Bob Brister killed a cape buff w/ brenneke slugs and it was deliberate, not an emergency situation. however, i'm sure he was able to pick his shots. in your situation, you won't have that luxury. but under the circumstances, short of a back-up carrying a heavy rifle (and in all likelihood you'd be between him and the target), it's the best you can do and would certainly be better than nothing.
i know askins the younger isn't popular but nonetheless, in his shotgun book, there's a chapter in which he was testing the latest auto's (latest as of late 1950's) and he ran 3" magnums thru them all just to see what would happen. not only did none of them blow up, none were even damaged and a few almost worked flawlessly. as noted, you won't be doing the gun any favors, but the gun may be a small price to pay for getting out alive.
roger
