Whenever I'v even mentioned an SBR AR in a gun shop, the salesman always tells me how unreliable they are (we're talking 223). Considering this came from a gun shop, I'm not sure how valid this is. I'v heard this from several people, however, but are they really that unreliable? Anyone own an SBR AR or anybody who knows what they're talking about share their thoughts on the subject? thanks!
Are SBR AR's as unreliable as I'v been told?
- Thread starter Colt117
- Start date
- Status
BADUNAME13
Member
I've shot M4's alot, does that count?
Seriousley, it's simple physics. The AR system starts the bolt and bolt carrier to the rear while the bullet is still in the barrel.
With a SBR, the pressure spike doesn't happen soon enough.
Make my AR 16 or 20 inches, and SBR something else.
Seriousley, it's simple physics. The AR system starts the bolt and bolt carrier to the rear while the bullet is still in the barrel.
With a SBR, the pressure spike doesn't happen soon enough.
Make my AR 16 or 20 inches, and SBR something else.
Eightball
Member
Don't always believe what you hear from a gun-store commando, regardless of which side of the counter they're on. If they can explain WHY they are unreliable, other than "bubba joe had problems with 'em"--If they can explain the physics of why, for example--then maybe it's credible.
Or maybe we should ask GunKid
Or maybe we should ask GunKid
Other than getting extremly hot when firing lots of rounds, and the unwanted interest from the ATF, they are as reliable as any quality AR. You can assemble a SBR just like its longer barrelled cousins.Use quality parts, you will have a quality gun.
bmwguy
Member
I've had one initial problem w/ my 10.5" LMT.
The problem was traced to weak extractor spring. I was getting FTE. Once I replaced w/ D fender O ring gasket and replaced the spring. It's been 100 % reliable.
BTW, the extractor spring apparently is a common problem w/ the 10.5"
The problem was traced to weak extractor spring. I was getting FTE. Once I replaced w/ D fender O ring gasket and replaced the spring. It's been 100 % reliable.
BTW, the extractor spring apparently is a common problem w/ the 10.5"
I think the problems with SBR's are exgaerated. Full autos are a little trickier to get to run with short uppers I think.
I have a 7.5 and a 10.25 in 5.56 and 1 5"9mm and all run like sewing machines on my Semi Bushmaster lower. I do run it with a 9mm buffer but that's the only change from stock. As someone above mentioned you might need to tweak the extractor with an o-ring.
There are plenty of high speed low drag types who use short uppers for serious purposes for me to discount them.
I have a 7.5 and a 10.25 in 5.56 and 1 5"9mm and all run like sewing machines on my Semi Bushmaster lower. I do run it with a 9mm buffer but that's the only change from stock. As someone above mentioned you might need to tweak the extractor with an o-ring.
There are plenty of high speed low drag types who use short uppers for serious purposes for me to discount them.
brass shower
Member
AR platforms that utilize the pistol and carbine length gas systems have a higher occurance of failures to extract and feed than those using rifle length gas systems. The reason being the gas pressure is higher when it is tapped out of the barrel and into the bolt carrier. The exact numbers of the pressure difference escape me but IIRC it's about a 20,000 PSI difference in pressure. They can be made to be as reliable, it just takes a little more work and it's easy to accomplish.
There are two problems that higher pressure causes on extraction with the short gas systems. The first is the higher pressure gas impacting the bolt carrier key causes it to be thrust rearward faster than that of a rifle length gas system. The faster unlocking motion causes more centrifigal force to act upon the extractor, flinging it outward and reducing the amount of surface area the extractor has over the rim of the cartridge.
2nd, the resistance the extractor has to overcome to to it's job is greater when using the shorter gas systems. The pressure within the barrel and pressing the cartridge case into the walls of the chamber is greater at the time of extraction, and the case has less time to contract from the walls of the chamber. The result is a case that is tougher to extract then one that would be extracted a few microseconds later in a rifle length system.
