New to 300 BLK - what range ammo?

So I’m brand new to 300 blackout and just got my first rifle. 8.5 inch barrel and will be shooting at an indoor range unsuppressed to begin. Will pick up a suppressor within the weeks to come. What is the best ammo to be shooting in this configuration and this type of range? Any specific brands and grain?

If I’ve learned correct, it should be subsonic.

Any help is appreciated.