[L473ncy]

Indoor .20 is fine, in fact using .25 isn't really recommended since .25's will hold their energy better over the BB's flight path than .20's and the goal isn't to hurt other people. Also the fact that there's little to no crosswinds in CQB means although you'll get better grouping from nature of a heavier BB you'd do just as well with a .20. Outdoors though, .25 or .28 is great. I personally use .28 and take the resulting slightly lower FPS hit but I just lead my shots a bit and I like the slight increase in accuracy of the heavier BB's (.30 is starting to get too heavy for most guns though IMO, unless you're shooting at like 450 FPS in an AEG or have a BASR or PTW .30's aren't going to give you terribly great results).

As for spring, probably an M100 if not an M90. Prometheus, Madbull, Guarder, Modify, etc. Just be sure you're getting the right spring based on the companies spring guide/chart. Different companies have different ways of measuring their resulting speed.

ie. I have a Modify S110+ and it gave me ~412 FPS before it was broken in and then settled to a nice 394-398 FPS. A Guarder SP110 on the other hand might only give you like 360-380 FPS.

If you want to do an indoor and outdoor setup you could buy a second upper and change them out depending on where you're playing (ie. outdoor you could have a 14.5" Carbine upper while indoor you could have a 10.5" CQB-R upper, otherwise just change out the spring using the split gearbox feature which is easy enough.