To be honest since I posted the last status about this I have forgotten most of the things UTJ told me about the A&K M24. But IIRC it comes with some very good stock internals that shouldn't need upgrading like the spring guide / trigger mech and TBB?! Saw a video but the guy said the piston isn't very good (it's weak) and the cylinder is hard to open. I don't know which one to get and there's only like a 2 euros difference in it.