I've been through this same set of questions before, and it's the right set of *questions*, MWM. If you have several to many long guns, I think you're on the right track, to not simply put "a tripod" on "a rifle" - get a system that interchangeable for all. But which one?
Well, things may have changed in the last few years, but the one I bought which "locked in" to the rifle using the sling swivel stud ended up being junky, and not working (the plastic failed under the stress of tightening the metal screw on the plastic that clamps on the ball to hold it in the position you want).
So, here's what I do now:
Get a regular high-quality lightweight field tripod.
Order the "V-notch" replacement part from the junky brand that failed on me (crap, what's the name of that tripod that's sold in Academy & Bass Pro? - Hang on; I'll come back here when I figure it out. It has different type heads, including some that can lock in and others). Attach the V notch head to the regular tripod (just screws on the threaded post), and carry it around with any longgun. Then you can adjust to any height - sitting in chair, standing, sitting on ground.
I highly recommend the ProMaster XC-525 or XC-525C tripod. You can remove the regular head leaving solely the round threading for the V-notch, for super light weight.
http://www.amazon.com/s/?ie=UTF8&ke...vptwo=&hvqmt=e&hvdev=c&ref=pd_sl_1rdzrow8a8_e
Now this solution is NOT a "lock in" where the longgun and bipod become one unit, so there's some limitations on stability, but it can be very stable.