[NorcalVP]

Took my newly assembled AR to the range today with my 10 year old son, along with a few other toys. and after about 20 rounds I had a FTF. I pulled the round out of the chamber and it looked line the firing pin didn't strike the primer. I noticed the denting and scraping on the case, and set it aside. On one of the mag's I unchambered a round and it had the same marks on it. After collecting the brass, I noticed that they all had the scrapes on them. I'm guessing the dents got hammered out during the ignition. All of the rounds chambered just fine, they all ejected nicely too with little force.

I'm relatively new to AR's, but I suspect that I need to do some polishing of the lugs on the barrel extension? or maybe knock off some sharp edges?

This is a PSA assembled upper chambered in 5.56, 1:7 chrome lined.

Thanks for any help!
Steve

 

[usmcdoc14]

show pics of chamber area

 

[NorcalVP]

Here is the chamber, It hasn't been cleaned, just wiped the lugs off a little to see the condition of the ramps

 

[TR]

Will a bullet chamber easily?

 

[hoverguy5]

those marks look like they are coming from either the bolt or the feed ramps.. shouldn't happen but only thing that looks that it could make those marks..

 

[NorcalVP]

Will a bullet chamber easily?

Yes, The round chambers smoothly with the bolt removed and me just dropping it into the chamber. It drops almost all of the way in under it's own weight with just gravity, I have to give it gentle pressure to get it fully chambered. It will not fall out under it's own weight. The round comes out easily with needle nose pliers (I can't grab it without the pliers with my sausage fingers...)

 

[NorcalVP]

If it's any consolation, the two rounds that are dented in the first pic, are also bent now slightly. (these were brand new and not bent when loaded into the mag) these two will not fully chamber. but came out relatively easily with a pull of the charging handle at the range.

 

[Roc Doc]

My Armalite M15 did that when I changed the buffer and spring. Looks to me like the bolt is smashing the round before it gets fully into the chamber.

 

[BeefCakeScout]

What mag? Pics?

 

[BeefCakeScout]

It looks like the bolt is contacting the next-in-line round while chambering. Maybe teh mag is a little out of spec and letting the chambering round unseat a little early, moving the next round into the feeding position before it should?

 

[NorcalVP]

What mag? Pics?

I was using two different FDE PMAGs (windowless gen 2's), pics of anything specific? I don't know if both mags did the same thing. These are a random selection of brass.

 

[FrankenToy]

You assembled the upper? Are these rounds handloads? Kinda looks to me that your AR may be under-gassed. What gas block did you use? Did you time it? The bolt may not be retracting completely or just barely so (that is if your buffer assy is good, as RocDoc pointed out).

 

[NorcalVP]

The upper was pre-assembled by PSA. The ammo is brand new Wolf "Gold". It's using the standard A2 front gas block that isn't adjustable. The rifle came as a kit from PSA. The upper was fully assembled and "Test Fired" The lower, I assembled with PSA parts and pieces.

I have another Buffer and Spring that I can put into this lower, and see if there is an improvement, but I'm guessing it's a standard buffer and spring. If it's undergassed, how would I go about fixing that? would the gas block need to be changed, checked, modified?

Do I just start by swapping parts? or is there a better way to test/troubleshoot which is the culprit?

 

[IEATRKS84]

It looks like a magazine issue. Try a different mag manufacturer and see if it continues.

 

[FrankenToy]

The upper was pre-assembled by PSA. The ammo is brand new Wolf "Gold". It's using the standard A2 front gas block that isn't adjustable. The rifle came as a kit from PSA. The upper was fully assembled and "Test Fired" The lower, I assembled with PSA parts and pieces.

I have another Buffer and Spring that I can put into this lower, and see if there is an improvement, but I'm guessing it's a standard buffer and spring. If it's undergassed, how would I go about fixing that? would the gas block need to be changed, checked, modified?

Do I just start by swapping parts? or is there a better way to test/troubleshoot which is the culprit?

If it's undergassed then my guess would be that the front sight (A2) is not timed correctly (rotationally oriented) or seated fully back onto the shoulder, a few thousands of a inch can make a difference. Give a different magazine a try, though, as others have mentioned (easiest way to eliminate a possible issue). Swap that complete upper on a different lower and see if the issue persists, too.

I handloaded a few rounds about a month ago to try out a new powder. I used the minimum powder charge initially. About 20% of the rounds would not cycle a fresh cartridge into the chamber and the round next in the mag was being deformed just like yours are. I increased the powder charge by 0.5 gr and no more problems. The AR has had hundreds of rnds through it without error BEFORE my handload experiment, so I knew it was my error.

 

[rockmup]

Looks like the bolt is dragging on the next round. your feed lips need to be adjusted

 

[Munchies]

Bolt smash. Magazine issue or maybe the lower is out of spec?

 

[[486]]

Try pushing up on the magazine rather than pulling down.

Reason I say it is that the bullet points are hitting pretty low on the feed ramp. They'll hit, bind, pop up in front and do a sorta "3-point-jam" thing like on a 1911. Put soft points in it and they'll probably catch on the edge of the ramp formed by the unfiled M4 ramps.

This is the reason I like metal mags. Can tune the feedlips to present the pointy end higher.

 

[IEATRKS84]

If it's undergassed then my guess would be that the front sight (A2) is not timed correctly (rotationally oriented) or seated fully back onto the shoulder, a few thousands of a inch can make a difference. Give a different magazine a try, though, as others have mentioned (easiest way to eliminate a possible issue). Swap that complete upper on a different lower and see if the issue persists, too.

I handloaded a few rounds about a month ago to try out a new powder. I used the minimum powder charge initially. About 20% of the rounds would not cycle a fresh cartridge into the chamber and the round next in the mag was being deformed just like yours are. I increased the powder charge by 0.5 gr and no more problems. The AR has had hundreds of rnds through it without error BEFORE my handload experiment, so I knew it was my error.

Its not a gas issue since the damage is coming from the round still in the mag.

If the rounds weren't feeding at all, then it would be a gas issue.

Lower mag well might be out of spec as well. Like the mag catch cut too high.

 

[NorcalVP]

I've got an extra lower, and all of the mag's I have are plastic and from the same lot, so I'd imagine that if one has an issue, maybe they all do... I'll try to get my hands on a different brand of Mag, and try my other lower with this upper, maybe change out the buffer and spring one at a time.

Thanks, I'll see what I can find and let you guys know.