Malfunction help

AR/M4 AR General Discussion
1

Sorry if this is a stupid question, but I couldn’t find anything on a search.

Last night I took a new build out for only the second time and I was having some weird stoppages. It would fire, cycle, chamber a new round, but the bolt would not go fully into battery (IE: the extractor was not latching onto the rim of the casing). To clear the malfunction, I would have to rack the charging handle and let it go in order for the bolt to catch the rim, and then pull out the unfired cartridge.

This happened in no specific order five or six times per mag with multiple mags (Lancer, Pmags, and USGI). I eventually pulled the bolt out, took the extractor apart, inspected it, and saw nothing out of the ordinary (no black o-ring). I put it back together and the malfunctions stopped.

Any clue what was going on?

BCM 14.5" upper
BCM BCG
Blue Sprinco buffer spring
H-buffer
Noveske Gen 2 lower

2

I’ve had similar issues with this happening on fresh builds. First problem was not sling shotting the charging handle, if you ride it what so ever the BCG will not go all the way forward.

The 2nd issue was my PMAGS did not like 30 rounds. I had to back off to 28 and I never had another issue. The mag wouldn’t seat all the way. This was at least the case in my Noveske Gen 2 lower.

3

Do you have an O ring in the extractor? If so REMOVE IT!

Other than that… it would likely be the ammo.

4

No o-ring. Ammo is Federal M855.

I’m slingshotting and using the bolt release. All mags seat properly.

5

Did you check for gas-tube interference following assembly?
If not, remove bolt from BCG, and the charging handle from the upper receiver.
Slide BCG back and forth in the upper. Good idea to check with the upper in the “correct/upright” firing orientation.

Next thought is that according to Sprinco, the blue spring should be used with an H2 buffer.

6

I did not check the gas tube. The upper I got direct from BCM, but I’ll take a look.

I was under the impression that an H2 was too heavy for a 14.5 barrel? I’ll pick one up and try it though. Thanks!

7

If the malfunctions stopped after you disassembled/reassembled the extractor, I wonder if the extractor spring was somehow installed incorrectly?

8

For a middy? Maybe. For a 14.5 Carbine gas, a must have.

9

Well I tried an H2 buffer, and a complete Colt lower. Same issue. I contacted BCM, but haven’t gotten a reply yet. I’m hoping it’s just the bolt that needs to be replaced.

Here’s a video of what’s happening…

[video=youtube_share;2KXxaCEglts]http://youtu.be/2KXxaCEglts[/video]

10

Check the cases that do this. I bet you see 2 deep grooves in them just behind the neck and a good bit of denting.

11

That forward assist was all like “Oh he’s fixing to actually use me!” Then you dropped mag & racked it. The forward assist was then like “I hate you”.

How many rounds do you have through it?

12

Had similar issue with my PSA premium which has about 700 rounds down range. The difference, happened after cleaning; greasing the BCG (lightly) then trying to load a round. I tried 4 times before I took the BGC and Charging handle back out, applied a generous amount of grease to the BCG. Loaded rounds smoothly afterwards.
I did notice a gouge in each casing that jammed/failed to feed.
Could the problem be that the tolerances are still very tight thus mandating extra grease (lube)? PSA has not returned calls nor emails so I go with a heavily greased BCG for now.
Kind of a bummer when all write ups and or videos show a “thin light” coating of oil or grease. I use high temp wheel bearing grease now by the way. The old trusty Hopped gun oil I use on my sidearms was too thin.
Thanks in advance, this is my first M4 which is mil-spec.
John

13

I believe this to be a common problem with mil-spec M4’s. Probably part of the reason some people view the platform as unreliable. Most barrel-extensions are not properly profiled, which I have encountered. Only my Daniel Defense is done correctly on the factory guns I have, although I have seen others. Seems to be several sources of barrel extensions and some are done right, some are not. This is overcome with stiffer springs, more lube, and other bandaids…or properly profiling the barrel-extension.

This is how a barrel extension normally looks:

This is the result:

(or what OP describes, bullet in chamber, but the denting and scraping slowed BCG velocity so much that the extractor did not snap over the rim as it BARELY chambered)

This is how it needs to look to be reliable:

Notice how pretty my Daniel Defense barrel extension is? No way for anything to get dug into or hung up on that! Feeds like buttah!
A file and some patients can get you there.

14

Hey thanks brother. What exactly am I filing? I am new to the AR15 community, still a lot to learn. Or, should I attempt to get PSA to repair the problem?
thanks again,
John

15

First, verify the issue. Second, it is the sharp corners on the sides of the feed-ramps. Maybe file is a strong word…polish…you can’t add it back…

16

Patients??? Sorry just thought that was funny. It’s early.

17

Sorry, I hardly ever write patience. I write “patient” dozens of times a night.

18

lol I can relate. 30 years as a Vol. Fire Fighter Heavy Tactical Rescue member, I too mix the spelling up.
Okay, So the two lower grooves (ramps) is the area to work with the dremel using the firm wool polishing Bob and some metal polish. I have used the red polishing stick from dremel when polishing the copper base of spent rounds prior to making them into necklaces for people (the 180gr HST has been a big hit). Should I use the same or maybe Menzerna medium cut compound?
Thanks,
John

19

I’d use a jewelry file or nail care kit or something. You’re just breaking a corner that’s gouging.

20

If its a factory BCM upper I would have them replace it first, if it happens with that one too then I start taking the jewelers files to it. Or just pick up a DD barrel or similar.