AR15 Short Stroking Conundrum

[CamoChameleon]

Hello, new to the forum, and not new to guns. I'm having a problem with the AR15 I recently built (my first). I have been having short stroking problems since the start; 20% of my spent casings are ejecting, and it never strips next round from mag. Last weekend, I had a friend help me diagnose what part(s) are the problem. My gun is built on an Anderson 80% lower, with a Hardened Arms 16" midlength freefloat upper, a PSA carbine lower parts kit, and supposedly a "Nitrided Toolcraft BCG", but I have my doubts because it looks to be wearing too fast for nitride, it looks like phosphate, and I got it from Delta Team Tactical <facepalm>. My friend has a PSA 16" midlength on a pistol lower with a Nickel Boron BCG from Brownells. His setup functions perfectly.

We tried various frankensteins of parts from our guns, and these are the results we came up with:

My upper + my BCG = no function
Friend's upper + my BCG = no function
My upper + Friend's BCG = function
My upper + Friend's carrier + my bolt = function
Friend's upper+Friend's carrier+my bolt =function
Friend's upper +my carrier+Friend's bolt=function
My upper + my carrier + Friend's bolt = no function, although the rifle would finally eject reliably and almost pick up the next round, but would jam it into the side of the barrel extention and crush the side of the case.

It seems from this list that the problem is with the bolt carrier itself, except for the result of test #6 above. Since it seems like the carrier is phosphate and thus softer than nitride, I am hoping there is something I can do to fix my current carrier, but otherwise I will just have to buy a new carrier.

Some more info about things I have looked at:
1: Gas block is aligned
2: Gas port is large enough
3: Gas key is solidly tight and properly staked
4: I am trying to contact Toolcraft to see if there is any way to identify if this bolt is really theirs. If so, I will just send it in for warranty replacement or upgrade or something.
5: Problem persists with both .223 and 5.56mm.
6: I have put about 200 rounds through it thus far, with only 10 fully cycling, and those were while using my friend's bolt carrier.
7: Gas tube and gas key are unobstructed.
8: BCG is well lubed with molybdenum grease (sure there's better stuff for long term use/storage, but it is what I had and what my friend is using.)
9: Bolt will support its own weight when standing on the bolt head, so the gas rings are good.
10: BCG is long past replacement period on DTT's policy page. I have been accruing parts for a while now, and I certainly know more about the markets and brands now than I knew at the beginning.
11: I cycled the action manually ~200-300x in the months before this testing.
12: When it fails to eject, you can see brass, so it extracts ok. When filmed with high speed, the BCG comes back just far enough to cock the hammer, but not too much farther than that.

I am at a loss. I can tell you that through this experience I am learning more detail about this system and its operation than I knew there was to learn!

 

[carbine85]

What ammo are you using?
What mags are you using?
Are you using the correct buffer and recoil spring?
Check the gas key on the carrier for proper torque. I have seen these staked in place but loose causing a gas leak.
Is the gas tube perfectly lined up in the upper receiver and not causing any binding? The carrier should fall home without any help or just a touch.
Is the receiver extension damaged in any way and is the the castle nut tight.
You said the gas port is large enough. What size is it?
Does the bolt hold open work on each and every single round?
"Short Stroking" is normally gas or binding.
Incomplete cycling is normally from magazines, alignment or binding.

 

[Blue68f100]

Toolcraft BCG may be your problem if it's anything like the AR-10 BCG I got from them. The machining of the body where the bolt gas rings ride was so rough that it was gouging the rings in less than 50 rounds. After 20 rounds the bolt got very hard to close or open. Very rough machining in a area that should be smooth.

Carbine85 covered it pretty good. Just need to check that every thing is correct. I had a barrel come in that the case port had not been cleared of chips and was not clean all the way through.

Like said magazines can cause this too when the follower is sticking/binding.

