#1. The QB 78D, 2078, and 79 all use the "fat" barrel...which by chance is the same as many RWS's.
#2. Got to agree about the RJ breech being a good one. For the $, it's really hard to beat the basic Crosman long steel breech. Do use a Crooked barn breech (old one...not double slotted) and it's a very good light weight unit, but doesn't have the length to offer a lot of scope mounting flexibility.
Will look...might have a picture of all 3 (crosman Steel, RJ, and a CB breech) on 3 different guns...would give you an idea of length.
OK...found one:
(click to enlarge)

Left to right:
Older CB breech.
Crosman long steel.
RJ riser
Crosman long steel
Really wanted that scope on the CB breech, but had to chop up a one piece mount to make it happen.

#3. Extension gas tubes do offer more volume. Haven't found a personal need to use one.