My Little Project Gun

So I've recently acquired a lightly-used JG M4A1 & I was hoping to make a neat DMR project out of it because I know that JG guns shoot pretty hot out of the box. This one matches that case, but its terribly inaccurate (as you can assume).

I know that you can get more accuracy with a longer and tigher-bore inner barrel & a good hopup combo. But I have questions about how I can improve the air seal in the GB & whether I should get polycarb piston/head or go with an aluminum head.

higher spring rates causes more air leaks. how much do you plan on spending for your DMR build?
low cost is to use teflon tape on the cylinder head o-ring. guarder o-ring nozzle works fine. i was able to find a stiffer hopup spring (can't remember the spring kit). you can replace the piston o-ring with a #14 o-ring from home depot. you may need to heat stretch it after awhile. no need to replace inner barrel/cylinder. stock inner barrel wrapped loosely with teflon tape will stabilize it. madbull blue works great but you may need to either wrap it with dental floss or teflon tape (depending on your FPS).

if you plan field limit 550fps, sp160 will get you there (it will be around 560fps until it breaks in). for a low stress setup, you can use 32:1 SHS helicals with element half teeth (i use this setup on my Tavor) or keep your stock gears, replace piston with full teeth rack (SHS 15 teeth rack). you can use an aluminum head for better energy transfer but i prefer poly heads to prevent gearbox cracking.

AOE correction is important. it will prevent your piston pickup tooth support from cracking. for DMR builds using aluminum heads, you can sandwich a layer of sorbo between the cylinder head and a harder rubber surface or with poly heads, just super glue a rubber washer onto the cylinder head (1/2L rubber washer from home depot). this will also add gearbox protection.

the stock JG blue end motor works fine. it will require at least a 7.4v LiPo, Mosfet, deans or XT60 connector.
don't forget to radius/round off the gearbox window corners with a round file. this helps prevent cracking.

you can also mod your hopup for better grip (flat hops requiring the bucking bump removed, rotated and a custom nub used).
for semi auto only, there is a selector plate mod which requires trimming the cutoff lever window longer. a computerized mosfet can also provide semi-auto function (but expensive). see reed for his mosfet.

other options include using a double o-ring cylinder head, metal nozzle with o-ring and 6.03 tbb barrel.