I have built a MK12 clone using a Centurion MK12 barrel. It’s a 1-7 twist 18” barrel.

I have been shooting 77gr MK out of it with sub-MOA results. No issues at all. The barrel is great.

Now I have a plan on trying out a new White Oak Match side charging handle upper with the MK12 Centurion barrel. Using a Sinclair single round feeding adapter.

With single hand fed ammo, what would be a suggested ammo for this. My thoughts are 80gr or 90gr. Has anyone done this or any input on max off the shelf loaded ammo for this project?

I’m not trying to build a bolt gun but to see if there is an opportunity to get more out of the MK12 barrel from a heavier load than the recommended MK262 ammo.

Thanks ahead

HSM is the only one I see with a current listing.

I don’t think you can find a 80 or 90 FACTORY load for the 223, except the HSM already noted.
I thought these were both hand loaded only otherwise.........

Barnes makes an 85grain 5.56 load.

https://www.midwayusa.com/product/101629790/barnes-precision-match-ammunition-556x45mm-nato-85-grain-open-tip-match-box-of-20