The upper button let gas inside the gun, the lower button let detonate it. If you let too much gas into the gun, it's power become lower. You can see the power at the right top of the screen. (max ist 2400).
There are some things, which you can shoot with it.
Sometimes there are bugs
Bugs:
Things getting stuck inside the gun (FIXED)
The Mechanic breaks if the things you put into the gun are too heavy. (That may be a bug, but in a real homade gas gun, it will be like that, too.)
If you pull the triggers too much, they will break (so just keep caution)
Update 1:
Most of the Getting - Stuck - Bugs fixed
Update 2:
Added some targets to destroy
