Pistol caliber carbine chamber blowback?

[slipknot]

I recently bought an AR-9 upper at a gun show and tried it for the first time on my lower. It uses Colt magazines and an adapter. The gun functions but the brass is so black and discolored I am guessing that the chamber must be very loose. Is this common among these? I don't expect it to be that accurate but 50 yards should be close. I shot out to 200 with it and it hit the beam all over the place so I am sure it is limited. I thought it would be good for steel challenge and steel shoots. If it is not normal I can ask the manufacturer if there is something they can do but I don't know since I have never owned one before.

thanks for any help or advice

 

[Uzi2]

What ammo?

Blowback chambers are usually loose, but not so loose that it blackens the brass all the way back to the rim.

Low powered ammo will blacken way back. I've experienced it with deliberately low powered reloads. Hot ammo doesn't do it.

Try some hotter ammo and see if the condition resolves.

 

[TomMontana]

I took this pic from a couple of years ago. Cheap UMC ammo. My AR9s have not blown up on me yet.

 

[Uzi2]

I took this pic from a couple of years ago. Cheap UMC ammo. My AR9s have not blown up on me yet.

Remington UMC......the classic definition of under powered ammo.

Just an FYI, the stuff is a runaway hazard in most submachine guns. For pistols or semi rifles....its cheap shooting fodder.

Haven't fire a round of it in almost 20 years.

 

[slipknot]

It was blacker than those.

I used same stuff I put in my G17 , Speer Lawman 9 mm 124 grain I believe

 

[DW357]

My brass gets pretty dirty when fired through my Sub2000. I would say as dirty if not, more dirty than TomMontana's brass.

 

[ScottS]

Did you just slap the upper on your lower and go? Usually, 9mm ARs need a heavier buffer to offset the blowback action. The blackening could be from the bolt opening too soon. I'd try a heavier 9mm-specific buffer (KAK, Spikes's, etc.) and see if that doesn't slow opening and fix the case problem.

 

[slipknot]

I got the heavier buffer along with the upper when I bought it and put it in. Good point, maybe it is not heavy enough. Thanks

 

[SJan]

As stated above, you need a much heavier buffer than you would use in 223/556. The 9mm bolt itself is also heavier, depending on the bolt you have (they vary greatly from one manufacturer to another) you want a buffer between 5 and 10 oz to achieve a total reciprocating weight of 18-20 oz. Sometimes a heavier recoil spring is used in conjunction, but is not a replacement for heavy buffer. The mass of the bolt+buffer keeps the fired round in the chamber long enough for the pressure to drop before extraction.

 

[milktree]

I recently bought an AR-9 upper at a gun show and tried it for the first time on my lower.
(snip)
I don't expect it to be that accurate but 50 yards should be close. I shot out to 200 with it and it hit the beam all over the place so I am sure it is limited. I thought it would be good for steel challenge and steel shoots. If it is not normal I can ask the manufacturer if there is something they can do but I don't know since I have never owned one before.

thanks for any help or advice

At 50 yards you should be able to easily knock over bowling pins. My MechTech and CX4 (both 9mm carbines) can pretty easily hit a 3" target at 50 yards if I take my time.

It should be *PLENTY* good for steel challenge and falling steel. Are you testing off a bench? Did it come with a cheap-shit red-dot that maybe doesn't hold a zero?

 

[slipknot]

It does shoot good st 50 yards
It hit steel about the size of bowling pin.
I only bought an upper, not a whole carbine rifle so I used my Aimpoint Pro on it.
I have not weighed the buffer yet but I will and if it is only 5.5 ounces, I will try a 7.5 oz buffer.


Thank you for the info, I will check it out to see what the total weight is and work towards that 18-20

 

[Tachyon]

When a cartridge is fed into the chamber, the fresh cartridge diameter is slightly smaller than the diameter of the chamber. As the cartridge is fired, it swells due to the pressure, until the outside of the case touches the chamber wall (and together they still swell a little more). This case to chamber snugness seals in the propellant gas. After the bullet has left the barrel, and the pressure dropped, the case retracts in time to be extracted. If the cartridge is mildly loaded, the case may not grip the chamber wall tightly enough, and some of the propellant gas can slip between the case and the chamber, blackening the case.

Tachyon

 

[Uzi2]

When a cartridge is fed into the chamber, the fresh cartridge diameter is slightly smaller than the diameter of the chamber. As the cartridge is fired, it swells due to the pressure, until the outside of the case touches the chamber wall (and together they still swell a little more). This case to chamber snugness seals in the propellant gas. After the bullet has left the barrel, and the pressure dropped, the case retracts in time to be extracted. If the cartridge is mildly loaded, the case may not grip the chamber wall tightly enough, and some of the propellant gas can slip between the case and the chamber, blackening the case.

Tachyon

Your last sentence describes Remington ammunition as a rule. It is the most under powered ammo I've ever fired and is almost a guaranteed run away in a subgun.

 

[DW357]

Your last sentence describes Remington ammunition as a rule. It is the most under powered ammo I've ever fired and is almost a guaranteed run away in a subgun.

Except for their 125 gr 357 mag loads and 180 gr 44 mag loads. Those are loaded HOT.

 

[EddieA]

That explains something to me that I didn't fully understand.
I was shooting some fluffy .45 reloads at a plate shoot, it was dim lighting. People were coming up to me and telling me that they were seeing some flame at the ejection port, kind of made me nervous, but I'd been using that gun and recipe for quite awhile with good results.

 

[92G]

It does shoot good st 50 yards
It hit steel about the size of bowling pin.
I only bought an upper, not a whole carbine rifle so I used my Aimpoint Pro on it.
I have not weighed the buffer yet but I will and if it is only 5.5 ounces, I will try a 7.5 oz buffer.


