms Technology Branch (FTB), Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF), on January 13, 2005. In your faxed letter, you seek clarification regarding the use of M16 machinegun bolt carriers in AR-15 type weapons.

As you are aware, since your provision of copies of relevant material in your previous faxes, ATF has previously addressed the use of M16 machinegun fire-control components in AR-15 type rifles in the General Information section of the Federal Firearms Regulations Reference Guide (ATF P 5300.4). (Please refer to page 115, item #3, “Important Information Concerning AR-15 Type Rifles.”)

However, we would like to direct your attention to a particular paragraph of item #3, which states the following:

In order to avoid violations of the NFA, M16 hammers, triggers, disconnectors, selectors and bolt carriers must not be used in assembly of AR-15 type semiautomatic rifles, unless the M16 parts have been modified to AR-15 Model SP1 configuration. Any AR-15 type rifles which have been assembled with M16 internal components should have those parts removed and replaced with AR-15 Model SP1 type parts which are available commercially. The M16 components also may be modified to AR-15 Model SP1 configuration.

Accordingly, based on previous FTB recommendations not to install this bolt carrier and the conclusions presented in the passage cited above, our Branch cannot specifically authorize you to install an M16 bolt carrier into an AR15 rifle. Also, we cannot definitively tell you that installing an M16 bolt carrier in an AR 15 will make that firearm fire automatically.

-2-

Mr. Carlton S. Chen


We can only inform you that if this installation were to create a firearm that fires automatically, it would be a machinegun as defined; conversely, if it did not result in the production of a weapon that shoots automatically, it would be lawful to posses and make.

We thank you for your inquiry and trust the foregoing has been responsive.

Sincerely yours,

[signed]

Sterling Nixon
Chief, Firearms Technology Branch

 

[Bartholomew Roberts]

The idea behind having more weight is that the carbine gas systems that everybody loves unlock faster than a rifle length system because they get more gas and they get it sooner than the rifle. As a result, some people seek to delay the unlock time by using a heavier bolt carrier, heavier buffer, or stronger buffer spring.

A heavier bolt carrier will also slow your cyclic rate a wee bit and may give marginal magazines a little more time to put the next round into position. If you aren't experiencing a problem with extraction (the bolt trying to pull the brass while pressure still has it stuck to the chamber wall), then the heavier bolt carrier isn't going to do much for you, especially if you aren't running other accessories (full-auto, suppressor) that increase the pressure.

One nice thing about the M16 carriers though is that they shroud the firing pin so that the notched AR hammer doesn't hit the firing pin collar and bend the firing pin retaining pin. Mostly this only happens with M16 firing pins in an unshrouded AR15 bolt carrier because the collar on the M16 firing pin is slightly larger but otherwise identical to the AR15. But on my long use AR15, you can see wear on the AR15 collar and the notch of the hammer where contact was occasionally made.

There are also a number of "enhanced" AR15 carriers that are shrouded like the M16 and have similar weight; but have the shorter back end of the AR15 carrier. I believe both RRA and CMT sell one.

 

[Onmilo]

Here in Illinois having an M16 bolt carrier in your AR15 is asking for a seizure and arrest by BATFE if they ever have just cause to inspect the weapon.

Play it safe if you live in a 'restrictive location' and use only AR15 components.

 

[DMK]

Here in Illinois having an M16 bolt carrier in your AR15 is asking for a seizure and arrest by BATFE if they ever have just cause to inspect the weapon.

Play it safe if you live in a 'restrictive location' and use only AR15 components.

Why would a federal law enforcement organization enforce law differently in different states? If you are breaking a federal law (and I'm not saying using an M16 carrier does) then it doesn't matter what state you are in.

 

[5.56]

Well, for my 2 cents worth. If one were to remove the diconnector spring from any AR15 rifle with ANY parts in it. The chances of a slam fire are very good. If the letters are accurate about the parts allowing the rifle to fire in full auto.
Then I would think a complete set of M16 parts still would not allow a AR15 to fire full auto unless a auto sear was used or the disconnector spring was removed to allow slam firing. Hence the soft primered ammo would fire. Meaning most US made primers. Imported ammo such as South African use a harder primer.
The best thing if your using M16 parts is to grind the spur off of the disconnector making it a SP1 disconnector. and do not use a M16 selector. But again, all they have to do to modify a AR to slam fire is to remove the disconnect spring.
I built a few rifles in the service. I am not an expert. It is just a humble opinion is all. My out is to own class III stuff I got prior to the may of 86 screwing we all got.

