well it is not a 2100 piston like it looked like it might be in the evp it looks to have its own piston, the 2100 piston is ~6" and the 760/66/13xx etc all share a piston that is ~ 2.5" long.I think the extra length is just to be able to use a longer pump arm with the 3.5" stroke it shares with the 760/13xx etc.Longer lever = reduced effort thru better leverage. That feeds a valve that looks to be larger than the 2100 valve but smaller than the 13xx valve so it is meant to build pressure quick.My 18.7" barrel 1377 tops out about 750 fps with a .13ci valve so I am guessing these with stock valve volume will top out around 680-720 fps or so with 7.9g pellets after mods.stuffing the piston will not do quite as much until the valve and pump cup match of course.
This post seems to have as much or more technical stuff on the MK than the rest so I figured I'd add my findings here since I just got mine today...I like it, feels good, less awkward than some people make it out to be... pump with left hand while holding pistol grip, butt pad in your right armpit if it makes it easier, cock with left hand and push the clip over to the right (opposite of normal) for the first 4 shots, reach over on the 5th shot and pull it to the last detent, fairly natural and reasonable balance.Now for technical information, I didn't try the chrony but might this weekend. I lined up 4 Crosman pumpers with the pumps open side by side, the piston is about 1/2" shorter than a 2100 piston, but the pump linkage is the same as a 1377... No I didn't disassemble it to look close or take calipers to it but form the looks of things it's the same as the 13XX/760 platform stroke wise. I have the MK, 2100, Recruit, and 1377 all side by side and that's what I noticed.If I get motivated to finish my garage as good as I can this weekend I might finish my M4-177 then might modify this MK...
Reading the EVP, the outer valve O-ring is 3/4 OD, so the tube may be 7/8x.058-.065. Would be interesting to upgrade a 2289 with a MK tube.
Support for the pump tube consists of holes at the rear of the tube which go over posts molded into the halves of the gun frame back by the trigger assembly. Not the strongest looking arrangement for going all out, crazy town pumper but there's other reasons I don't believe that's much of an option with this gun. The front of the pump tube is held with the flash hider section and the pivot pin. The barrel band is part of this section and it holds the barrel very well.The interesting thing ( to me ) is the transfer port seal and breech or receiver setup. A flat o-ring type gasket shoved in a counter bored pocket at the exhaust port. The barrel butts into a plastic breech like part which has tube off the bottom of it and this basically just sits on this seal. This breech part is held in position by posts on either side of it which go into holes molded into the frame of the gun right above the barrel. There's all sorts of wiggle room here. It holds it down OK when assembled but I think there's room for improvement here for better seal.When I took mine out, I noted the seal was only contacting the TP tube from this breech piece about 70% and I'm sure air was blasting past it each shot. I suspect the valve was not accurately indexed 180 degrees from the bottom in the tube for a perfectly flush seal. The seal had a deep impression in the area it was contacted by the breech ( crooked ). I flipped the seal over before putting it back together and made sure to get the valve perfectly straight up.Piston is around 5.345 long or so with seal.