I don't put much stock in shooting gel blocks as I've never been attacked by one. If you've ever actually shot bullets into living flesh (deer for example) and tracked their path and destruction, you'll know that they rarely do what they do in gel. But in the interest of your post I found a couple of videos in which they shot 55 gr. FMJ's into gel and actually recovered the bullets. Both did tumble but neither fragmented. And it bears mentioning that a .30 caliber bullet starts out about 40% larger in diameter than a .22 caliber bullet, and in the case of the 55 gr. FMJ, the .30 caliber bullet in question is twice as heavy. All that said, why on earth would anyone choose FMJ in either caliber?
35W
Fragmenting of 5.56 FMJ is well documented not only in gelatin but on the battlefield and elsewhere (in a highly-publicized recent incident, see the unfortunate Gaige Grosskreutz, I doubt .30 Carbine FMJ would have anywhere near similar effect). Even if it fails to fragment it still is likely to tumble and cause effective wounds. Meanwhile small wounds from .30 carbine FMJ is also well documented not only in gelatin but also on the battlefield. It acts exactly like you would expect for a FMJRN. Larger diameter and weight is only relevant if both had a similar bullet design, it is .308" but a 55 gr 5.56 turned sideways is about 3/4" and moving 1000 fps faster. However soft points the carbine is very respectable. Jim Cirillo said it was an extremely effective stopper. I don't know why someone would choose FMJ in either for defense, but apparently they still do.