i have a G&G SR25
i have fitted a Polarstar jack to it
in all honesty every guide i have read recommends getting the A&K SR25, as it has the v2.5 gearbox, which is the same as the v2, except it has a 19tooth piston and longer cylinder etc, which ensures correct cylinder porting for the longer barrel length.
the main problem with the G&G is it has a propriety hop unit ... and the nozzle is huge lol
the only other issue i have found is with 0.4 bb's and above not feeding in the 120rd mags, as there double stack type, 50rd and the hi-caps are fine.
i did ru a 7.4v lipo, if going 11v, i recommend a mosfet, unless you like replacing trigger contacts.
one last problem i had with mine, if your quick on the trigger release, you may lock the gear box, and as it has no full auto, i had to remove the motor to poke the anti reverse latch into un jamming the box.
other then that its a nice gun i wont be selling mine