5,56

 

[TexasRifleman]

Here in Illinois having an M16 bolt carrier in your AR15 is asking for a seizure and arrest by BATFE if they ever have just cause to inspect the weapon.

Play it safe if you live in a 'restrictive location' and use only AR15 components.

This is another one of these wacky scare stories. Why in the world would ATF, a Federal department, have more cause to arrest you in Illinois than in Texas?

That's just silly. ATF is federal. If your state law says you can't have M16 parts that's one thing, but I'd be shocked to see such detail in state statute.

The M16 bolt carrier only, with no other M16 parts, is a non-issue.

Can you buy a new Colt AR15 in Illinois from a dealer? If so, according to you, you're going to prison. Colt is shipping M16 bolt carriers in many models now. So it's conspiracy from Colt to get us all sent to prison?

 

[Onmilo]

Yeah, yeah.
It's silly, it's crazy, it's wacky, on and on.
It isn't a scare story,it's the way it is.
I didn't say you would do hard time if the charges came before a Federal court here, though you might.
I said, in not so many words, one of the wacky left wing, in the pocket of the powers that be around here agents may, and in all likelyhood will, find cause to seize your weapon and press charges against you.
Move up here and test the waters if you want to, we need more pro gun people.
The Attorney fees and court costs to play the game will most likely bankrupt you.

I haven't seen a brand new, recent production, Colt AR15 for sale around here and I work for a gun shop.
Buyers aren't willing to pay Colt prices for what they consider an overpriced, overrated, but original manufacturer weapon.
Rock River Arms is in the quad city area.
So is Les Baer and Springfield Inc.
D.S.A, is down Barrington way and D.P.M.S. is up the road in Minnesota.
Most AR15 shooters around here seem to buy their rifles from up Maine way, myself included.
Bushmasters come with an AR15 type bolt carrier, as do the other AR manufacturers rifles I listed. keeps everyone safe.
We don't see a lot of Colt anything around these parts except for used guns.

This is Illinois guys.
We have NO type 3 allowance, none.
It is even near impossible for law enforcement to obtain permission to possess class three firearms and following the stringent regulations placed upon them gives Police Captains ulcers.
The only folks who have class three here are bodyguards of the delegates of the United Nations, bodyguards of and friends and collegues of political powers who tend to make their residence in the north eastern part of the state.

Did I mention it was Chicago politicians who were at the forefront of the 1937 machinegun ban and Gun Control Act of 1968?
No?
Well I just did.

 

[middy]

Did I mention it was Chicago politicians who were at the forefront of the 1937 machinegun ban and Gun Control Act of 1968?
No?
Well I just did.

How can you stand the overwhleming stench of Commie around there?

 

[Too Many Choices!?]

M16 carrier in an AR-15...

The M16 carrier, even though it is required for,"rock and roll", is not part of the FCG(ie not part of the lower wich house the true fire control components ), and therefore not regulated as a MG part. The auto sear, selector, disconnector, and if you beleive the BATF, a hole in the lower receiver are the fire control parts.

 

[Onmilo]

If the pro gun people leave.
If the Illinois State Rifle Association packs up and moves to Iowa.
ALL is lost here.
Besides I love my wife and she wants to stay.

 

[Jon Coppenbarger]

Most folks use the Tubb carrier weight system or make on as you did. I do have a couple of the carrier weight systems and have experimented with them this year.
What you are trying to achive is to delay the cycling of the bolt slightly. By having the lock time reduced you get a large decrease in pressure that allows you a few advantages of the guy not using them if you wish to go a certain direction.
First you will see less recoil and be able to get back on to your next shot faster. What you achive may only be a second or less per shot but that allows you more time in your rapids and the rapids being a time management thing every second you spend making a good shot adds up.
Also it allows you to try hotter loads or loads you see pressure signs on.
I do not use it in cooler temp. matches but during the summer it is the standard fare in my rifles now except one and that ntit rifle does not need it.

Does the average joe need it? I do not think so but if you live on the edge and are trying to push it? The extra weight will not save you from bad habits.

