AR15 New build

[Guest coquiboy]

I purchased a complete CMMG 300blackout upper and installed it on a Spikes lower. I installed all the lower parts but the bolt catch does not retain the bolt carrier back. What should I do to fix the problem?

[atlas3025]

Wait, do you mean that it does not retain the BCG back even when you manually engage it, or just when firing the last round?

[Dolomite_supafly]

It is handloads or factory ammo?

Is the ammo supersonic or subsonic ammo?

What is the barrel length and what is the gas block location?

Try manually holding down the bolt hold open as you fire it..

If it locks back then the bolt is coming far enough back and it is likely a magazine issue.

If it doesn't lock back then the bolt is not coming back far enough. That can be caused by a few things. It could be a gas issue or possibly an issue with a weighted bolt. It could also be ammo related or possible just a break in issue. A lot of people think they need heavy buffers on a new gun but that is rarely the case. I tell people all the time to fire the gun first to see if it needs one. About the only AR's that need heavy buffers are blowback guns like 9mm, 45 ACP or other pistol calibers.

I would warn against drilling the port larger first. If you have access to an adjustable gas block then you could drill it out and use the gas block to adjust the gas impulse until it cycles. Or you could measure the port size you have now and step it up one but be aware if you do go to far you will need an adjustable block to correct it. It is possible to drill and tap the port then install a setscrew then drill it out but it is not ideal.

And adjustable gas block simulates a longer or shorter gas system. That is if the port pressure of a carbine is 34Kpsi and a rifle is 19Kpsi you can use an adjustable gas block to limit the carbine length system to the same pressures as a rifle length system. It allows you to tune the gas system to pressures you can't get with a port alone. You could tune the pressures to somewhere between a carbine and midlength where it would create a hassle if doing the same thing with a port location.

If you know the port size that would be helpful as well.

And before you do any irreversible modifications call CMMG. They are a great company that will jump through hoops to make sure their products meet the customer's need. The same can be said for Spike's as well although they might move a bit slower.

Dolomite

[Guest HCRoadie]

I came across one AR where the screw holding the adjustable but stock to the buffer tube was just long enough to keep the buffer from fully traveling to the rear of the tube, which in turn kept the bolt from going back far enough to be caught by the bolt catch. This is rare though

[Dolomite_supafly]

I came across one AR where the screw holding the adjustable but stock to the buffer tube was just long enough to keep the buffer from fully traveling to the rear of the tube, which in turn kept the bolt from going back far enough to be caught by the bolt catch. This is rare though

You are probably talking about an A2 fixed buttstock. It uses a screw to hold the stock on the buffer tube. Again this is for a fixed stock not a collapsible.

There is not a screw that extends into the buffer tube on an adjustable stock.

Dolomite

[Magiccarpetrides]

You assembled the lower yourself correct?

Have you tried putting a different upper on top to make sure the bolt catch works right? Start there. Does the gun have a BAD lever installed? if so remove it and see if that helps. If still not working attempt to manipulate it with your hand while empty...

Report back to us and we can probably get this worked out quick.

[Guest HCRoadie]

You are probably talking about an A2 fixed buttstock. It uses a screw to hold the stock on the buffer tube. Again this is for a fixed stock not a collapsible.

There is not a screw that extends into the buffer tube on an adjustable stock.

Dolomite

Actually...it was on a Magpul PRS Stock which is adjustable and uses a screw that extends into the tube.

Edited April 18, 2012 by HCRoadie

[Dolomite_supafly]

The PRS uses the A2 buffer tube which does use a screw to hold it in place.

Sorry about the confusion. I am always in the rifle and carbine state of mind. And with that an A@ stock or buffer is generally not adjustable while the carbine is.

And if they had a screw going that far into the buffer tube they were using the incorrect screw. There are actually two different lengths of screws for use with rifle length buffer tubes depending on the stock used.

Dolomite

[Guest HCRoadie]

And if they had a screw going that far into the buffer tube they were using the incorrect screw. There are actually two different lengths of screws for use with rifle length buffer tubes depending on the stock used. Dolomite

That was the problem. My friend bought the stock second hand, and without the screw. He found a screw with the correct pitch, but just a #### hair too long. He brought the gun over for me to figure out why she did not stay locked back. The quick fix was placing a washer between the screw and the stock. Works like a champ now.