So i am going with an M4 style in my build and will be either going with a 14.5 inch or 16 inch barrel. Anway i am seeing 1:7 twist and 1:8 twist but i thought 1:9 was the best for the standard 55 grain bullets... Whats the deal? The only 1:9 barrels i am seeing are full length rifle barrels.

Second question, How hard is it to install a sight block on a virgin barrel?