M4 AGM GBB venting every time trigger is pulled

[gordon]

Hi There got a M4 A1 a while back and been struggling to get it working.
every time i fire i will get one shot at best that goes about 8 meters and then my whole mag will vent on the second shot, i have replaced the hammer spring and hammer, had the whole gun in bits and replaced it. tried various different bolts and nozzles and mags. every time it just vents

any help would be really appreciated as id like to get it to at least fire without venting before i start upgrading it,
thanks!