[L473ncy]

Try holding the mag at a different angle (ie. shim it forward or backwards). I know some mags are really finnicky about how they align with the hole at the bottom of the hop up and I had to push my mag forward to get it to feed properly (shim with a piece of soft velcro on the back of the magwell and you're good to go). However that doesn't solve your shoot 3 feet then immediately drop problem. For that it might have to do with the BB hitting the edges of your flash hider. I've only seen it once with an offspec PDW flash hider but it could be... Try removing the flash hider and firing and see how the BB's behave as they exit the barrel.