I recently bought a BCM M4 Mod 2. It has a 16" 1:7" twist barrel. I've only reloaded pistol cartridges so far so this will be my first attempt at reloading rifle cartridges. Should I look at 55 grain bullets for that barrel? Heavier? Flat base or boat tail? I'd think boat tail would be easier to load. I'm looking to load a lot so cheap is great, but also want accurate. Which bullets should I consider?