I just purchased a Bushmaster AR15 which has a post ban lower and a pre ban Colt HBAR upper with a 20" barrel and a 1 in 7 twist.  I have heard a few opinions as to the best ammo to use.  It seems like the 50 or 55gr ammo is more favored, but how does it perform with the faster 1 in 7 twist?  I would appreciate any comments or opinions before I buy a quantity of ammo.
I've also heard that the 1 in 9 twist is more accurate than the 1 in 7, if this is true, how much more accurate is the 1 in 9 twist?  Would it be worth it to re-barrel with a post ban 1 in 9 twist?

Very funny.

I assume you are a troll...

But if you are just a newbie: it is illegal to have a pre-ban upper on a post-ban lower.

You can sell the upper, or grind off the bayo lug and permanatly attach a muzzle brake to make it legal.

Av.