AR-15 failing to go into battery

[HiredHand]

Well, I just tried the upper on another, known good lower, and had the same problem of the bolt not going fully into battery with a magazine inserted. Then I managed to remove the JP silent capture buffer, and I tried two different springs, a standard and a heavy duty, and two buffers, standard and heavy. Releasing the bolt on an inserted magazine partly stripped a round but stopped; the furthest forward the bolt ever got was with the JP buffer. The problem appears to be in the upper, and somehow seems related to the presence of a magazine, loaded or not.

I did notice that if I loaded a fresh round, then dropped the mag after the failure to go into battery, and I ejected the one round, I noticed faint striations spiraling very slightly along the length of the bullet from the case mount toward the point. Like the dashed line on a divided road. The marks weren’t there before the attempted chambering.

Interesting. How do the feed ramps look?

 

[Perplexed]

The two things that I notice are the mag isn’t fully inserted and the bolt stop looks like it has some heavy wear marks along the top edge, but that may just be the lighting.

I was inserting the magazine so you could see the movement of the bolt stop as the mag was seated. The mag was fully seated at the end of the video. The upper rear of the ramp on the bolt stop does have a bit of wear along the edge, and it’s brand new. The bolt, also brand new, has wear marks along the underside on both sides of the center ramped part.

 

[Perplexed]

Interesting. How do the feed ramps look?

Best I can do. New KAK barrel.

 

[mr ed]

So, How bad has the bolt carrier's gas key beat the gas tube?
Does the tube slide easily in and out of the gas key ?
Gas key look straight not out of round?
Not hanging up on end of charging handle?

 

[HiredHand]

I was inserting the magazine so you could see the movement of the bolt stop as the mag was seated. The mag was fully seated at the end of the video. The upper rear of the ramp on the bolt stop does have a bit of wear along the edge, and it’s brand new. The bolt, also brand new, has wear marks along the underside on both sides of the center ramped part.

My bad. I didn’t click on the embedded video to see the whole thing play.

 

[Perplexed]

So, How bad has the bolt carrier's gas key beat the gas tube?
Does the tube slide easily in and out of the gas key ?
Gas key look straight not out of round?
Not hanging up on end of charging handle?

The gas tube looks great. No marks on it. When I slide the bolt forward slowly, I don’t feel the key hanging up on the end of the tube. I’ve also tried two bolts in it, including a known good bolt from another AR.

 

[RangerRick]

Best I can do. New KAK barrel.
View attachment 260702

Just looking and comparing to my Rock River and Adams arms piston looks like the barrel in not far enough into the upper. just a guess

 

[swampratt]

Can you Trim the P mag top?
Can you take a sharpie and color everything under the bolt carrier black and do the same for the magazine top and bolt catch.
Or anything else that bolt assembly bottom touches.

If you follow what I am saying in the above then do this.
I would then cycle it and look for wear marks in the sharpie colored parts.

I would then massage those parts to eliminate some contact.
Sand and or chamfer and then polish the parts.

Works without the mag so it is in that area as you know.
Wish you were here I like tweaking things to make them work better.

Look at these pictures.
https://www.ar15.com/forums/ar-15/Bolt_Carrier_Binding_on_Bolt_Stop/66-417271/

 

[dennishoddy]

The problem occurs when I load a magazine and then hit the bolt release. It fails to go fully into battery, even with repeated use of the charging handle. But if I drop the mag with the round still in the chamber and then use the charging handle again, the bolt goes fully into battery.

Reloads? I had the same issue with an AR. I called RCBS about why I was having issues not coming to battery. They said that the issue was in the standard .223 dies. The instruction included with the dies say to bring the ram all the way to the top of the die, back off a bit and screw the die in 1/8 turn.
RCBS told me to take it to 1/4 turn. It fixed all issues. They now sell small base dies to correct that problem.
If you're using factory ammo, we were told in the Army to not load a full mag. load one less so the round rested on one side and I don't' remember which. . Back in the day it was a 20 round mag, now 30 for standard issue.

Edit: Read where you are using factory ammo.

 

[Perplexed]

Can you Trim the P mag top?
Can you take a sharpie and color everything under the bolt carrier black and do the same for the magazine top and bolt catch.
Or anything else that bolt assembly bottom touches.

If you follow what I am saying in the above then do this.
I would then cycle it and look for wear marks in the sharpie colored parts.

I would then massage those parts to eliminate some contact.
Sand and or chamfer and then polish the parts.

Works without the mag so it is in that area as you know.
Wish you were here I like tweaking things to make them work better.

Look at these pictures.
https://www.ar15.com/forums/ar-15/Bolt_Carrier_Binding_on_Bolt_Stop/66-417271/

The problem occurs both with PMags and with aluminum USGI mags, and with two bolts, one of which came out of another AR that runs just fine, and the problem also occurs with a second lower, out of that same functioning AR. So I’m pretty sure the issue is in the upper. What I’ll try next is to mate the first lower with the known good upper and do a function check, just to definitely eliminate the lower as being part of the issue, and go from there. You’re right, tinkering and tweaking will be the order of the day. I’m starting to suspect the side charger as being the issue, or perhaps the barrel’s locking lugs being slightly out of spec - or something!