A 7.4v 2000mah lipo will act sort of like a large 8.4v battery....no crazy ROF goosing of the system, but decent staying power for lots of shots.
Also...less arcing = longer switch life
Don't be too thrown by MOSFETs...some are IC controlled with tons of features...others are not much more than a pinky nail sized chip with a couple of resistors soldered together. Depends on what features/functions you're looking for.
Best of luck
(If it was my own gun, I'd go with the 7.4v lipo that fit the space, keep the fuse because I wouldn't need to remove it to make space and just swap the switch if needed down the road)
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