Quoted:
I'm currently building an SBR with a 1:7 twist, so I'm looking at buying several boxes of different ammo to see what shoots the best compromise of accuracy, reliability, and knockdown power.  From what I hear, the 1:7 twist combined with a 10.5" barrel is best used with heavier bullets, up into the 70's for weight in grains.  I haven't completed my build yet, but I thought I'd start picking up different manufacturer's offerings of 65-77 grain bullets, but my Dad, who is an old school shooter/reloader, keeps telling me that the really heavy bullets are too long to use in a semi-auto .223/5.56, and that they are used primarily in bolt guns for target shooting, because the added weight requires added length.  Can anyone comment on whether or not the bullet weights will fit in a standard PMAG, and if they will cycle reliably in a semi-auto AR?  I don't want to go dropping money on ammo that I can't use.  My dad typically knows his stuff when talking about guns, but it seems like so much of that ammo is aimed at the AR shooter, it must work.
Any help is appreciated.