I throw my spoon to this also, I have Bolt Sopmod version and have done some modifications to its internals.
At first it caused me lot of headache, as it was shooting semi only. Well, finally noticed that selector plate was modded so it would not move cut-off arm. I bought the gun from Germany, and due their laws here it was modded by Bolt to fulfill the laws... I was not aware that law, and it took while to find the small mod done to selector plate. I replaced plate with old unknown maker plate and now it rocks happily.
My friend did a metal spring guide from old metallic non-bearing, basically it was just drill a hole through it. Before that we tried to drill a bigger hole to spring guide with bearings, only to found that its bearings inner diameter are just 7mm... And we were after that size hole, so guide snapped.
I filed down the second teeth from the piston, just to be sure.
Oh, and I put the Gate PicoSSR 2 mosfet inside the gearbox. That fet is amazing small, and fit to space where wires come inside box. No need to more wires outside the box
Also I changed the wires to better, if I remember right they are now non silicon AVG18.
I the first or maybe second game the fake bolt carriers actuator arm broke. I had assembled it wrong and it stuck when returning back to front position. Next cycle snapped arm clearly and it was later found inside the box. My friend machined new one from steel, and it is again functioning.
Gun shoot a little too powerful with 1,69J (130m/s with 0.2g) so I replaced the original spring with M115 spring. Now it shoots around 120m/s.
Lastly I replaced the regular ris with Iron Airsoft URX4 13". Bad pictures here: http://arniesairsoft.co.uk/forums/index.php?/topic/197001-ar15-variants-and-derivitives-picture-thread/?p=2652247
Love how light the URX4 is!
Otherwise really nice gun, blowback is so nice that I doubt I will newer buy any new gun without one.
I'm running 11.1V lipo in it, and now I think 7,4V would be safer. What you think?