Trying to decide on my next SBR. I already have a 10.5" LMT, and I’ve got paperwork in right now for a 12.5", it’ll be a Noveske or Denny’s Operator barrel. All are in 5.56. Although I’m not crazy about having to buy more mags, I think a SBR in 9mm might be really fun. Thinking of a Colt 6450 and chopping the barrel after paperwork. O’course the Colt 6933 has always caught my eye, and the 6921 is pretty cool since it’s as close as I can get to a real M4 (can’t afford the FAs). Sure, I’d love to get em all, but with all the $ I’ve spent lately, I can really only afford one more, at least for a while anyway.
I have a 6921.
I wish I had a 6933.
I have 2 6933’s and a 6921. I sold the 6921 upper. I use my guns and I am not a collector. If I had to pick 1 single AR and that be all I could ever own, it would be a 6933. It is my favorite AR by far. Ask Dean Caputo and he’ll tell you the same, the 6933 is all anyone really needs.
6933 - short, light, and you know it will run.
6921 - close to an M4 but aside from the essoterics not that much different than a 6920 given the hoops to jump through.
IMHO
Good luck
I’ve got a 6933 inbound, waiting on dealer to dealer paperwork. Only difference is that this one’s made a trip to SAW for some “massaging”. Had to vote for it of course.
Reasons for a 6933:
Quality. When’s the last time someone claimed that Colt was pure junk? Overpriced maybe…
Factory SBR. No extra engraving required. And if you’re truly anal about things, no need to re-anodize the lower to fix where the info was engraved.
Collectible and somewhat rare. Don’t know about your intentions, but mine will never be a “safe queen”.
Fills that empty slot between your 10.5" and 12.5". 
ETA pictures
I’m surprised that noone has voted for the Colt 9mm. Do they not make a good SBR? Now that C Products makes mags, I wouldn’t think that mags would be too expensive. ???
The 6933 is the way to go. I don’t see the point of the 6921. When i want a short barrel i want something a good bit shorter than a 6920.
No. 6
Thats a bit different then my 6933’s. Did SAW replace your barrel? also my 6933’s don’t have a bayonet lug
no need to re-anodize the lower to fix where the info was engraved.
One should never do this as it changes the pin hole diam. Just use some Alumablack or laser inside the trigger guard area.