Hello Folks,  New to this forum though I've been reading threads quite a bit off google links.

I have the guts of an Armalite AR10-A2, everything but the upper and lower receiver.  I recently purchased over Gunbroker a Matrix Aerospace M-762 upper/lower set, which is essentially the same upper and lower as the Ruger SR-762.

I have all the parts assembled except the barrel and it does cycle with snap caps and Armalite mags (Steel, w/ button tab thing).  Oddly it doesn't like modified M-14 mags (steel tab ground off, hole cut in side) that I was given along with the parts and know did function in the Armalite.

The problem I am having is when the bolt is cycled pulling all the way back and releasing, it sometimes appears to slam fire (trigger already pulled when bolt rams home), or I pull the trigger and get a dull thunk followed by the hammer actually moving the moment I touch the charging handle.  I did cycle the assembled AR10 at my CHP father-in-law's house and know it works. but I've also read/been told that a lot of the Armalite stuff is proprietary or clunky in other builds.  

What could cause the trigger to behave the way I describe?  I'm 99% sure the lower is assembled properly, and the trigger/safety functions flawlessly without the upper.  Could this be a sign of Carrier Tilt (I'm not sure what that is, but read it in relation to the M762/SR762 discussions)?  

Any info you all could supply will be greatly appreciated.
Thank you very much!

The receiver block I ordered is being delivered today, so I'll have the barrel installed by this evening.  I don't see how having the barrel installed or not will have any effect on how the trigger functions though, and that's the problem I'm fighting.

If the bolt isn't resting on the barrel extension, I'd suspect that the hammer is hanging up on the rear of the bolt carrier, and/or being knocked forward by it. I'm assuming the only thing stopping the bolts forward travel is it whanging into the (softer) aluminum upper without a barrel? I.e. bolt is too far forward?

The Matrix Aerospace receivers are designed to be used with pmags/ DPMS pattern mags not armalite mags.

You need to use the Armalite "A" Bolt catch on the MA lower.
Also MA's are designed to use PMAGs.
Armalite AR10 LPK's ALL contain the "B" bolt catch.
Hope this helps.

I am currently building a Matrix Aero upper/lower. All you should need to buy is an Armalite AR10A bolt stop and mag catch. Everything else is the same. Check the link. The parts are on there. I just ordered the same parts.

762SASS

Thank you for the replies all!

It appears having the barrel installed will correct the trigger hang issues, as I cycled and put pressure on bolt with my thumb making it work just fine every time.

I'm not sure about the A or B bolt catch.  I installed the one I had in the 10-A2, and it is functioning fine without gouging into anything.  For reference, the Bolt Catch/Armalite Mags I have are Gen I per http://www.armalite.com/images/Tech%20Notes%5CTECH%20NOTE%2071%20%20AR-10%20Magazines%20090603%20Rev%200%20Final.pdf

Still haven't been able to swap the barrel over as the barrel nut on the armalite upper is frozen, so I'm letting it soak overnight in some PB blaster then will try the freezing it in the morning before I break any more teeth off.  Which leads me to my next question:  I'm assuming I'm going to destroy the barrel nut I have now, are .223 and .308 barrel nuts (delta ring style) the same size?  I cannot find a 308 barrel nut online that's made for the delta ring/plastic guards.

Edit: Scratch that last, my google fingers failed me.  found the replacement barrel nuts from Armalite and Rainier arms.  Those of you with the MA receiver, are the barrel nut threads the same between the MA and Armalite?

One other difference I've found is the rear takedown-pin detent requires the AR-15 style spring.  The Armalite Rear Takedown is held in place by the pistol grip, while the MA is held by the buttstock.  I used what I had by fashioning a pin out of a small finishing nail, cut to length, and working down the sharp edges with a grinder.

Remember, the MA is designed for PMAGS. Your original Armalite is designed for Armalite/M1A mags. You can not use your Armalite mags. If you are satisfied with how the original parts work, go with it. If you start experiencing problems later, change them.

Ok so the barrel swap didnt go too badly once I froze the old receiver to break the barrel nut free.  Barrel is now mounted in new upper, only thing holding back is getting the buffer ports in the delta ring/spring set to line up.  I spent the better part of 3 hours yesterday trying to get them to line up with no luck.  I just need to keep trying.

I did function test both of the mag type I had and both hung up, so I will be acquiring some PMags sometime soon, as well as the A-type bolt catch.  Does the generation of PMag matter in this build (Gen 2/Gen3)?

Also I am still having the problem with trigger function.  If I ride the charging handle forward, I usually have no problem. When I pull back and release letting the buffer spring slam the bolt home, sometimes the bolt comes forward with the hammer already up, and sometimes I hear a dull thunk when I pull the trigger, followed by the hammer strike the second I touch the charging handle.  I am not familiar enough with these rifles to diagnose a trigger/hammer problem, my only guess is to use a candle and soot up the bolt carrier, cycle it, and see if the hammer takes any soot off the BC in places it's not supposed to touch.  I could widen out some small portion of the hammer channel in the BC if that does appear to be the problem.

Anyone have any advice on this trigger function issue?

Does not matter what Pmag you buy. Don't think I can help with your other problem.