By the sound of it I'd guess the rear piston has enough damage to reduce compression (i.e. at least one of the rings isn't free anymore) but not enough to stop it running.

Get the barrel off and have a look, get the front off too if you can (it's the easiest one anyway) but at just 3000k's old I wouldn't bother replacing anything that's not damaged. If the rings are free and there's no sign of seizing.

If the barrels have been plated you need to measure the clearance, the letter on the barrel now means nothing.
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