I recently completed my M4 clone and I have some issues.

The first time I took it out, I didn't have a charging handle. I was shooting real M855 out of a mixture of NHMTG, OKAY and LaBelle mags.  I opened the weapon, put a round in the chamber, closed the bolt, cocked the hammer and safed the weapon to ready it. I had a loaded magazine inserted so it would theoretically not require me to repeat the loading action again for the rest of the day. The only mag it would actually recoil far enough to load the next round was the LaBelle. I shot with that magazine the rest of the day since I was only zeroing. Also, the mag release was REALLY hard to press at first. It has since smoothed out.

This weekend I picked up a charging handle, completing my carbine. I bought 180 rounds of Remington UMC .223 55gr rounds at wal-mart. The weapon would reliably chamber and fire each round, but didn't lock back on an empty until the final magazine which was an OKAY I think. Also, when I broke the rifle down at home, it was INCREDIBLY dirty, far dirtier than any weapon I have ever used in the military, even after long exercises. The bolt was very stiff in the carrier. I think this is due to the ammunition because it wasn't nearly as dirty after the Winchester 855 and the Remington UMC did the same thing to my Husqvarna 1907.

Basically, I am wondering what is causing my rifle not to lock open. I think it could either be the buffer breaking in or a slight bind on the gas tube/carrier key interface. I'm planning on removing and re-installing the barrel to fix the slight rub.

Here is a relevant parts break down.
Colt M4A1 military surplus upper reciever, complete with matched colt BCG.
Phantom 28 5C2 flash hider
RRA charging handle
Low S/N Stag Arms lower
CMT LPK, single stage trigger
Enidine Carbine hydraulic buffer
Carbine Spring

A-2 stock?

Sorry, M93 stock with a carbine spring and enidine buffer.

Yes, the bolt will lock back if I manually hold the catch and pull the charging handle to the rear. The bolt has only locked back once out of the 240 rounds I have fired so far.

Can you swap a standard buffer and spring and recheck?

I'm going to see if I can't borrow a regular buffer, the only guy I know with a carbine is being a cock about it. The catch will grab the bolt if I have an empty magazine in and I pull the charging handle to the rear.

Bolt won't fall out of the carrier, even if I shake it up and down. Carrier will also not go from unlocked to locked position when I place it bolt face down on a table and lightly tap the BC. Gas tube pin is present. I have to delay any actual test fires until I can get to a range.

Military CLP from the arms room. Yes, the bolt moves freely in the carrier when I clean it and it'll slide foward and lock at a very low angle. At 45 it is moving fast enough that it actually bounces back a little.

I'll be retorquing the barrel and tweaking the gas key this weekend. Tuesday I should be able to take a magazine or 7 to the range and test fire. Now if I could just snag a regular buffer.