Any other receivers other than m4 platform for gel blasters?

Any other receivers other than m4 platform for gel blasters? Thinking of building one from scratch and already have 3 blasters with m4 receivers.

Ump45 or m249.

Ump45 you can get an empty shell. Takes a gen 8 Gearbox. Not a lot of cosmetic changes available though. Can get metal rails, aliexpress have metal handguard extensions.

MK47 Mutant - might still be too “M4” for you. UMP45 as mentioned above is a much cheaper starting point.