You'll just be able to increase velocity ~ 30 to 40 FPS is all with the 700mm. Maybe just a little less of a harmonics fight with the 600mm. The efficiency won't quite be as good with 600mm verses the 700mm for the same velocities, but who cares with a bottle gun and a compressor to fill. I chose 600mm for my .25 cal for the length and 700mm for my .30 cal for max power. Both are lasers with high shot counts (580cc bottle).