[Lazamataz]

There's no real way to silence a 223 or any other sort of a rifle round (other than 22 rimfire). Much of the sound involved with rifles is the sonic crack of the bullet and that can't be silenced. Other than that, you might could muffle the escaping gas on a rifle to some extent but even that's harder than with pistols.

Actually, while surfing around on this very topic, I ran across a very interesting article on Soviet 'closed circuit' rounds.

The Russians, during the Cold War, developed something called the Closed Circuit round. It had a tiny bolt inside the shell to kick the round out, and an insignificant amount of gunpowder was all that was needed to drive that miniature bolt. Usually they had to be tapered casings. They were first produced in 7.62, but eventually were produced in all major calibers. They were classified weaponry in the Soviet Union, and were a much lighter load because they don't need as much powder, since the gas pressure never leaves the shell. Its just enough charge to send the round out at 900 fps. The only sound heard is the click of the hammer and the pling of the ejected casing, and the whining whoosh of the round downrange.

[Lazamataz]

Actually, something occurs to me about this description: There would be no pling of an ejected case, since no gas would be available to the gas system in most semiautomatics that would allow the cycling of the mechanism. It would probably not cause an autoloading of the next round in delayed-blowback mechanisms either. It might function okay in recoil-operated weapons.

However, all you'd need to do to get the next round up and ready is to cycle the action manually.