All these devices are are "bump-fire assist" devices, and learning to bump-fire without them is much easier, cheaper, and works on most semi-auto guns without an annoying device bolted on.

-Troy

speaking of which,

i have seen videos of bump firing.

can't really practice near my town, slow fire only.

but specifically how is it done??

forward pressure on VPG???
how is pistol grip held.
buttstock tight or loose in shoulder????

Bumpfiring is done like this:

Hold the rifle down by your hip, using your off-hand to support the rifle by the handguards.  Place your primary hand's index finger straight across through the trigger guard, and hold it straight.  Now, keeping your primary hand absolutely still, and NOT moving that finger or pulling the trigger in any way, use your off-hand to pull the rifle forward until the trigger is depressed by your stationary finger.  If you maintain forward pressure on the rifle, what will happen is that the rifle will fire, and the recoil will move the gun back just far enough for the trigger to reset before your forward pull on the rifle will cause the rifle to fire again.

With a bit of practice, you can rip off full mags of ammo, and after you get some practice, you can move the rifle up to your shoulder and learn to duplicate the practice there.  It's much harder to learn the mechanics if you start that way, though.

Note that this is no substitute for full-auto, as you can't really control the rifle well.  It is mainly for ripping off ammo and impressing your friends, and/or making an ass out of yourself at the range, depending on your audience.  

-Troy