m28 upgrade

Can anyone recomend me a better Piston Cylinder +head spring guide trigger i have already bought the maple leafy hopup and a m107 spring and inner barrel so can you link me to upgrades avalible to get the best proformace

I decided to get an Echo 1 m28 about 3 months ago, for two reasons: It was cheaper than a lot of the same quality options (I got it on sale), and I need tech experience. About halfway through each game, my shoulder and upper arm become fatigued from the stiff bolt pull of the rifle, and I'm...