 

[CamoChameleon]

I have used both Federal .223 and Winchester 5.56mm, both 55gr. I have Mission First Tactical mags, but was using these same mags when I used my friend's bcg. The carrier requires a light touch to put into battery when not under spring pressure (with receiver tilted to 45°), but when I ride the charging handle forward slowly, or drop it with the receiver vertical, it goes into battery every time. I am using a standard carbine buffer and spring which cycled reliably with my friend's bcg. Castle nut is tight, receiver extension is not damaged. I do not have the necessary vices and wrenches to remove the handguard to get an exact measurement on the gas port, but I was able to snake an allen key in to remove the gas block and look at the port. I found that it was a little smaller than the flats of a .0938 (3/32)" allen wrench. I'm estimating its in the .075" - .085" range, which seems to be within the reasonable range on other forum posts. My bolt catch doesn't have its full range of motion and hasn't worked from the beginning. Even if I push up hard on it with my finger it won't engage the bolt. That is what I will work on fixing next once I get it functioning. I will double check the gas key bolts when I get home from work in about an hour.

 

[CamoChameleon]

Just checked the gas key bolts, they are definitely tight. I could not get them to budge.

 

[entropy]

Replace the BCG. Remove the bolt catch, clean up the inside surfaces where it sits, replace the bolt catch roll pin and spring at least, and clean up the surfaces on the bolt catch if you keep it. (replacing it is cheap insurance)

 

[GunnyUSMC]

Call Glen at Delta Team Tactical. Tell him the trouble you’re having and what you have tried. Glen helped me with a 300 Blackout upper kit I bought from Delta. It had a carbine length gas system. We talked a few times, trying something different each time. Lighter buffer, shorten the buffer spring, polish the chamber, changed the gas block and opened the gas port a little more. I must of taken that upper apart 5 or 6 times. I even messed up a screw on the rail and scratch it too. Glen had me sent the upper to him. About two weeks later my upper made it home. They replaced the barrel with one with a pistol gas system and to my surprise they even replaced the hand rail, all at no charge.

 

[carbine85]

Just checked the gas key bolts, they are definitely tight. I could not get them to budge.

I would check the actual size of the gas ports in the barrel and gas block and fix that mag catch. If that does work I'm at a loss.
Did you try another bolt carrier group? I didn't go back and reread everything.
You can open the gas ports. Just use a good ream and go small amounts.

 

[Blue68f100]

I have used both Federal .223 and Winchester 5.56mm, both 55gr. I have Mission First Tactical mags, but was using these same mags when I used my friend's bcg. The carrier requires a light touch to put into battery when not under spring pressure (with receiver tilted to 45°), but when I ride the charging handle forward slowly, or drop it with the receiver vertical, it goes into battery every time. I am using a standard carbine buffer and spring which cycled reliably with my friend's bcg. Castle nut is tight, receiver extension is not damaged. I do not have the necessary vices and wrenches to remove the handguard to get an exact measurement on the gas port, but I was able to snake an allen key in to remove the gas block and look at the port. I found that it was a little smaller than the flats of a .0938 (3/32)" allen wrench. I'm estimating its in the .075" - .085" range, which seems to be within the reasonable range on other forum posts. My bolt catch doesn't have its full range of motion and hasn't worked from the beginning. Even if I push up hard on it with my finger it won't engage the bolt. That is what I will work on fixing next once I get it functioning. I will double check the gas key bolts when I get home from work in about an hour.

If your mag lock is not working manually, how do you expect it to work when fired. If not going up fully check to see if it's hitting the upper housing on the inside as it rotates up. If so you will need to removes some metal from somewhere, preferably the catch or replace it with one that will work. Most mag catches have a generous clearance. I've take a few to the grinder on AR10 which do not have the same clearance as the AR15's do.

The gas ports on a Mid length system should be in the range of 0.078"-0.081", if carbine 0.0625"-0.070". Need to check that the gas port is not being partially blocked by misalignment. Some require you to set a gap of 0.025" between the block and barrel step. This is to compensate for the metal ring associated with plastic hand guard hardware. Then some mfg tells you to make contact due to design/size of the opening used. If you need to open the port up use a set of Number Drills bits. If you have to drill these it's best to use a drill press/endmill so you can control the depth. Run your drill bit at high speed, this will give you a cleaner hole. Also run a brass rod down the barrel to protect it when you go through. You will have some flashing that will need to be removed after wards. Just use jags and cleaning brush till its gone. If you go to large you will need an adjustable gas block.

