I have a 16" Bushmaster M4 type flat-top I bought a couple months ago.
I am thinking about doing the ATF form 1 to allow making an NFA weapon, and buying a short barreled upper to put on my lower.   (Rather than buying a whole new short-barrel carbine). Bushmaster  sells a upper with 11.5" barrel for $565.   Is there any other vendors I should consider getting it from?   I want a quality upper for this, and I guess I have a warm and fuzzy feeling buying it from BM since it will be going on a BM lower.   I have seen a Lewis Machine Tool 10.5" upper, but its more expensive and has a 1:7 twist (I think).   I like the 1:9 of the BM.    I saw one from Model 1 sales......any good?  Also, I will likely put a LaRue or Daniel defense free-float rail fore end on this shorty.
Any recommendations?

While I am not in the market for an SBR at this moment, my general thoughts are that as barrel length diminishes, caliber should increase.  Therefore if choosen to go SBR I would think that a 9mm subgun is the way to go.

I'll be shooting 55gr and 62 gr bullets.      1:7 is the way to go?
With the short barrel, it changes things enough that you need a shorter twist to sabilize?

Actually, I was thinking 1:9 because I have read that it is optimal for stabilizing the 55 and 62 gr bullets.    Since those are the bullets I will be shooting, thats why i was thinking along those lines.   But it does seem that most every short barrel offered by vendors is a 1:7.    ok.....1:7 it is.

then what I want to make sure of......will a 1:7 twist work well with 55gr bullets out of a 10-11" barrel as far as stabilizating the bullet when shooting no longer than, say, 100 yds.?  (I mean when just considering stabilization and not fragmenting for self defense).     Would a 1:9 stabilize a 55gr bullet well, even though the barrel is so short?
Sorry if I am being redundant here, but I am a little confused about this.

I recently went through some of these same issues on a build.  

Do you like the fit between your current lower / upper receivers?  If the answer is yes, then simply buy a barrel from a reputable dealer (like the one commenting in this thread) and build it yourself.  Your going to be installing a full float handguard anyway.

Yes, the BM I have is fine for fit..... nice and tight.     Thanks for the idea.    Since  I got this rifle, I kind of wished I had gone with a mid-length 16".     I have the patrolmans carbine with the short handguards.
 Maybe I should just talk to Randall about cutting it down.    (after getting a Form 1 completed, of course).        I'll send you a note about it, Randall.