Thank you for the info, I will check it out to see what the total weight is and work towards that 18-20

unlike a locking bolt design, on a blowback the weight of buffer is critical and pretty much the only determinant of cycling. the spring has almost no role in keeping the breech closed. the buffer/BCG needs to be heavy enough to keep the bolt closed until the bullet exits and the pressure drops. so one definitely wants to ensure they have the correct bolt weight in the gun. underweighted bolt could be no bueno.

personally i would not worry about fouling on the brass. a lot of gas comes out a blowback. it's going to get onto the hot brass. if the brass physically looks OK and the primers aren't bulging then i would keep firing the gun and enjoy. my only concern with blowbacks is that they get regularly cleaned to ensure the bolt can fully go into battery (i.e. the bolt can completely "close").

reality is that blowback firearms are inherently less safe than a locking mechanism. you're relying on the brass to do its job. a case head separation or brass blowout on a blowback gun can be a nightmare. i don't mess around with +P or extra hot rounds on a blowback. i would find an ammo that runs well and try to just use that.

it seems a lot of people run FMJ at the range then throw in JHP's for HD. my experience is that many blowbacks don't do well with JHP's. not sure if that's due to short cycling time or fixed barrel (as opposed to tilting) but they don't all play well with hollow point ammo. further the gained velocity from a rifle can lead to jacket separation or early expansion, so an FMJ may be a better option overall.

--
excellent website for blowbacks, Orion's Hammer:

Blowback Gun Design

 

[ScottS]

Your last sentence describes Remington ammunition as a rule. It is the most under powered ammo I've ever fired and is almost a guaranteed run away in a subgun.

I don't understand this part. Could you explain what you mean here?

 

[Uzi2]

I don't understand this part. Could you explain what you mean here?

In a blow back operated submachine gun, if the ammo is not hot enough to push the bolt reward to sufficiently catch the sear when the trigger is released, the bolt short strokes and just keeps on cycling......aka "runs away" until the magazine is empty.
Similarly, in a beltfed that is experiencing "cook off", the gun will cycle until the gunner or assistant gunner twists the belt stopping the feed.

 

[ScottS]

In a blow back operated submachine gun, if the ammo is not hot enough to push the bolt reward to sufficiently catch the sear when the trigger is released, the bolt short strokes and just keeps on cycling......aka "runs away" until the magazine is empty.
Similarly, in a beltfed that is experiencing "cook off", the gun will cycle until the gunner or assistant gunner twists the belt stopping the feed.

Thanks. I was familiar with cook off, but didn't know that under powered ammo would cause a subgun to run away.

 

[92G]

In a blow back operated submachine gun, if the ammo is not hot enough to push the bolt reward to sufficiently catch the sear when the trigger is released, the bolt short strokes and just keeps on cycling......aka "runs away" until the magazine is empty.
Similarly, in a beltfed that is experiencing "cook off", the gun will cycle until the gunner or assistant gunner twists the belt stopping the feed.

i would be shocked if that's an issue with any modern blowback 9mm. the cocking occurs earlier in the cycle than ejection, so by definition if the spent brass is ejected and fed a live round, then the bolt has to have moved sufficiently rearward to drive the hammer back under the disconnector.

i follow your logic and understand how this could certainly be a problem in older designs but not sure it's an issue in any modern blowbacks.

 

[SJan]

i would be shocked if that's an issue with any modern blowback 9mm. the cocking occurs earlier in the cycle than ejection, so by definition if the spent brass is ejected and fed a live round, then the bolt has to have moved sufficiently rearward to drive the hammer back under the disconnector.
.

In an AR9 it is possible for a situation where the fired case will eject and the bolt will not move back far enough for the disconnector to catch the hammer. The result is the hammer will follow the bolt forward and the trigger will be dead. This will only happen with a few combinations of bolts and drop in trigger assembly with non standard shaped hammer.

 

[92G]

In an AR9 it is possible for a situation where the fired case will eject and the bolt will not move back far enough for the disconnector to catch the hammer. The result is the hammer will follow the bolt forward and the trigger will be dead. This will only happen with a few combinations of bolts and drop in trigger assembly with non standard shaped hammer.

ejection aside, this would require the bolt to move sufficiently rearward to chamber a live round. in other words, the bolt moves back rearward of the entire magazine and picks up the next round without bringing the hammer to the disconnector? if so that is a shit design. in such a situation more likely it's a defective disconnector that's failing to grab the hammer.

on every lower i've examined, the BCG is pushing on the hammer immediately upon chamber opening. for this reason one also should not be running lighter hammer springs on a blowback. the hammer spring plays a role in slowing down bolt travel in a manner similar to the buffer spring. the hammer is immediately engaged upon bolt opening.

so by what you folks are saying the same phenomenon should occur with a 223. from this perspective it's no different. so if i run weak 223 it will go full auto? no it won't. i think if we are going to cite weak ammo leading to FA we should list some specifics (i.e. lower, trigger, ejector, mag, ammo, etc) otherwise this is a lot conjecture that has nothing to do with the OP. i'm sure it happened at some freak situation with freak components but there's no way this is currently an issue. there are other safety concerns related to a blowback that are far more relevant.

 

[SJan]

Like I said, it's only a thing on select combination of drop in trigger group and bolt. Some 9mm bolt have a very long shallow ramp where the hammer is reset on, along with a particular shaped hammer, the hammer will begin to cock back immediately upon bolt opening, but if the bolt travel is limited slightly (purposely or by weak ammo or other means) the hammer may not go back and down far enough to catch.

These are trigger groups that work flawlessly in a 223 application.

No, this condition I described does not create full auto or even doubling, it creates a dead trigger.

yes, not completely relevant to the OP.