 

[CamoChameleon]

Did you try another bolt carrier group?

Yes I did, with detailed description of the results of every combination of my friend's parts and my parts. It is the second section of my original post. My rifle cycled perfectly with my friend's BCG, and my bolt inside his bolt carrier. I got ejection followed by failure to feed when using my carrier and my friend's bolt.

If your mag lock is not working manually, how do you expect it to work when fired

I don't. I was just saying that trying to have the mag lock back on empty won't be a valid test. Once again, with all things the same besides the bolt carrier, I had perfect function. I was mentioning the magazines in response to your concern about the magazine/follower sitting too high and impeding the bolt.

check to see if it's hitting the upper housing on the inside as it rotates up.

Replace the BCG. Remove the bolt catch, clean up the inside surfaces where it sits, replace the bolt catch roll pin and spring at least, and clean up the surfaces on the bolt catch if you keep it. (replacing it is cheap insurance)

It doesn't rotate enough even with the receivers are separated. I will follow your tip though and try removing some metal from the catch itself to try to free up the motion. If I can't figure out the specific problem with this carrier, it looks like I will be going the replacement route as well.

Replace the BCG. Remove the bolt catch, clean up the inside surfaces where it sits, replace the bolt catch roll pin and spring at least, and clean up the surfaces on the bolt catch if you keep it. (replacing it is cheap insurance)

Were you able to polish this area, and did it help? Also, are there any special ways to polish bolt carrier rails? I'm noticing that they seem to not be perfectly smooth, but you can see some tooling marks. Is there any detriment to doing this (other than if I go overboard and grind them out of tolerance)?

I will see if I can figure out a way to get an exact hole size, but I don't think that is the problem since I had perfect function with replacing the bolt carrier.

Any tips on how to touch up the carrier and give it the best last chance of running before I replace it?

 

[entropy]

Were you able to polish this area, and did it help? Also, are there any special ways to polish bolt carrier rails? I'm noticing that they seem to not be perfectly smooth, but you can see some tooling marks. Is there any detriment to doing this (other than if I go overboard and grind them out of tolerance)?

I've never had to clean up a bolt catch, military or AR. Use the 2000-3000 grit sandpaper I mention below to do it. Have you inspected the bolt gas rings and the 'tunnel' they ride in? Just because the bolt carrier stands upright on the bolt does not necessarily mean the rings are good, if they are all scored from the 'cylinder' area they run in. (The bolt is the piston, complete with rings, the area in the bolt carrier where it sits is the cylinder) Is that nice and smooth as a baby's butt? It should be.

The BCG 'rails' are radiused, not the easiest to polish. Shouldn't be necessary; read below about correct lubrication. If you were in Florida, glopping moly grease in it might make sense for storage, but definitely not in NM. If you must polish it, I'd recommend lightly touching it up with 2-3000 grit sandpaper. (Walmart sells it in the automotive bodywork section)

BCG is well lubed with molybdenum grease (sure there's better stuff for long term use/storage, but it is what I had and what my friend is using.)

Incorrect lube. This is probably part of your problem, though certainly not all of it. If you want to use any grease, use a tiny amount on the BCG rails, I have used TW-25 on the rails. Have a bottle of Break Free CLP handy when you shoot; when you get malfunctions (after everything else is fixed), squirt a drop or two at the base of the BCG. (There's a reason you see pictures of soldiers with a bottle of LSA or CLP in their helmet bands)

Any tips on how to touch up the carrier and give it the best last chance of running before I replace it?

Inspect the areas I just mentioned, and if the ring edges are scored and the 'cylinder' is rough, you can try to smooth it up using the same methods one would hone a chamber, but if you get it out of round, you will lose gas pressure and it will not function. You of course would have to replace the rings in that case. OTOH, that's where your at now, so you have little to lose.

