How far will you actually be shooting at deer? As one poster here puts as his tag line "A man needs to know his limitations" or something like that.

I would try to make sure at the range you are darn close to how well you do with a centerfire rifle. If you can't group well with either practice, practice, practice. After that it is all up to the hunter because most bp guns are as accurate as centerfire rifles. Yes they have less range which brings up my first question but all other principals apply.