What would be a good selection of 556ammo to try in a 1in9 twist rate 20inch barrel?

Ar15 rifle platform.

55gn or 62gn FMJs should be fine. What ever you can source.

But I would just run .223, ie Hornady 50 round packs of 55gn soft points have good brass to keep if you reload, or sell if you don't.

I shot 75 grain Hornadys out of my 1/9 barrel and they shot into just over .5 moa.