I have a complete BCM upper (w/ BCG etc).

Issue is that sometimes the BCG gets stuck on the return from the buffer tube.
Almost at the bolt hold open position but not quite... but a bit further.

it doesnt do this 100% of the time but maybe 20% of the time as a magazine empties.
In the beginning it only did it so rarely I thought it was  afluke and was easily cleared.
But now it does it much more often and can be cleared only with difficulty

...sometimes all the way up to mortaring to knock it back forward.
It will go back when I try to clear it but will get stuck again at same place maybe  not quite an inch past the bolt-hold open postion.

I thought at first maybe this is an issue with the lower. So I switched lowers (from Bushy to Sig516 lower) took it to the range another day..
Still does it, rifle completely locked up within a magazine.

Then I thought..
Well the only moving part really is(are)the BCG and components so I switched out the entire BCG .

Now I am at home and I cant test it on shooting but even with this other BCG it still does it when I check it via hand cycling.

Same symptoms as on range but I did notice it seems to happen more when the rifle is held canted towards the left..

Anyone have any ideas?

Round count is lowish... upper may be a couple years old but I only now broke it out of my stash and decided to exercise it.
Round count is still fairly moderate maybe under 2000 rds I think and the issue seems to increase in frequency as of late.

its bad enough not cannot even be used in training (never mind as a go-to)

Ooops. Sorry should be in trouble shooting forum.
Admin/Mod please move to T/S thanks!!  

I don't know if this got answered, but I would think the bolt catch has a issue either from the magazine pushing up on it, roll pin is not smoothly letting it move, or the bolt catch spring isn't set right.

If you can, try a different magazine. The bolt catch may be out of spec and pushing upwards with the rounds. Did you happen to get a 9mm bolt catch or a nonstandard?

Thank you for replying!
This has me stymied

It does it with lowers both ALG and milspec triggers notched and unotched

With or without magazine inserted.

FA broken?

Hmm..

Good idea.. that would explain the symptoms. I will look into it and post the result on here.

I thought based on your description that the mag catch is hanging up on the bolt. Is that correct?

If the trigger notch is catching it, then it's most likely an out of spec lower or the trigger spring installed wrong somehow.

Sorry if I was not clear enough.. the bolt gets caught well before the magazine catch.
It also does it w/ different lowers.

Tonite I will check the condition of the FA :)

Okay great. Did you install the FA by yourself? It might be that you pressed it in too far. It's a tricky item to get right if you installed it by yourself.

A funny story is on my first build I wanted a teardrop FA, and after installing it, it doesn't "bump" as easy. I actually had to remove it and install it again since the springs weren't in right and it is smooth as butter. Every other upper I made never suffered the problem since it was a learned lesson.

If the upper is assembled by factory, I'd return it. BCM will make it right and they'll be quick. Usually they ship the item back in 1-2 weeks. I had a BCG that had a loose gas key and they replaced it no question asked.

The upper is factory.

I'm sorry havent gotten to checking it out yet.
Havent installed an FA before except  a couple years ago back in armorers school ....havent had to do it since then so I will probably check youtube first.

Just moved and dont have  a nice work area either ready either.. so I will wait until I am really in the mood for it on the weekend.

Finally got around to fixing it.

Was going to disassemble the FA but my punches seemed to be either too large or too small, figures.

However when fingering the FA from the inside I noticed the front tip of the plunger seemed real loose going in and out w/o any relationship to the FA button.

Real odd and then I was actually able to pull it out!!
How could the plunger tip become loose like this?
I bouth this rifle used with very low round count ( as verified by inspection) and understood it to be factory.

And the bolt and BCG were both BCM (BCG marked of course and the bolt had the correct appearance) which seemed to back up the claim.

However can one ever be sure?

Kudos for GregoryK for calling it correctly!!

So then I just attached lower and upper back together and called it a day.
I never use the FA anyway.

Glad to have my rifle back (even if it was just a back-up)