Built this up this afternoon and have a problem. The bolt won't lock back on the bolt stop. It doesn't go back far enough.

I don't think it is a problem with the bolt stop or the receiver being out of spec, as I tried this upper on another completed lower with carbine stock/buffer and it worked just fine and locked back smoothly.

So I think it must be a problem with either the buffer or the buffer tube.

What am I missing ?

Here is some pics. Last pic shows how far it naturally goes back, as you see not far enough to engage bolt stop.

Also I have one other question. I plan to switch to an A1 stock for the vintage look. I believe the same rifle buffer will work with that but I may need a different stock screw ?

Used:

Anderson lower
Anderson lower parts kit
I believe Colt A1 upper and barrel
Not sure about BCG and stock parts. Just used parts I acquired.

Your buffer is a bit weird-looking. It's supposed to have a polymer or polyurethane tip on the back end to absorb the shock of the buffer hitting the back of the tube.



Your tip appears to be missing. What happened to it? Did you remove it?

Also, why is the front of your buffer all chewed-up looking? It looks like it's beating the hell out of the buffer retainer pin every time the bolt goes forward..

This is normal. Buffers will hit the buffer retainer.

A agree about it missing the nylon bumper. Get one of those.

Also have you tried a GI mag?  I've had some magpul mags not quite push the bolt catch up before.


With the mag in posh the bolt catch to catch the bolt. If i catches, then the issue is with cycling and shouldn't be a spec issue.

No I did not remove it.

Honestly I didn't even think of that. I am not new to AR's but never had a rifle length one before so I guess I didn't catch the difference.

It isn't the mag. I just tried it with a GI mag.

But that wouldn't make sense because like I said it doesn't even go back far enough. Like the last photo. It isn't an issue of not engaging the bolt stop, its a issue of not going back far enough to do so.

The question is why. Only thing I can think of is the buffer or buffer tube.

Considering it works great with a different lower.

You think that could be the problem ? I wonder...seems weird it would cause this being its shorter now.

Just tried a carbine spring and carbine buffer in it. It seems to work. I realize it might not actually function correct with live fire due to the shorter nature of it, but it at least easily goes back all the way.

I don't have any other rifle length buffers to try, but I suspect for some reason the buffer may be the problem.

Ordered a new buffer. Guess I'll see if that fixes it. I suspect it may.

Some A2 stock screws are too long and protrude into the buffer tube and cause this issue. But the fact you have no rubber end cap for the buffer is crazy.

Shine a light down that extension and look for that bumper, it might be broken and stuck in there or other junk in it. Also length of spring, buffer (as it sits) and depth of tube?

my thought/advice as well...

If the polymer bumper is completely missing from the buffer all the weights and spacers would be gone or rattling around.

As already mentioned if the rear screw is too long it can protrude into the tube and keep the buffer from fully retracting.

If you shoot a carbine buffer in a rifle length stock tube the back of the bolt carrier key will slam into the rear of the lower receiver and run a risk of damaging it.

Yeah, that's bad. Don't do it.

As to the buffer being chewed up by the buffer retaining pin, that is NOT normal. With the the upper pinned in place,the buffer should be resting against the bolt carrier, not the buffer retainer pin. Either the bolt carrier is a bit too short, the retainer pin hole is too far to the rear, the barrel receiver extension is too short, the barrel nut is not tightened correctly, or the barrel collar on the upper is too long. Eventually you're going to break the tip of the buffer retainer off, and it's probably going to jam something at an inconvenient moment. I just inspected the buffers on 8 different ARs. None of them are chewed up like that, and some of them have fired thousands of rounds. If all the parts are in spec, that will not happen.

Check to see if the stock screw is protruding into the buffer tube. If so, that could be your problem. It shouldn't need to be more than 1/2" long unless you're using a stock longer than the buffer tube and have a spacer. A long screw intended for use with a spacer used without a spacer would cause exactly the problem you're experiencing, and would tear up the rear tip of the buffer as well..

Yeah I have no intention of doing that. Just put it in to see if it functioned better by hand.

I will be checking it soon. It does have the A2 stock spacer at the end of the receiver extension if that is what you are talking about.

I may have found the problem.

There was two little pieces of the broken buffer cap inside the tube. It seems to work now. Still will be replacing buffer.

You probably already realize this, but you do not want to fire the rifle with that buffer, even if it does function manually now.
without the cap, the buffer is now too short and could have the same affect as using a carbine buffer.
Jim

When the weapon cycles, the buffer will not contact the buffer retainer.

Due in part when the upper receiver is pivoted downwards, when securing it to the lower receiver via the takedown pin, the carrier actually pushes the buffer off of the buffer retainer a very small amount. The only time the buffer can "hit" the buffer retainer is when the upper receiver is shot gunned open, or is removed from the lower receiver.

I had the same problem with my 1st 308 AR 80% build. I bought the upper and lower from Custom Built Arms, and the rest from all over. The first problem was using an AR15 A2 standard buffer and spring. The spring is too weak, and the buffer too long and too light. Chopped the buffer to the correct length, added weight, ordered the correct 308 spring.
Next problem was the CBA upper receiver protruded into the area where the bolt catch lever resides and prevented the catch from  covering more than a fraction of a mm of the bolt producing an unreliable hold back function. I removed about 1.5mm from the bolt catch lever where it contacted the lower edge of the upper, problems solved.
I am not satisfied with Custom Built Arms because of their response, or lack of response to my communications, and for their poor customer service policy. They have adopted a policy of never answering the phone, and when I purchased my second 80% lower, they strung me out without notice the part was no longer in stock. Ten days after I paid for the order, no part. Two days after asking for a tracking number or a refund, they assured me a refund would be in my account within 3-5 working days. 6 working days later, no refund.
I have disputed the charge with my bank, and will never do business with CBA again, ever.

Thanks yes I will be waiting to shoot it until the new buffer arrives.