I run stand alone 40mm as a primary at UA regularly. Lost of fun and pretty effective if u carry enough ammo. For in door propane shells can be bought for as low at $20 each. Outdoor co2 Shells actually get better range then u think if u plump enough psi in.
If ur looking for the best gun u don't run 40mm. But u can get ten kills with a AEG, but the one with a 40mm is the one u will talk about.
Below how i have fun with 40mm
https://www.youtube.com/watch?v=X-ItvWgvUfY