Thanks. The crown is not original. I pulled it down to replace the sagging ceiling, and will reinstall the same. The plaster is brittle, so I was just wondering. I'll use my 15 ga as planned...or maybe my 16 ga angled, come to think of it.Since all trim (base, casing, crown) used to be hand nailed with 4's to 10's in size, I can't imagine where the size of gun shot would be an issue. Assuming it's painted, it gets caulked , & caulk ends up holding as much or more than nails anyway.
I see more and more guys using 18's to nail all sorts of trim these days, where 25 years ago most used #15 ga senco shots. The difference is in the caulk used. Todays caulks are also excellent adhesives!
Add wood behind it and nail your crown to it as usual... we do this all the time on plaster walls and the job is always superb... not to mention crown doesn't get messed up from nails nails that hit something and don't go in marin up the wood, etc.