Probably going 8” barrel
Will add a suppressor eventually
300 or 6.8?

Have any specific use in mind for the SBR?  I'd go 300 if 8" and 68 if 10".

300BLK for that short bbl.

The 6.8 shines with 11.5” to 12.5”.

FWIW, I’ve owned both.

Thanks for the info.
No specific use for this.
Been considering an alternate caliber for a while for a build

If you need to go under 10", .300 is the way to go for sure.   It's designed for that.   But make sure you understand you're getting something on the low end of the intermediate round spectrum.

If the build is just for fun and you don't already have a 5.56 SBR upper, just do it in 5.56 and make it 10.3" or longer.   If you do already have a 5.56 and don't have a particular use, I'd suggest looking at what ammo you want to shoot out of it, comparing cents per round, and picking a barrel length to match the round.

I have an 8in 300blk sbr with a form one can, awesome gun.

What do you want to do with it?

8.5" subs 300BO is essentially 45 ACP and wicked quiet.

A 10.5" 6.8 devlops about 80% of the round's potential.
A 12.5" 6.8 is almost optimal.
Both supersonic.

If you want to shoot supers, 6.8 out performs 300BO.

I have both.

I went 300BO. I'm in the 8" barrel range and currently unsuppressed. runs subs and supers no issue, obviously POI is different. with subs it's a 45 and with supers it's ballistically a 7.62x39 or 30/30. I went 300 because if I end up not liking it, the only thing I need to change is a barrel and it's a 5.56 again. so far I really do like it. I'm not burning a ton of rounds so the cost doesn't super matter.

I am filing for a can shortly. subs will be fun for plinking, supers make good HD and hunting. all said and done it will still be shorter than a 14.5 pinned upper so makes it handy.