What is a good grain for 1:9

I in 9 is good up until 69/70 grains depending on the individual barrell.
some 1 in 9 have done well up to 75 gr rounds but that cannot the counted on wiht every gun.

Most folks agree that 69/70 is ususally the upper limit for 1 in 9.

I have both 1 in 7 and 1 in 9 AR's and fire 62 and 55gr thru them.

1 in 9 is not a bad twist , there aremany excellent 55gr rounds out there and even 50 gr and lower that are just great rounds.. but a 1 in 7 might have a hard time with anything under 55

Most 1-9 twist 223/5.56 barrels will shoot up to a 75 grain bullet,but it will depend on the length of the bullet. A 75gr BTHP is shorter than a 75gr A-Max,and shoots great out of my Savage 12,but the A-Max's don't fair to well,the twist rate doesn't stabilize them very well.

Since your shooting an AR,the 75gr BTHP will be the largest bullet that you can load to magazine length. A 75gr A-Max would have to be shot like a single shot rifle.The max length for a magazine is 2.260",and the 75gr Amax should be around 2.390"
I shoot them out of my 1-8 twist RRA varminter,as well as 77-80gr match bullets,but it is a single shot rifle when I do.

55gr-69gr bullets will shoot great out of your rifle,and all will fit in the magazine without any problems.

55-62 grain.

My Savage bolt has a 1:9 twist and will shoot 50-65 gr bullets well. SA rifles can be a little more picky with loads and will take some experimenting with powders.