I just ordered a MK18 SBR, it's in paperwork hell, unfortunately.

What kind of accuracy can I expect? Is it a sub-moa rifle with match ammo?

Is 75 or 77gr. the best to use in a 1o.3" barrel for accuracy?

I plan on suppressing it as well, will I need a different buffer and spring?

If the answers are posted elsewhere, my apologies!.

69-77 grain match grade ammo will provide the best accuracy results, but expecting sub-MOA accuracy out of a 10.3" carbine isn't very realistic.  1-2 MOA is more reasonable.

Shouldn't need to change your spring, but you could benefit from adding an H2 buffer.

Thanks for the response, much appreciated!

I have a couple of DD uppers (14.5" and 10.5") and have found them to be pretty accurate.    Haven't really shot the 10.5" for groups, but the 14.5" is capable of fairly consistent 1.5" five shot groups at 100 yards using 77gr. or 69 gr. match ammo.  And I suspect that a more consistent shooter could probably shrink those groups by .25".

The 10.5" upper should be nearly as capable, but the point of this gun is really for CQB at 100 yard and in.  For me that means that almost all of my shooting is offhand, with a red dot and XM193.  If I can get consistent hits on an 8" steel plate at 60-70 yards (and I generally can), I'm a happy camper.  However, I have no doubt that someone with better eyes than me (I have an astigmatism that makes using red dots for precision shooting pretty challenging) could get hits on a 10" steel plate at 2-300 if they benched it.  I can certainly do so with my 14.5" upper and a LPVO.  And honestly, that's about the max range I'd try to shoot out of a 10.5" barrel anyway.