You can soak the nipple end of the barrel in a metal container with some transmission oil. The transmission oil can then be heated on a hot plate and allowed to cool to accelerate the process, but it still may take days to loosen it up.

Another idea is to put the end of the barrel with the stuck nipple in boiling water to heat it up. The drawback is that water will get in the barrel that will need to be dried out if you still can't get the nipple off.
A hot barrel will dry quicker than a cold one. Swab it to dry it out and then oil it to prevent rust or go back to soaking it in penetrating oil.