With a smoothbore bore barrel liner adapted from a Recruit and some work on the magazines you might could make up a pretty decent CO2 BB "rifle." Then you could see about valve mods if you needed more power.
A Recruit barrel liner may also work to make a 2100 into a smoothbore BB gun.
Crosman 760 smoothbore barrels could also be adapted to 2240's and 1377's.
My personal favorite BB guns are the Daisy Red Ryder and the Crosman 760 and 788.
I plink, therefore I am.