SpudFiles

does anyone know which is mre powerful, a metal cannon with pure oxygen or a metal hybrid?

well, if you want to live to shoot it again a metal hybrid, but still you'd want to get high pressure metal, hardware store stuff is sometimes not the greatest.

and i dont think just oxygen would ignite, wouldnt you need a fuel to start it, rather than just a spark?

Pure oxygen needs to be mixed with something to ignite. I say go hybrid since O2 hasn't been played with much and hybrids are plenty strong.

i knew i needed fuel, what if i gave just enoug oxygen so it would fire safely and be somewhat fully automatic, without open a endcap

you'd still need to open the endcap because the fumes will still be in the chamber, and O2 isnt played with much so i wouldnt try to find how much is "safe" to fire

The main issue with using a O2 enriched atmosphere is that without the nitrogen to slow the flame front, it reaches super-sonic speeds much easier. That is, your mixture detonates.
I would not suggest being near a launcher, made of any material what so ever, if this happens.

all i want is "artifical air" without using other useless gases. and wont the fumes be somewhat vented when the projectileis shot?

what useless gasses? without the nitrogen in the air acting as a buffer for the fuel (to slow it down) our launchers would most likely detonate without adding everything else