AICS style magazines are what you want. Other mags, ie. M-1, FAL, won't fit due to Rem 700 bolt/feed lip geometry. Mods to feed lips lead to expensive headaches that would pay for a different style rifle. PTG bottom metal or other Badger copy, bedded with the pillars that come with it (they provide the proper spacing), is about the least expensive route. Most chassis systems have integral AICS mag bottom metal, a lot of their appeal. There are a couple of other designs out there that are closer to BDL bottom metal but rarely "drop in". You're gonna have to bed it anyway, just do the Badger style.