'm in the middle of putting together a fun little plinker and running into some trouble. I'm using a complete upper from a Colt 6951 and a dedicated 9mm lower (cmmg MK9). The upper has a ramped bolt which is what cmmg lower is supposed to be made for since it's got a regular AR hammer. Everything goes together and fits perfectly except the charging handle is extremely tough to cycle. You have to put the butt on the table and pull the charging handle with both hands. Once the bolt gets free from the closed position, then it slides easier but still not as smooth as it should be. Any idea on what to check to figure out what's going on?

Are you using a round hammer or does it have a notch?

It's a rounded hammer

I am wondering if the bolt is getting hung up on the ejector.  The ejector sort of rides in the left side groove of the BCG.  If the ejector is too high, it may be rubbing on the top of the groove.  Are you having trouble getting your take down pins in?  I mean do you have mash your upper and lower together with some force to get the takedown and pivot pins in place?

Check to see if the bolt catch is causing interference.

I had that problem with one of my builds, and had to file a small amount of the bolt catch.

I'd put money that this guy nailed it.

All of the above mentioned items can cause an issue. BUT, you said you were using a CMMG Mk9 lower with a Colt upper.    Just for fun, check to see if there is a drag mark on your BHO. (The spring loaded piece that pushes up and locks the bolt back when the mag is empty. CMMG did use a slightly different height on that part. I know I ran into a problem once when using a CMMG upper on a Rock River Lower and while researching I recall hearing about this. I do recall I did have one of my 9mm carbines that actually had a "witness mark" where the bolt was rubbing the BHO. I couple of file strokes to the BHO cured the problem and all was good.

One quick little check you can do regardless is to push out the lower pin while you are having the binding occur and slow raise up on the lower to see if the bolt gets freed up. If that happens you can then locate which part is causing the issue. Its super simple to push out the hammer pin and remove the hammer to eliminate that part.

Good luck with it.

That's the issue! The bolt catch is a lot taller than the standard colt bolt catch. I wonder why Cmmg decided on this height. Wonder if I should just buy a colt bolt catch or shave down this one.

Try taking a little off the one you have first...
File in small steps and test.

Dave S

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just get a new one, once you remove the surface hardening, it's all down hill.

It's a bolt catch, not a H/T/S where hardness is surface only.