As i think you know, i just recently finsihed buying the parts for my m249.
There are two distinct versions i find for how you want your SAW to be. Uber fast with a high speed or a high torque SAW.
High speed is high speed gears, high speed motor and very low spring, m70-m90. Along with a Lipo, Pushing out BBS so fast i cant describe it. more of a CQB/ CQC kinda thing where you dont need distance as much.
torqe up mode that i have mine set up to. standerd torque up gears, M125 spring and magnum motor. i use a lipo that does have an INTENSE rate of fire. not as intense as literal High speed, but slower with still ROF. Having a higher spring has more FPS and little more distance.
But the thing with distance is if you have a good air seal, good piston head, double o ring cylender head, tight hop up, BBs go the distance. SAWs are soposed to be more of a "high speed" head ducker. but im a rebel and i brake the rules with a torqu up high speed. i keep heads down and hit them to. if you want to know more like witch parts are better, comparisons and etc. i can easily help you out. give me a pm if u got any questions.