Quoted:
Pure fantasy...Oh it is definetly doable but no civilian will ever see it.
You don't need caseless ammo for a BB gun
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Oddly enough, there was once a U.S. made commercial caseless gun on the market. Back around the 1960's Daisy (believe it or not) made some type of gun to use caseless ammo. I'm not sure, but the propellant may have been ignited Diesel-style by an airgun type mechanism. It didn't go over too well.
101_proof, I don't know the internal workings of any caseless firearms, but the goals are to greatly reduce ammo weight and to make possible super high cyclic rates. As akrazy said, the high rates of fire are made possible by eliminating the extraction and ejection steps from the operating cycle; as they say, economy of motion is the essence of speed. I do know some designs took advantage of the high cyclic rates to deliver mechanically limited bursts with less dispersion of the rounds than usual. This was done by allowing the whole barrel and mechanism of the weapon to recoil back within an outer shell, not hitting any type of stop until after the last shot of the burst was out of the barrel. The shooter only felt a single recoil impulse.
The snags encountered have to do with cookoff problems when the bare propellant lands in the hot chamber, the difficulty in making a breech tight enough to seal without a case but still able to operate when dirty, and the need to have some means of extracting inevitable dud (or unfired) cartridges in a design that is specifically intended to eliminate extraction.