Below is the link to the unissued version which are LNIB for $550. They also have A, B, and C graded conditions for those looking for a bargain on some used models.
http://www.surplusammo.com/colt-556-hbar-upper-U-grade/
if suppressed, the general thinking is that barrels less than 9 or 10" will keep 147gr ammo subsonic. there is some variability to this. my 7.5 keeps 147s at subsonic velocities.
I'm sure there's a lower limit but I've seen 4" that work when suppressed (and maybe even not suppressed)
other...
some of my very not professional pictures of a chinese red head that I keep in my basement:
these pictures are so bad...need another photo shoot in better lighting with a better camera.
I've heard that some after market triggers don't work with the 9mm bolts - and the ramped vs non-ramped compatibility has me all confused. My 9mm build will never be a bench shooter so I left the standard, crappy mil spec trigger in with my ramped bolt.
Had a half a mind to try it on my other...
Need to add pictures of my Mak-90 with the red timbersmith furniture. Took a lot of sanding to get it fitted but I think it turned out decent. Was able to get it 922r compliant while still leaving the Chinese hammer in.
also, forgot to mention but I've yet to break a bolt catch through about 1000 rounds. mine is a 7.5" adco barrel that I almost always shoot with a tirant9. 147gr stays subsonic.
I went with a hahn top loading dedicated block because I have another sbr for different calibers.
I use a 9mm specific buffer which negates the need for two fiddy in quarters in the buffer tube.
my set-up is 100% reliable.
like others have said, if you're not comp shooting at long range then it probably won't matter too much.
I use 1x7, x8, x9 and focus more on the barrel profiling, lining (or lack therefore), finishing, gas system length, and mfg reputation.