I looked that up today (Gyro Jet). Very intersting. Seems like with a few refinments, it would work. A cartridge that leaves the barrel at a relatively slow pace and then picks up steam as it goes along is very intriquing.
I also found some stuff on liquid fueled rockets. You're right, there are some serious problems. Heat for one. 2,500 degrees and up. What would that do to a gun barrel? Liquid rocket fuel is also unstable compared to solid fuels, plus it's very corrosive. Apparently at some point the availability of Hydrogen Peroxide dried up, and people are no longer building rocket engines in their garages as they once did not so long ago.
Back to the drawing board. Propellants or methods of propelling a bullet are probably as good place to start as any. Some mentioned in earlier posts about using a 72 grain bullet for the .223. Actually Black Hills Ammunition came up with a round for our Special Forces in Afghanistan which weighs 77 grains. Obviously they needed a propellant that would stabilize the heavier bullet wihout creating exceedingly high pressures. I bought a box of their Cowboy loads not knowing what it was, so that's how I came across them.
Thanks for the reply. This really is an intersting subject.
P.S. Those cowboy loads were kind of cute. Very light recoil and they even throw off a little smoke for that cowboy effect. :-)
The problem is that that means that the low velocity projectile won't cause much damage at close range, and can easily be blown off course. An all-burned-on-launch type would have prodigious recoil and dangerous back/muzzleblast. Rocket exhaust tends to be spectacular, hot, and stupendously toxic.
The next leap forward in infantry weapons development will be the man-portable railgun, IMHO. The only problem is the power supply (which could be contained in a magazine).