I can tell you that through this experience I am learning more detail about this system and its operation than I knew there was to learn!

You haven't even scratched the surface.....

 

[carbine85]

As mentioned above, don't use grease.

 

[Blue68f100]

Contact Toolcraft CS and tell them your problem and what you've tried. They will want to look at the BCG and may/will replace it. They back there products.

 

[4v50 Gary]

Make sure those gas rings on the bolt are not perfectly lined up such that gas can escape. You want them staggered.

Colt or USGI magazines? Too much spring pressure can impede the bolt's movement.

 

[BilliamB.]

Are you holding your rifle firmly enough? I prefer a soft hold (less muscle tension, fewer yips), but hold her too lightly and she'll short stroke. FWIW, of course. Also, I find that everything is better behaved (and stays cleaner) when I degrease the bolt and trigger in favor of TSI. Again, this is based on my experience only. Good luck!

 

[boom boom]

Everyone above covered much of what could be wrong. My take is the process that you are using can confuse issues. ONLY CHANGE ONE VARIABLE AT A TIME when trying to isolate problems.

For example, AR's generally like to be run wet--get rid of the grease by thoroughly degreasing then reoiling with the appropriately light oil. Try it, note any failures, etc.

Parkerizing is designed to be rough as to soak up oil to prevent corrosion. That is why it is a pain to remove old cosmolene from old milsurps as the cosmolene (milsurp storage grease) thoroughly penetrates the parkerizing, it just keeps seeping out unless thoroughly degreased. You will probably find the same problem removing the grease from your carrier. Boiling water and degreasing solvents are your friend. Then to the next problem if any. BTW, be careful with your greases and avoid any with chlorine as these have been linked to stress cracks in metal at junctions.

Use bog standard store bought ammo, a consistent shooting method, and only change one variable at a time such as the buffer weight until you find what works. Keep detailed notes on what you did.

One of the reasons that I do not prefer working on semi-autos is that the chain of events required to run properly gives more opportunity for gremlins and thus these are harder to troubleshoot.

 

[entropy]

Are you holding your rifle firmly enough? I prefer a soft hold (less muscle tension, fewer yips), but hold her too lightly and she'll short stroke. FWIW, of course. Also, I find that everything is better behaved (and stays cleaner) when I degrease the bolt and trigger in favor of TSI. Again, this is based on my experience only. Good luck!

Horse hockey. I've fired them full auto, extended like a pistol, with nary a hiccup. Two at a time, even.(Gotta use up all that extra requesitioned range ammo somehow... )

ONLY CHANGE ONE VARIABLE AT A TIME when trying to isolate problems.

Sage advise.

 

[CamoChameleon]

Thanks for all the replies! I was out of town, hence my tardiness with my replies. Let me try to touch on the things everyone has said:

Have a bottle of Break Free CLP handy when you shoot; when you get malfunctions (after everything else is fixed), squirt a drop or two at the base of the BCG.

I will certainly be buying some soon, as I hear nothing but strong recommendations in favor of it. As you mentioned though, it is likely not the cause of the malfunction, especially seeing as a different bolt carrier (with same bolt, upper, and lubricant) ran perfectly. I am guessing the reason moly grease is not as optimal in NM is because of the possibility of sand gumming up the works?

Make sure those gas rings on the bolt are not perfectly lined up such that gas can escape. You want them staggered.

They are indeed staggered.

For example, AR's generally like to be run wet--get rid of the grease by thoroughly degreasing then reoiling with the appropriately light oil. Try it, note any failures, etc.

My bad, I forgot to mention that failures have persisted while using Hoppes #9 as well (I don't know if this counts as a sufficiently light oil). The reason I made the switch to moly grease for this testing is that a) I ran out of Hoppes and couldn't get more before testing, and b) I had plenty of the moly grease left over from an airgun project and my friend's rifle was demonstrating perfect function with the same lubricant. I will be obtaining CLP in the near future, but I am certain that this is not the main cause of the failures.

