I have the option of picking up a bushmaster carbine with integral sights, or a colt carbine flattop.......Now I would like a flat top,,,,but within the limits of the law, I would like to add various components and possibly swap topends from time to time, and have heard that colt has non standard parts or pins etc...., is this true and what type of problems could I expect if i went with the colt??,,,,I also have an option to pick up a pwa ar, although I dont know what quality they are or if there would be a mismatch with this lower.....tks fubsy.