...if you only have FMJ, I wouldn't lose sleep.
I would! Hardball is a notoriously poor stopper, particularly if it's smaller than a .45. It zips through flesh with very little disruption. For some reason, this is lost on a great many shooters who will argue and say that the military used it for years and that it works. Hogwash! Elmer Keith wrote of it in 1936 and it still holds true today. A good semi-wadcutter beats hardball every day of the week.