Use bog standard store bought ammo, a consistent shooting method, and only change one variable at a time such as the buffer weight until you find what works. Keep detailed notes on what you did.

The reason I did my testing in this manner is to conserve ammunition ( and thus $$$). My friend and I are both married mechanical engineering grad students, and at least on my end, a few hundred rounds is more than I can shoot wisely at a given time, especially in a rifle that isn't working. We shot in the method we did to narrow down by subsystem/part what the malfunction was. First, we concluded that the upper receiver assembly was not the problem, due to perfect function of my receiver with my friend's BCG. This same step showed us that the problem was within the BCG assembly, as Friend's BCG functioned in my rifle, and my BCG failed in his rifle (these were the only parts switched). Then, narrowing even further, we tried our respective rifles, with only our bolt carriers being swapped (all other parts left the same). My rifle functioned, as did my friends, which surprised me, as I had begun to suspect the carrier. Seeing how Friend's rifle functioned with my carrier and Friend's bolt, I wondered if perhaps the malfunction was a result of a simple tolerance error between my carrier and my bolt. Shooting my upper with my carrier and Friend's bolt still malfunctioned. Every combination with my bolt carrier in my upper malfunctioned, and my BCG malfunctioned in my Friend's upper in the same manner as it would in mine. Reading this over, I see that forgot to include that Friend's upper + Friend's carrier + my bolt also functioned. Are there any combinations I am leaving out that you think would better shed light on the situation? Do you think there is just some compounding tolerance issue at play?

 

[CamoChameleon]

Are you holding your rifle firmly enough?

I am holding the rifle firmly.

Parkerizing is designed to be rough as to soak up oil to prevent corrosion. That is why it is a pain to remove old cosmolene from old milsurps as the cosmolene (milsurp storage grease) thoroughly penetrates the parkerizing, it just keeps seeping out unless thoroughly degreased. You will probably find the same problem removing the grease from your carrier. Boiling water and degreasing solvents are your friend. Then to the next problem if any. BTW, be careful with your greases and avoid any with chlorine as these have been linked to stress cracks in metal at junctions.

Good tips for when I go to clean the grease in preparation for CLP. I will reference this in the future.

Contact Toolcraft CS and tell them your problem and what you've tried. They will want to look at the BCG and may/will replace it. They back there products.

I sent an email service request about a week ago, but haven't heard back. I may have to prod them a little bit or call them. Since the BCG was a gift, I do not have the receipt. Are there ways to identify if the carrier is in fact by Toolcraft? I haven't been able to find anything conclusive yet. Mine is unmarked and full auto profile.

 

[Blue68f100]

I quit using CLP decades ago when they changed the formulation. I still have the old and use it only for protection, not lube. I use only syn lubes today. The best one I've found so far has been the new Amsoil Gun oil, very slick oil. I also use a lot of MP-10 oil prior to testing the Amsoil. Syn ATF actually works a well as any thing I've used.

You probably need to contact them again. I got a response from them with in 24hr. I suspect the issue is the Carrier where the rings ride. I don't know what it is about theirs but after 50 rounds if you pull the BCG the bolt is supper hard to move in or out. I don't have this problem with other brands of carrier. I will probably never my another one from them from the issues I have had with mine.

 

[boom boom]

I quit using CLP decades ago when they changed the formulation. I still have the old and use it only for protection, not lube. I use only syn lubes today. The best one I've found so far has been the new Amsoil Gun oil, very slick oil. I also use a lot of MP-10 oil prior to testing the Amsoil. Syn ATF actually works a well as any thing I've used.

You probably need to contact them again. I got a response from them with in 24hr. I suspect the issue is the Carrier where the rings ride. I don't know what it is about theirs but after 50 rounds if you pull the BCG the bolt is supper hard to move in or out. I don't have this problem with other brands of carrier. I will probably never my another one from them from the issues I have had with mine.

Believe if you get the CLP collector, it is the original formula. Have to look for it though.

 

[4v50 Gary]

Magazines?