My take on the 7.62x39 vs 30-30;
- Given similar ballistics, the shot placement is a bit more critical with the x39
- 30-30 will give you ~1200-1600 lbs @ 100yards vs ~1000-1150 for the x39
- 30-30 sectional density hovers in the .220+ area, while x39 is generally under .2
- Accuracy is a tossup, highly depends on the actual rifle.
In general, the 30-30 has a superior energy transfer and expansion properties, along with a solid edge in available energy. Even given the top hornady loadings in both calibers.
My verdict is that the x39 is a solid 100 yard gun, and the 30-30 is a solid 200 yard gun. But that is just based on numbers. I think everything matters when it comes to what game can be taken with which rifle. In the end, if you feel confident with the shot (completely different from feeling "up to a challenge"), then its the right rifle for the job.
Now I have never shot a deer myself, but I've seen some shot... with smaller calibers than both these rifles. 22-250 and 44-40. Shots were around 40-80 yards (I'm guestimating, we didn't measure). Both bucks went down pretty quick. 22-250 layed it down within seconds with a upper neck shot (IIRC), and the 44-40 left a short blood trail, maybe ran 100-200 yards (it was in heavy brush so not sure).
When you see what a gun can do and gain experience with it, or get direct mentoring, only then can you really know what your rifle is capable of. Numbers are numbers until you get in the field and deal with real world situations.