My BA that was detuned to a 500fps (with .2's) spring is not heavy to pull at all. The spring that was previously in it wasn't all that hard to pull neither and it was over 517fps on .36's!
A BA will be the most consistent over an AEG and gas (even HPA)
locking an AEG to semi isn't that difficult though as it's just a matter of modding the selector plate to prevent full auto. simple mod that can be done with a box cutter.