is at least half of the airgun fun. Myself I am a CO2 lover. with the crosman stuff to take it one step further I have made a few guns bigger and that shoot harder.
If I was into pumpers, (its a physical thing that I hate them!) I would love to create one using crosmans designs but with a much bigger tube, stiffer, longer and thicker, and with a much,much longer pump handle and also a reservoir to get a few follow up shots and a full wood stock using a marauder trigger. no one seems to want to go much beyond a 1377 very far. a very good design that can be expanded on. making a check valve/reservoir 5" or 6" in front of a 1377 type valve would be no big deal and would yield a pcp/pumper. Possibly interchangable caliber/barrels too. There is no law that says a pumper has to be a ,177 or .22 either.
There once was a fellow here who created a beautiful 1377 with hardly more than hand tools so it can be done with out a hefty investment in machine tools. buying bolt on pieces gets old as far as I am concerned.
Springers on the other hand are pretty much you got what you got. you can "tune" and make them smoother, a few have changed caliber, but for the most part out side of a stock for looks, there is little you can do to double or tripple power. john