Benjamin Gen 1 .25 marauder

I imagine the breech riser is there to allow a moderator to fit above the reservoir tube.
You aren’t going to make 100fpe with that 12” barrel on the marauder. You can estimate each inch of barrel added will provide approximately 10fps to the pellet speed. I would be looking for a good custom barrel about 26” long to begin with, and then go from there. You want to shoot slugs right? Because pellets are going to be trash accuracy at .25 cal 100 fpe levels. So you need a barrel with a twist rate and no choke that is going to handle the heavier slugs. Don’t skimp on your barrel. What good is 100fpe if you have no accuracy?

Have you considered just buying an Airforce Texan? Or even a Condor? It seems like they would be more in line with what you are trying to achieve and you could quit being Sid from Toy Story and stop mangling all the Marauders. View attachment 490976
I got a longer shroud and barrel band and the regular length breech screws coming just need the barrel now.
 
Everything you said is true. You just forgot to mention the great white shark tank you have to drag along.. very nice gun .your money. Go for it .
Yeah i got a 66 cubic foot 4500 psi scba tank I'll be getting a 88 cubic foot tank later on and I got the twin piston tuxing compressor and a booster hand pump... the addiction is real!! Here's a video of me this morning shooting my pbba .457 3000 psi with a 350 grain slug