Phil, if you look at the photo of the 16" AR of mine above that's a OPS 12th mounted on it. It is very quiet, and the suppressor reduces group size by as much as 50% at longer ranges, it can be hard to tell at 100 yards sometimes.
The OPS 12th (also mounted on the SPR 18") is probably the state of the art in suppressor performance. Also you get very little blow back with it.
You can buy smaller suppressors, but not really any shorter 'cause the OPS telescopes back over the barrel, haven't heard a quieter suppressor yet than the 12th. Plus it' very durable, at least one example (in the military) has over 100,000 rounds through it. The secret is the brake which the OPS use, it acts as the first blast baffle.
The 12th would be my first choice, although due to the mounting system it isn't really suitable on below 12.5" SBRs.
However, if I was going to build another SBR, I'd look to use a 12.5" barrel and either the OPS 12th or probably the 15th (a little shorter and slightly louder) if it was to be shot suppressed almost exclusively.
Once you start shooting suppressed, you will not want to go back!
Now the SPR 18" with the OPS is as quiet as a 5.56 is going to get with full power ammo. It's a pleasure to shoot, awesome accuracy (.4 MOA at 500 yards once! Really), and as close to the real thing as you can get.
The 16" is my favorite rifle, some criticize it's 3.5 mag ACOG, it's a TA11C, which has a BDC reticle almost perfectly matched to the 77gr MK262 ammo fired through the Kreiger barrel.
My answer to the complaints about the 3.5X mag is below, never leave home without the Geovid 10X42 laser rangefinding binoculars.
Use them to ID and range your target, then put the appropriate cross on the reticle on the target and squeeze the trigger.