I have a colt smg.  It has no upper.  Therefore, I need to build one for it.

What is different about building a 9mm smg upper from a rifle upper?  

Obvious that it's blowback.

Planning on an
1.   8" barrel
2.  Flattop upper
3.  Clark custom carbon fiber f.f. Tube
4.  Adco ramped rra bcg

Ramped bolt?  Is this needed!

Anything else?

Looks like a compete setup other than a heavy buffer and the obviously needed mag block.

The ramped bolt is not required but seems to be a very good mod and easier on the gun.

You may also look at some kns pins or something similar. The blowback design can be rough on those parts.

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).

Sent you an IM OP.

nvm

I would go with the Colt 9mm pins if you don't already have 'em.

No mag block needed.  It's a colt smg lower.

This one to be exact:


That upper s a friends upper.

Got you. I have seen people refer to a short 556 as a SMG before. Just making sure.

That video looks like a hell of a good time. My SBR setup seems inadequate now.

Yeah, but even Colt makes things to high or low sides of spec.  I think the slot in question can vary by at much as .010" from one to the next, which can have an effect on the bolt catch itself (which can also vary).

As for the A5 buffer (just the buffer), it functions more smoothly than a 9mm buffer and helps take that excess slop out when locking back on an empty mag (which also heavily contributes to broken catches).

Stop being so queer about it, it's bastard child of an AR15 anyway.