trigger ass is really tight on the newer M14's, i use the tool in the but stock to put it in the hole in the guard and really gotta yank on it, loosens up after awhile and gets easier, there are youtube videos of it. 2nd you need to swing the locking mechanism off the front of the gas tube, it rotates off counter clockwise, shifting it 90 degrees is enough to clear the piston. mine was stuck a little from factory and i just used a plastic screwdriver handle to tap it loose. it is a very tight fit with the shorty. just look up a video on M305 shortie dissasembly and it will show how to do it.