Quoted:
It may be hard when you're comparing it to a pool of one, but I'd check the bolt stop for being too loose. Many manufacturers either cut the slot relatively wide, or use a bolt catch that's a bit small. If you have the opportunity to sit in front of a pile of stops with a set of calipers to cherrypick a fat one, I'd recommend it.
A5 buffer w/ regular spring & tube = awesome. Works like a boss.
Many upper receivers are cut in such a way that they restrict the final bit of magazine travel. Doublecheck with your upper for "happy" function. Jam a mag in a couple of times and you'll see if it contacts the upper at all.
You may have missed the part where the OP said he has a
Colt SMG lower. That is the standard to which all 9mm's were built (except Olympic).
I guarantee you his lower is in spec.
Also, he does not need the A5 system for a 9mm subgun. A standard buffertube, standard bufferspring, and 9mm buffer is sufficient, factory correct, and has been proven on many thousands of factory SUB MACHINE GUNS (which is exactly what the OP owns).