Failures to feed are also more common. The faster motion of the bolt carrier group gives the magazine less time to feed a new cartridge into position in time for the bolt's return trip. Resulting in bolt closed on an empty chamber. Extra power magazine springs can help rectify this problem.
There are a number of inventions designed to overcome these problems. Stronger buffer springs, and heavier bolt carriers and buffers slow down the motion of the bolt carrier. Extractor spring aids like the Dfender provide more positive extraction. Gas tube designs like the Pigtail and Fatboy slow the impact of the hot gas into the bolt carrier key by a few microseconds. The midlength gas system moves the gas port about 2" forward of the carbine length system so the gas pressure in the barrel is signifigantly lower by the time it is vented out and rearward into the bolt carrier. I've played with them all and find the midlength system is probably the best remedy available for 14-16" barrels without getting into a couple hundred $ more for custom parts.
There are two problems that higher pressure causes on extraction with the short gas systems. The first is the higher pressure gas impacting the bolt carrier key causes it to be thrust rearward faster than that of a rifle length gas system. The faster unlocking motion causes more centrifigal force to act upon the extractor, flinging it outward and reducing the amount of surface area the extractor has over the rim of the cartridge.
2nd, the resistance the extractor has to overcome to to it's job is greater when using the shorter gas systems. The pressure within the barrel and pressing the cartridge case into the walls of the chamber is greater at the time of extraction, and the case has less time to contract from the walls of the chamber. The result is a case that is tougher to extract then one that would be extracted a few microseconds later in a rifle length system.
Failures to feed are also more common. The faster motion of the bolt carrier group gives the magazine less time to feed a new cartridge into position in time for the bolt's return trip. Resulting in bolt closed on an empty chamber. Extra power magazine springs can help rectify this problem.
There are a number of inventions designed to overcome these problems. Stronger buffer springs, and heavier bolt carriers and buffers slow down the motion of the bolt carrier. Extractor spring aids like the Dfender provide more positive extraction. Gas tube designs like the Pigtail and Fatboy slow the impact of the hot gas into the bolt carrier key by a few microseconds. The midlength gas system moves the gas port about 2" forward of the carbine length system so the gas pressure in the barrel is signifigantly lower by the time it is vented out and rearward into the bolt carrier. I've played with them all and find the midlength system is probably the best remedy available for 14-16" barrels without getting into a couple hundred $ more for custom parts.
GunNut
Member
A properly setup SBR can and will run just as well as a carbine.
Take your carbine barrel and chop it off to 11.5" and there is a good chance that it will not function correctly. Now open up the gas port and use the right buffer setup and you will not be able to tell a difference.
Get a CMMG 10.3" upper and it will run like a champ.
Steve - who wishes he could own a SBR
Take your carbine barrel and chop it off to 11.5" and there is a good chance that it will not function correctly. Now open up the gas port and use the right buffer setup and you will not be able to tell a difference.
Get a CMMG 10.3" upper and it will run like a champ.
Steve - who wishes he could own a SBR
It depends. If it is set up right, it will run just fine. If it is not set up right, it won't. I'm no expert, but I gather that a lot of people don't tune the gas system and recoil buffer to the shorter barrel length. That'll make it run poorly, fo shizzle.
Mike
Mike
Zak Smith
Member
Like any AR-15, it has to be built properly, and some aspects are more critical in SBRs.
That said, my 12" SBR has been one of the most reliable AR's I have owned.
I have run a couple two and three-day high-volume classes with it, 500+ rounds per day, without cleaning and only adding a couple drops of lube daily, and it keeps running. At the last class I shot, it never malfunctioned.
-z
............... Larger version of above photo.
That said, my 12" SBR has been one of the most reliable AR's I have owned.
I have run a couple two and three-day high-volume classes with it, 500+ rounds per day, without cleaning and only adding a couple drops of lube daily, and it keeps running. At the last class I shot, it never malfunctioned.
-z
............... Larger version of above photo.
Still 2 Many Choices!?
Member
- Joined
- Apr 17, 2006
- Messages
- 455
While not an actual SBR..