A few of you might of heard the story but did not know I was involved in it. I will not mention the other persons name but this is a example of something that can go wrong.
We were shooting a palma match and we were using his rifle and the loads he wanted to try. The rifle was a service rifle legal ar15. He had changed the loads because of the new Lapua brass had more space this year that before. By bumping the load to get the same velocity we figured it would be a safe starting point.
He fired his 5 sighters and 15 shots for record at 800 yards and lost like 5 primers. I fired the same rifle and ammo right after he did and lost 2 primers.
Back at 900 yards we decided to add the carrier weight as it should reduce the pressure. I fired first at 900 yards. My two sighters and 15 shots for record and had no bad signs of pressure and no lost primers with the carrier weight. He lost the primr on shot two I believe and the gun blew up on shot 4 or 5. He took a trip to the eye doctor before the match ended. He turned out ok but it was touch and go for a few weeks.

Now did the carrier weight as you are using make a difference?
I believe it did for me and the way I used it. I did the following he did not. The temp that day at the time was more than likely in the high 90's and the temp on the mat was like a little over 100. I kept my ammo wrapd in a towel and in the shand inside my stool and always kept in covered and out of the sun and directly off the hot mat.
I keep the round in my fingers by the port and then read the wind and put the round in fast and fired and my shots go off fast in slow prone. Results no signs of pressure.
He keep his ammo in the hot truck, took it out and layed it on his mat. Then he would put it in the chamber without closing the bolt, read the wind and then close the bolt and re-check the wind and then fire taking more time than myself. Results BOOM!
Now were we playing Russian roulet? Maybe Was it stupid? Yeah.

My whole point to this was why use the carrier weight if you do not need to. Now as far as the carrier weight goes I really do not see why you could not use it all the time if it is legal. I think it would help a few things.

No I will not tell what load we were doing but trust me it was moving! LOL

 

[saltydog]

1) My understanding of the law is for the Courts to show "intent" for a person to make a Full Auto. AND:

2) My understanding is that ATF must make the weapon, that was confiscated, fire more than one round with a single pull of the trigger.

1 and 2 go hand in hand. Is a full auto carrier bad? Depends how honest Law Enforcement is. Under normal conditions 1 and 2 are not violated. If said law enforcement agency, by any means they wish, engineer a condition 2 (which can be done to any semi auto) that would allow said weapon to fire full auto then you have a problem, carrier or not. Condition 1 usually always backs the law enforcement agency.

Point is-->law enforcement agency needs to show the Courts that said weapon can fire full auto, period. Is there room for abuse? Yes, we all know this. If law enforcement wants to make "any" semi auto fire full auto they can easilly do this with no extra parts, carriers, etc. even though a safety condition would exist to the shooter. Remember, 2 rounds going off with one pull of the trigger is all law enforcement needs for a conviction even if the event never occurs again. My understanding is that this condition 2 can "sometimes" happen with "ALL M16 FCG parts installed, including carrier" and that a few people have been charged and convicted with this violation and, not with just the carrier alone.

 

[Trumpet]

FWIW,
All of Colts new LE rifles are being shipped with M16 BC's.

 

[Jeff White]

Onmilo said;

Here in Illinois having an M16 bolt carrier in your AR15 is asking for a seizure and arrest by BATFE if they ever have just cause to inspect the weapon.

Play it safe if you live in a 'restrictive location' and use only AR15 components.

You are either sadly misinformed or rumor mongering my friend. The BATFE does not have different laws for different states. A federal law or in this case an administrative ruling by an agency charged with making such a ruling to guide in the enforcement of the law has the same weight across the USA. All 54 states and territories.

There is nothing whatsoever in the Illinois Revised Statutes stating what parts constitute a machine gun either.

Yeah, yeah.
It's silly, it's crazy, it's wacky, on and on.
It isn't a scare story,it's the way it is.
I didn't say you would do hard time if the charges came before a Federal court here, though you might.

Do you really think that a federal prosecutor would waste his/her time with a case like that, even for political reasons? If you have an M16 bolt carrier in your AR15 and your AR15 doesn't fire multiple shots with one trigger actuation, you are fine. You are not in violation and no amount of wishing by an antigun politician or prosecutor will make it so.

This is Illinois guys.
We have NO type 3 allowance, none.
It is even near impossible for law enforcement to obtain permission to possess class three firearms and following the stringent regulations placed upon them gives Police Captains ulcers.

Yeah they are so hard to get it took me all of 20 minutes at the computer at work to produce the required paperwork to get a full auto M16A1 free from the federal government under the 1033 program.

The only NFA items not available to law enforcement in Illinois are suppressors.

