how about somthin' like www.spudfiles.com/forums/9mm-smg-project-and-other-hpa-bbmg-experimentation-t19367.html
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Yes and Yes. Such weapons are also just as illegal to possess as a standard submachinegun and even less safe to use. However, If you are asking for entertainment value or as an idea for a roleplaying game, I suggest googling
'Metal storm' or 'Dread gun'. The videos alone will leave you in awe and I can feel secure in the knowledge that you can't reproduce them in your garage.
you could buy:
A paintball gun and crank it up to like 400 fps
build a sniper rifle
I have built my own sniper rifle and since im not old enough to own a gun it works perfectly