My gun won't feed

AR/M4 AR Technical Discussion
1

I need some guidance here.

The problem: my gun has started to fail to feed. It was shooting just fine but then started to experience failures to feed after every shot. Since the failures started I’ve noticed that it also has had 2 failures to eject and 3 failures to close the bolt in the last 50 rounds.

The ammo: All hornady red box 77r OTM. The gun has only had this ammo put through it, and only about 500 rounds total. It has been fed with Magpul and Lancer 20 round mags.

The rifle: BCM 410SS 20" Service profile barrel with BCM BCG. Rainier Ultramatch upper receiver with rainier “match” gas block assembled by Rainier. Gas block has a set screw (barrel comes dimpled for set screw). The issue has occurred on two lowers: one with rifle gas using rifle buffer, the other with the Vltor A5 system.

Shooting prone the brass ends up around 4 o’clock 4 feet away. The gas rings look good, i switched buffer springs, and tried different mags. What else can I do to try and diagnose?

Thanks,
-LOP

2

I really have no idea but I’ll take a crack… Some match chambers are very tight. Get a wire chamber brush and clean it very well. And try some different ammo. That ammo should run fine, but still try some other just for grins in case you have a goofy batch.

Never heard of a MATCH gas block, but it made the hair on the back of my neck stand up. If you never figure it out, get a BCM gas block as a final attempt to fix. I would do it right away, but it may be a waist of money.

Good luck.

3

Thanks Pappabear. The only thing special about the “match” gas block is that its something like 0.05" greater inside diameter so its easier to install without scratching up a stainless barrel. I just took off the handguard and I don’t see any obvious signs of gas leakage or any crookedness but it is something to consider after everything else has been eliminated.

Thanks,
-LOP

4

Is there a gap between the shoulder of the gas block journal and the gas block? It should be around .028.

5

We had’em back in Nam.

6

I swore by those Mattel match gas blocks. It was a sad day when they stopped making them.

7

i remember now. They were painted in marine camo. Smelled like napalm. Nice touch guys.

8

Yep, that’s them. You were pretty cloudy back then. Takes a little jogging to bring it all back.

9

Does it also fail to lock back after last round fired?
Your gas block might be loose.

10

There is a gap, my eyeballs say it looks like more than that but I’ll measure tomorrow.

11

The thing that has me confused, is that you are referencing several different issues (failure to chamber, eject and failure to feed). All the components are known variables (except the gas block). My inclination is to replace that gas block first. Assuming that the ammo is good, mags are good and everything else is correct.

12

It is much closer to 0.28" than 0.028". Should I upload some pics?

13

Well there’s your problem! Yikes!

14

Well it certainly would help.

15

Nearly 3/10s of an inch seems pretty significant, does it look like the block moved at all since installation (set screw properly set into dimple?)?

16

Apparently I had too many Montucky Cold Snacks last night. It is 0.028" confirmed with feeler gauge. I also verified that the gas block is aligned properly and that the gas tube isn’t loose. Here are some pics of the GB and chamber:

Gas Block

Chamber/Gas tube

17

I can’t see any problem there. I would do the following.

Lube the weapon and procure a small amount of M855 or M193 and see what happens.

If the failures continue, then I would grab another BCG and try it.

18

I have some M193 and will try this. I have another BCM BCG in my other gun that has been flawless and can use it as a test if I need to. I’ll report back in a couple weeks with the results. Thanks for the help!

-LOP

19

I was finally able to do some more testing. The plot thickens.

  1. I thoroughly cleaned my barrel/chamber/BCG and lubed up with slip2000.
  2. I put 20 rounds of Federal XM193 into a 30 round Lancer mag and shot it without issue. I put 20 rounds of Black Hills 77SMKs into a 20 round Gen3 pmag. In 8 rounds I had 6 rounds that fired and cycled properly, and 2 malfunctions. They were failure to feed, the fresh rounds would get stuck where the back half of the cartridge was in the mag and the other half is on its way up the feed ramps. At this point I realized I was out of time and had to stop shooting.
  3. I went shooting again (no cleaning after last time) and experienced the following:
    •20 round gen2 pmag loaded with 75gr Hornady TAP fired 20 rounds with 2 failures to feed
    •30 round Lancer mag with Black Hills 77s fed 3 rounds properly and then had failures to feed (top round in mag undisturbed), failure to eject (spent shell stuck in ejection port) and even 1 failure to extract.
    •30 round gen3 pmag with XM193 would then start frequently failing to extract spent shells and locking the gun up tight. I had to mortar the gun a bit to get the spent shells out. This happened maybe 10 times with one mag.
  4. At this point I wanted to start ruling things out. Swapped the lower with another known good lower, also with rifle buffer system. Issue persisted. Swapped lower with known good lower with Vitor A5 recoil system. Issue persisted. Swapped BCM BCG with known good BCM BCG. Issues persisted.

Anything else to check at this point before I take it to a smith?

Thanks,
-LOP

20

I would contact Rainier Arms. Me thinks the issue is with the barrel.