The only folks who have class three here are bodyguards of the delegates of the United Nations, bodyguards of and friends and collegues of political powers who tend to make their residence in the north eastern part of the state.

It actually would be harder to find a police department in the state that doesn't have at least one machine gun sitting around somewhere. They may not be used for political reasons, they may be collectors items like Thompsons or Reisings, but they are out there. Have been for years.

Did I mention it was Chicago politicians who were at the forefront of the 1937 machinegun ban and Gun Control Act of 1968?

The National Firearms Act was passed in 1934 and it didn't ban anything, it just taxed things. Back then the politicians respected the constitution enough not to even use the word ban.

Jeff

 

[Onmilo]

I am not rumor mongering my friend and I resent the accusation.

I have stated my view based on the state I live in, hey it is your state too.

You have your opinion and I have mine.

Have you yourself done all the things you have printed or are you just trying to prove me a simpleton for my view and opinion on one subject?

The point of my thread wasn't to say, no the M16 bolt carrier cannot be used and will do anything to benefit or break the AR15.

I know an M16 bolt carrier can be used in an AR15.
I see no benefit that the heavier M16 bolt carrier will provide.

The point of my thread was to be absolutely certain the part is absolutley legal to install in the firearm before you use the part.

The point is to be absolutely certain of what the individual is doing before they attempt an experiment that ends up getting that individual in trouble with law enforcement.

If you trust Federal employees who have the job of determining acts that break laws in this state, and yes, they can bring charges against an individual based on state or federal violations, that is your opinion and view.
I am not so generous with my trust of governmental law enforcement.

 

[GoRon]

Well at least you know if Jeff is at the same range as you and you have an M16 bolt/carrier in your gun HE won't arrest you.

As a LEO he may have an idea whether it is legal or not.

 

[Bartholomew Roberts]

1) My understanding of the law is for the Courts to show "intent" for a person to make a Full Auto. AND:

According to the Staples decision, you only need to show knowledge, not intent. If the government can show you knowingly added an M16 bolt carrier and that created the situation you describe in #2 (even if you weren't aware that it was possible), you are probably going to court over it.

Point is-->law enforcement agency needs to show the Courts that said weapon can fire full auto, period. Is there room for abuse? Yes, we all know this. If law enforcement wants to make "any" semi auto fire full auto they can easilly do this with no extra parts, carriers, etc. even though a safety condition would exist to the shooter. Remember, 2 rounds going off with one pull of the trigger is all law enforcement needs for a conviction even if the event never occurs again. My understanding is that this condition 2 can "sometimes" happen with "ALL M16 FCG parts installed, including carrier" and that a few people have been charged and convicted with this violation and, not with just the carrier alone.

Well, I look at it like any other issue - what are the costs and risks compared to the benefits. The benefit is a slightly slower cyclic rate and delayed time to unlock. I can get both of those more reliably by switching to a heavier buffer. The costs are minimal and the risks are minimal as rare; but if you ever get in the type of situation where ATF is closesly examining your rifle with an eye towards prosecution, the M16 bolt carrier won't help you. On the other hand, despite the complaints with gun laws, for most of us it isn't that hard to keep out of situations where ATF is closely examining your rifle with an eye towards prosecution.

I wouldn't sweat the legal considerations in my own rifle; but I also don't think you gain much with an M16 bolt carrier compared to some of the enhanced AR15 bolt carriers with the firing pin shroud.

 

[TexasRifleman]

but if you ever get in the type of situation where ATF is closesly examining your rifle with an eye towards prosecution, the M16 bolt carrier won't help you.

I think this is the most important take away from this discussion. If you've done something to warrant this much attention to your AR, I suspect your goose is cooked anyway. At that point it won't likely matter what parts you have installed in your rifle.

 

[saltydog]

If the government can show you knowingly added an M16 bolt carrier and that created the situation you describe in #2 (even if you weren't aware that it was possible), you are probably going to court over it.

Your are right but it would be hard to prove "knowingly". Much easier to show the judge or jury your fully functional full auto AR15. There was a case, sometime back were an individual "unknowingly" bought a full auto AR15 from and individual. Trial by Judge. I don't have the case number but I followed it on the news. Judge ruled in favor of the defendent because he did not know the weapon he purchased was full auto. The defendent walked but of course the weapon was confiscated.

The big deal with M16 parts is because of the 10's of thousands of drop in auto sear's that were sold in the early 80's. If its wasn't for that, I don't think any of this hype would be a big deal.

