This is an idea I have been kicking around in my head for quite a while now and would like some feedback.  At work we use a tool called a trakfast to shoot material to concrete and steel, it uses a butane cylinder and a rechargeable battery both good for about 1000 shots.The butane is released into a chamber where it is ignited by a spark from the battery. The explosion drives a pin into the steel or concrete. I figure if this could be used in a firearm you could carry magazines full of bullets only,no brass or powder, therefore you would be able to carry many more rounds. Any thoughts?

I have a Paslode nail gun. I pin the safety back and play at work. never thought of making a gun from it. Not enough velocity

I think the Army toyed with artillery using a "fuel air" mixture as a propellant.

The caseless ammo system has been tried, as well as fuel/air propellant.  The propellent tank is too heavy, and you really ought to send a tracer or incendiary round thru a butane tank.... it is TRULY impressive.