AR bcg wont lock back?

[supermario]

Hello,

Got an AR15 that shoots well. Not a single malfunction, however, the bcg will not lock back after an empty mag. I cant even get it to lock back manually by pressing the bolt release and holding the bcg with charging handle. It shoots fine though.

Ammo is fine. Mags are Pmags. It feels like the bolt release is not depressing enough when i try manually.


Any help would be appreciated by people who know what they are talking about. The gun is lubed, good ammo, good mags.

[NewbieDave]

Check to make sure you have correct buffer in tube. Rifle sock should have a longer buffer while carbine stock is much shorter. Also check you have a clear buffer tube and nothing snide tube to block BCG rearward travel.

NB

[jcwatchdog]

Hello,

Got an AR15 that shoots well. Not a single malfunction, however, the bcg will not lock back after an empty mag. I cant even get it to lock back manually by pressing the bolt release and holding the bcg with charging handle. It shoots fine though.

Ammo is fine. Mags are Pmags. It feels like the bolt release is not depressing enough when i try manually.


Any help would be appreciated by people who know what they are talking about. The gun is lubed, good ammo, good mags.

Did you put the lower together yourself or buy it from someone who put it together? Check to make sure the “hold open” part of the bolt hold open isn’t damaged. If not damaged, then there might be something binding it. Check the roll pin to make sure it was installed correctly. If that looks ok, there may be something in the channel that holds the detent/spring if it’s not pressing down correctly. The channel might have a burr that needs to be removed. I doubt the wrong spring was used, but if it isn’t factory, anything is possible. You might have to punch out the roll pin partially to remove the bolt catch to see what’s going on. But overall it sounds like the problem is a result of that if you can’t even lock the bolt back with the charging handle/bolt catch.

[ar15barrels]

Got an AR15 that shoots well.
Not a single malfunction, however, the bcg will not lock back after an empty mag.
I cant even get it to lock back manually by pressing the bolt release and holding the bcg with charging handle.

Any help would be appreciated by people who know what they are talking about.

Buffer or spring lengths are probably a mismatch for the buffer tube.
You should be able to get the bolt to lock back manually and then have an additional 3/16" to 1/4" of carrier/buffer travel before the snubber on the base of the buffer bottoms out in the buffer tube.

A common cause for your problem is using a rifle spring in a carbine buffer tube.
Rifle springs have a longer coil bind length than carbine springs.

[SVT-40]

You don't "press" the bolt catch / release to engage it. You pull it out.

[ar15barrels]

You don't "press" the bolt catch / release to engage it. You pull it out.

You press on the top to release.
You press on the bottom to engage.

Most aftermarket bolt catches even have a lower pad to make them easier to find by feel:

[supermario]

Its my friends AR. He assembled it. I told him what to try. Starting with spring 1st. If i remember correctly, when i pulled his bolt back, it didnt appear to go as far back as its suppose to. That makes me think out of spec spring. I have no idea what lower parts he used. But it shoots fine.
Also i do understand how to lock a bolt back.

[Ki6vsm]

Yeah, I was going to say, examine your spring carefully. Same thing happened with a carbine-lower build I was putting together a couple of years back. Strangest thing. The bolt wouldn't go back quite far enough to lock. I figured out pretty quickly that the spring I'd used had a few too many coils! Couldn't compress sufficiently in a carbine buffer tube. I discovered this by comparing it with another, bona fide, carbine spring I had. The relaxed length was an inch or so longer than standard. Difference was obvious right away. I don't know what this spring's application is or where I bought it. I usually components components separately, made by reputable brands. How I acquired this spring is lost to memory. Maybe I bought a buffer "kit" though once, that contained this spring.

I still have that spring, in my parts box. I should probably post a pic of it next to a standard spring. Maybe someone can identify its application. A5 maybe?

[SkyHawk]

Also be sure the correct spring and plunger is behind the bolt catch. If you somehow used the wrong spring or plunger, there will not be enough travel in the bolt catch and it cannot come up high enough.

[ar15barrels]

I still have that spring, in my parts box.
I should probably post a pic of it next to a standard spring.
Maybe someone can identify its application.
A5 maybe?

A5 uses a standard rifle spring.
The A5 buffer has the same amount of spring space on it as a rifle buffer but the front of the A5 buffer is much shorter so it will fit into an A5 buffer tube.

[SanDiego619]

Randall you're the best. Thanks for always sharing your knowledge.

[Ki6vsm]

A5 uses a standard rifle spring.
The A5 buffer has the same amount of spring space on it as a rifle buffer but the front of the A5 buffer is much shorter so it will fit into an A5 buffer tube.

Hah. Thank you. Well there goes my theory. It was a WAG on my part. I've never really looked at an A5 setup. I just knew it was different in some way from standard. I might pull out that spring this weekend and have another look. It would be nice to solve the mystery anyway.