Reply by Loren

18 ga nails may crumble if they hit steel in the wall. 15 ga nails don’t
crumble nearly so easy. I never use 16 ga. so I can’t comment.

The main things to look at is if your nail can penetrate the wall (some
plaster is really hard) and if your nail is long enough to get perhaps
3/4” of penetration into the wall. Crown isn’t especially heavy in
general. Sometimes you have to bend the crown a bit to conform
to the wall and a longer nail helps hold it tight.

So, 18 ga. can be fine but sometimes a beafier nailer is good to
have on hand. The 18 ga nailers also don’t do the angled
collation you find in the bigger nailers.