If you had the choice of a high end ar in 556 or 6.5 grendel or a 6.5 creedmore or 308 bolt gun for 300 yards or less.   -unknown distances.   -targets are as small as 1" squares  and time is a factor in scoring which caliber would you pick and why?  Same glass and assume high quality ammo shooting from prone.   I'm trying to decide if I should build a quality 20" 556 upper or run my larue 18" grendel or buy a tikka t3x a1 in 6.5.

If I go with the grendel I could spend a lot on glass.    I've never tried to shoot a 1 or 2" plate at 300 I generally shoot 8" and 6" circles

I would go 5.56/223, but you have the Grendel - run with it.

Your biggest factor at that range is going to be wind.  I'd be concerned with too much wind drift with a 5.56 on a target that small.  6.5 creedmoor is likely the best of your listed chamberings for beating the wind, with grendel as second best.

I'd be inclined to use the grendel and get the best glass that you can.  But if you really need an excuse for a new rifle you could probably get a bit more velocity out of the Tikka in 6.5 creedmoor.

I own a 6.5 x 284 norma but I don't want to shoot 100-150 rounds a day of that at these ranges.  I like the glass and grendel idea.   Vortex razor 2?

I guess it depends on how the game is scored. Is accuracy more important than time, or vice versa?

I know the internet will swear up and down that ARs will have no problem consistently hitting a 1" square at 300 yards, but relative to bolt guns they've always lagged way behind in the accuracy dept for me. Hell, even in dead calm conditions only the best bolt guns are going to be holding a 1.5" circle for 60 rounds; my last 300 yard tourney nobody shot a 600. Several scores in the 580s. If the game weighted towards accuracy, I'd be running the a .308 bolt gun. Probably work up a load with 155.5s (which should be stupid easy to find a load for) for minimal recoil.

If it's weighted more towards time, then I suppose the AR would be the ticket. Of the ones you've got, I'd be shooting whichever one is the most consistently accurate. Unless you're shooting in hurricane level winds, the wind isn't going to be a massive factor at those distances. If .223, shoot the heaviest bullet that's accurate.

My 2c.

If time wasn’t a factor I’d say a bolt action 6.5 Grendel or 6.5x47 Lapua. Due to the low recoil. Right now I’d lean towards the 6.5 Grendel with a 108gr or 107 gr bullet and the best glass I could afford with a moa wind reticle. A mil dot I think would have a little to much variance on a 1” target at 300 yards. I have a 2”x2” swinger at 250 yards and on a windy day the .223 gets challenging. Even my AR10 in .308 gets challenging because the recoil breaks your site picture and you can’t call wind as easily. So I think a AR10 in 6.5 Creedmoore would do the same unless you build it as heavy as the RPR. Another cartridge if you decide to build would be the Valkyrie or the 6mm Grendel.