The upper was built by a local shop for me, it's a 24" stainless full profile barrel, anderson upper, and a keymod tube.(that I cannot remember for the life of me who manufactures.....)

Took it out to the range today, and it refused to chamber a round from the magazine. If I single fed it, without the magazine, it fired perfectly. for some reason I never though to single feed with the mag in

I was not happy(the other three builds ran fine....just took that as a good sign), and was not screwing with the gun on a firing line in 90+* weather. When I got it home and cooled off, I started looking more closely. I figured it would be one of a few things:

Ejector out of spec
Extractor spring to stiff or extractor stuck
mag release not adjusted correctly


Turns out, the BCG is binding on the top of the mag. This bind is hard enough to keep the bolt from being able to travel into battery, period. I've tried:

Adjusting the mag catch, on two different lowers, from the point it barely retains the mag to the point I can't tighten it
A total of four different BCGs, all known working units.
A total of four different lowers, all firing and feed well on other uppers
Three OTHER uppers on that lower, all of which feed and cycle cleanly
5 different magazines, 4 pmags and one metal GI mag


The same BCG issue is manifesting on all 4 lowers, with all four BCGs



So I'm completely stumped. The only thing I can think is maybe I've gotten hold of an upper that is slightly out of spec?

Upper has M4 feed ramps and the barrel doesn't?

Can the BCG go into battery without a round in the mag? What area of the the mag do you think the BCG is binding on?

When you're single feeding I assume you're sliding the round into the chamber and then closing the BCG?

Adjustable gas block?

A picture in case OP isn't sure what to look for:

quote:

---

Adjustable gas block?

---

The way I read it ,its not even chambering the first round

After drinking a little more coffee and rereading your original post, I have another question:

Have you tried any of the other BCGs in the new upper?

If you haven't, try old BCG in new upper/lower combo and let us know what it does.

Another thing to try would be taking the bolt itself out of the new bcg and seeing if it will go into battery. If it still won't, that will let you know something is funky with the bolt carrier or upper.

quote:

---

A picture in case OP isn't sure what to look for:

---

Thanks Bender! Most uppers especially mass produced like Anderson typically have M4 feed ramps. Using that with a rifle extension as pictured creates a nice flat edge for the bullet to catch on when being magazine fed.

It would not surprise me at all if this is the issue especially since it was a built upper.

quote:

---

quote:

---

Adjustable gas block?

---

The way I read it ,its not even chambering the first round

---

I just read it again more carefully. It does sound like an out of spec upper or feed ramp discrepancy if it's happening with all BCGs on all lowers with different mags.

OP, can you see the feed ramps? Are the cartridges getting mangled on the tips? Or are they not even going in that far?

Op, I had a similar issue and it was due to a misaligned gas block. Somehow, it shift slightly and would only allow a half to one-third cycle. I took to a professional and had him realign & pin it, and its been great ever since.

Edit to add: Smithy realized the gas block had lost its algnment, not me. But it works perfectly now.

quote:

---

A picture in case OP isn't sure what to look for:

---

Am I ******ed? What is the difference in pics 2 and 3?

The straight-on shot doesn't show it well.

The problem is that mismatching it creates a small ledge that the cartridge tip will catch on instead of smoothly sliding into battery.

The grooves in the barrel and ramp line up much better in #2 than in #3.

quote:

---

quote:

---

A picture in case OP isn't sure what to look for:

---

Am I ******ed? What is the difference in pics 2 and 3?

---

The problem is this lip between the barrel and upper receiver. If yours is off, the cheapest fix is to get a new upper that matches your barrel. Try and post a pic.

Others covered it with better pictures. Yes, it creates a lip that can cause feeding problems.

Sorry working nights, just woke up. The feed ramps on both are m4, checked them when I got the lower.

I guess I should have specified better on failing to feed. The bolt comes forward, feed the round, and the BCG binds against the mag. The round feeds cleanly into the chamber, and if you drop the magazine, the BCG will move into battery and cycle the round( if you pull the trigger), and cycle back into battery on an empty chamber.

It looks like the entire BCG is dragging on the top of the mag. With the mag release all the way out, you can actually use the FA to.get the BCG in battery, but the whole system is so bound it bent a feed lip.

Stupid question: did you try more than 1 mag?

Total of 5, 4 pmags and a metal mag. All known working/feeding

Post a pic of the bolt carrier and the bolt face, please.

So that upper with every BCG isn't feeding on every upper?

That's what I'm getting from OP just making sure.

quote:

---

Total of 5, 4 pmags and a metal mag. All known working/feeding

---

It's entirely possible the upper is out of spec. Post some pics.

Correct. I have access to 4 completed uppers, 3 carbine and one rifle, same issue on all 4, with 4 different bolt carriers. I did a lot of shuffling and testing last night lol

I'll post pics as soon as I get home in the am, or I'll bring the rifles to work with me and take some pics in downtime.

Yeah, your upper receiver is probably the issue. Do you see any worn marks inside where finish is gone?

Yeah that has to be the upper. No other variable left.