My 10.5"(or is it 10.25"?) barreled AR pistol runs like a top. Even with Wolf
! It isn't even built out of the best parts(Model 1 Sales kit on a Rock River receiver), but it does what I want it to do, and is quite accurate at the ranges I have shot it at.
So I don't buy into all the nonsense about short barreled AR's being problematic, or needing a crap load of work to get running properly. Then again, I did decide to go with the carbine length buffer in stead of the anemic little cute pistol buffers most people decide to run, and I refused to go below what I see as the minimal usable length of barrel for a .223 firearm
. Any less than 10" would/could probably be trickier to get going reliably IMHO, and YMMV!
Still 2 Many Choices!?
My 10.5"(or is it 10.25"?) barreled AR pistol runs like a top. Even with Wolf
! It isn't even built out of the best parts(Model 1 Sales kit on a Rock River receiver), but it does what I want it to do, and is quite accurate at the ranges I have shot it at. So I don't buy into all the nonsense about short barreled AR's being problematic, or needing a crap load of work to get running properly. Then again, I did decide to go with the carbine length buffer in stead of the anemic little cute pistol buffers most people decide to run, and I refused to go below what I see as the minimal usable length of barrel for a .223 firearm
. Any less than 10" would/could probably be trickier to get going reliably IMHO, and YMMV!Still 2 Many Choices!?
HorseSoldier
Member
I've seen a large number of them used on the flat range and for the most part they seem to run just as well as their longer barreled relatives. I've seen one that was built improperly and could not get through a whole magazine without multiple stoppages. Like Coronach said, they can be done wrong and won't run. Buy one from Lewis Machine and Tool or Noveske or other quality manufacturer and I don't think you'll see many issues.
Bartholomew Roberts
Member
The big issue is that an AR15 is designed around a 20" barrel with a rifle length gas system and a certain gas port size. When you start changing those parameters, you change the way the system was intended to work and weird things can happen. brassshower described the issues well.
In SBRs, you have a shorter gas system, higher pressures at the port and chamber and a very short dwell time after the gas port. All of these things combined generally mean that an SBR has a smaller "window" where it will run reliably compared to the longer barrels.
Finally, you have to remember that as the barrel heats up, the time it takes for brass to shrink away from the chamber walls increases by microseconds. This mean resistance to extraction increases as the barrel gets hot on any AR - on an SBR, you are already at the edge of function with a normal extractor spring and you have less barrel mass, so you'll hit higher temperatures with less rounds fired.
SBRs can be made to run reliably; but that "window" where they run reliably will never be as big as it is for its longer barrelled cousins.
That's like cheating though because you have between 1.5" and 0.5" more dwell time than most SBR ARs and I'd bet the uppers are MSTN which is definitely stacking the deck reliability-wise
You can make a reliable SBR definitely; but a lot of people don't. Even LMT had lots of issues with their SBRs at first until they got the hang of it.
In SBRs, you have a shorter gas system, higher pressures at the port and chamber and a very short dwell time after the gas port. All of these things combined generally mean that an SBR has a smaller "window" where it will run reliably compared to the longer barrels.
Finally, you have to remember that as the barrel heats up, the time it takes for brass to shrink away from the chamber walls increases by microseconds. This mean resistance to extraction increases as the barrel gets hot on any AR - on an SBR, you are already at the edge of function with a normal extractor spring and you have less barrel mass, so you'll hit higher temperatures with less rounds fired.
SBRs can be made to run reliably; but that "window" where they run reliably will never be as big as it is for its longer barrelled cousins.
That's like cheating though because you have between 1.5" and 0.5" more dwell time than most SBR ARs and I'd bet the uppers are MSTN which is definitely stacking the deck reliability-wise
You can make a reliable SBR definitely; but a lot of people don't. Even LMT had lots of issues with their SBRs at first until they got the hang of it.
Zak Smith
Member
Close-- Noveske. Only have to pay for quality once...
- Status
