Usually the breech itself is not Damascus (never seen a Damascus steel breech). I have used the mutitude of penetrating oils and after weeks of soaking those,I did CAREFULLY drill out the old nipple. I started small and then worked up in size. Almost all nipples in shotguns, that I am aware of, start out as 1/4-28 thread. You can try the easyout like Mike suggested. I drilled out the nipple until I could collapsed the old nipple and withdrew it that way. Special oversize taps and nipples are made, in .005 increments up to about .285. Of course try not to go larger in size, but make sure you do have good threads to hold the nipple safely.
