Modify springs are easy to compress and cheap and put out predictable and relatively consistant fps. Just watch out for the rating though, they tend to be about 10m/s under. Example, I installed bushings, Modify 100 spring and bearing spring guide into a stock TM AK47.................. net result was 370fps using 0.20g BBs, high rate of fire, little mechbox abuse, perfect for V2 guns (yes, I know the AK is V3, I stopped talking abiout that after 'high rate of fire')
I have a MOdify 90 spring in one of my MP5s, bearing spring guide and tightbore, I'm getting fps in the 320-330fps range.
Jugglez sent me a study done by Modify on M4s, one for each spring value, all upgraded with some Modify parts. Results were chronied with 0.20g BBs, then the guns were used for 30,000 rounds then chronied again. All results were within a few fps of the new condition fps. Pretty impressive, eh?
And yes Saint, instead of having 12-15A running through the trigger system, you end up having well under 100mA running across the contacts of the trigger, therefore eliminating that arcing (cutting) problem.