If you are shooting from a bench get a heavy leather bag with ears and the flat bottom, you can adjust your sight alignment and fine tune the POA elevation by sliding the rear bag back and forth on the rear stock. I use a ProtekTor. You can get them on sale unfilled and pick your poison. I like good old sand or steel pellets. Keep in mind this is for bench shooting. If you are shooting on various surfaces you probably want a type of competition squeeze bag type: I recommend some sort of silent retention like velcro straps or a bungee or even paracord and a small plastic carabiner. I use a Large TAB Rear Bag v2 with lightweight fill, with a poly steel wire attached to my buttstock. I use a SOPMOD buttstock on my SPR, it's surprisingly well suited for bench and ground use. The TAB is more of a squeeze type bag. If you are going to be stationary you want the heavy poly fill. I use the lightweight fill because my SPR setup is designed to be rapid deployed: can't have 2lb bags slappin around. But the added weight is preferred by competitive shooters, and of course not preferred by folks lugging gear in the field as much.. Additionally: Caldwell's "rabbit ear" rear bags are all decent. Even the cheap nylon classic is decent, friend has one and I like it. But I would go at least with Caldwell's leather Universal Deluxe model. That said, get a ProteckTor leather flat bottom for bench on sale and fill it and you'll be set for life. You can get higher end one's if you're a bench aficionado but the designs are all similar, it;s the thickness of leather and size and weight. Before you buy anything, get setup and make sure you get the right size bag. Size is everything. For bench it's less complicated but for field use, you might want a four sided squeeze bag that offers three height adjustments plus fine tuning based on fill material and compression.