Personally, when it comes to AR15 parts, I could care less as I prefer the AK class of weapons....

 

[Bartholomew Roberts]

There was a case, sometime back were an individual "unknowingly" bought a full auto AR15 from and individual. Trial by Judge. I don't have the case number but I followed it on the news. Judge ruled in favor of the defendent because he did not know the weapon he purchased was full auto. The defendent walked but of course the weapon was confiscated.

Yes, this would be due to the Supreme Court decision in Staples v. United States decision, I referenced earlier. It must be shown that you knowingly possessed a machinegun; but the decision allows some leeway in the circumstantial evidence used to prove knowledge. Just saying "I didn't know" may not save you.

 

[Jeff White]

I'm not convinced of the utility of the M16 carrier in a semi-auto carbine. I have an older Colt R6920 (purchased new on a letterhead in June 1999). It came with small fire control pins, no receiver block and a 3/4 moon AR15 carrier. It also has the M4 "H" buffer. I have over 10k rounds through this carbine and have yet to have a malfunction that wasn't intentionally induced my me for training purposes, ammunition related (had a squibb load with Federal XM193 in Pat Rogers class in 2003) or operator induced (forgot to "push/pull" and failed to seat the magazine, took a long time to reprogram myself for 20+ years of the Army teaching slapping the magazine into place).

I'm not sure that the extra weight of the M16 carrier is going to make that much difference on a semi-auto. I do know from personal experience that the "H" buffer is often necessary to get a full auto with a short gas system to function correctly. Last year I converted 3 M16A1s that the Sheriff's dept received on the 1033 program to carbine configuration for the tactical team. The county bought Rock River Arms Parts, 16" barrels and collapsible stocks. Two of the carbines wouldn't function on automatic. They would pass the function check, but load them up, put the selector on auto and they would fire one round. The problem was bolt bounce. A call was put in to Ken Elmore at SAW LE sales and three Colt "H" buffers were soon enroute. They slowed the bolts enough that they functioned on full auto. I replaced the Rock River buffer on the carbine that functioned to avoid any future problems.

I think the only advantage you get in a semi auto with an M16 carrier is the shroud around the firing pin.

I'm not sure that Colt's suddenly getting permission from BATFE to ship their rifles with M16 carriers isn't more motivated by economics then it is by a desire to make their weapons more reliable. Colt has shipped AR15s in so many variations that I swear they just use what parts are in the bin that day, no matter what rifle they are supposed to go on. It makes collecting them interesting and causes a lot of heated arguments on the internet about what parts go in what rifle.

Onmilo,
I have done all the things I mentioned in my earlier post. Based on what I have seen from teaching patrol rifle courses to other officers here in Illinois, you'd be hard pressed to find one that knows the difference between an M16 and AR15 bolt carrier or any other internal part for that matter.

The letter from BATFE technical branch trumps all anti-gun politics as far as being prosecuted for simply having an M16 bolt carrier in an AR15.

I don't think we do our cause for freedom here any good by posting bad information. I apologize for insinuating you were rumormongering. But you simply aren't correct in your assertion that installing an M16 bolt carrier in your AR15 is and of itself going to land you in legal hot water in either state or federal court because you are in Illinois. Ask an officer up where you live how hard it is to get the US Attorney interested in filing NFA charges against a criminal for possession of an unregistered NFA weapon. It's pretty hard to get them interested if you have a bunch of state charges to prosecute them with. The meth cook or crack dealer is most likely never going to see the inside of a federal courthouse for having a sawed off shotgun or an illegal machine gun, unless those are the only charges you can convict on.

Jeff

 

[Werewolf]

In order to avoid violations of the NFA, M16 hammers, triggers, disconnectors, selectors and bolt carriers must not be used in assembly of AR-15 type semiautomatic rifles, unless the M16 parts have been modified to AR-15 Model SP1 configuration. Any AR-15 type rifles which have been assembled with M16 internal components should have those parts removed and replaced with AR-15 Model SP1 type parts which are available commercially. The M16 components also may be modified to AR-15 Model SP1 configuration.

It appears to me that the ATF letter is saying that an M-16 bolt carrier is only Okay if it has been modified to meet an AR-15 configuration specification.

Are the AR-15 and M-16 configurations identical? If not then it seems to me the ATF is saying you cannot use the M-16 Bolt Carrier in an AR-15 unless it is modified to AR-15 specs. That's what it says in the letter.

Am I missing something here? What am I failing to understand?