Tat,
I am a fan of the .357 mag, but you will need to do some additional research. The .357 mag out of a rifle barrel is not anywhere close to a .30-30. I think that with an ordinary 158 grain bullet, in a normal.357 factory load, you are only getting about 1,200 foot pounds of power at the muzzle, and that drops very quickly to about 750 foot pounds at 100 yards. If I remember correctly, the .30-30 has about 1,600 foot pounds at the muzzle, and about 1,400 foot pounds at 100 yards. So, the .357 mag out of a rifle has only a little more than half of the power of the .30-30.
You have to remember that the .357 magnum is a pistol round, whereas the .30-30 was the first modern high-powered rifle round.
Now, all of that having been said, I think that either Buffalo Bore, or Garrett, offer super loaded .357 magnum rounds, for use only in modern Marlin rifles or modern single shot rifles, that will give you power that "approaches" the .30-30. You can go to their websites, and you can see all of the specifications for these. These rounds are very very expensive, and if I remember correctly, they get you to about 90% of the power of the .30-30, at least for the first 100 yards or so. After that, the .357 bullets slows down alot faster that the .30 caliber bullet, so they shed energy much faster.
The rounds sold by Buffalo Bore and Garrett are proprietary (they aren't going to tell you how to make them), and they employ the use of compressed powder charges (so I doubt you could make them even if you knew how.)
Core-bon makes (or use to make) .357 mag "hunting loads", that have a 180 grain bullet, and a somewhat more powerful powder charge than a regular factory load. These are nowhere near as powerful as the loads by the other companies, but they are cheaper and they will work OK on deer but only out to 100 yards. And you have to avoid taking any kind of shoulder shot (too much bone to penetrate), and shoot behind the shoulder for a lung shot or heart shot.
The .357 Mag out of a rifle is a nice gun, out to 100 yards, but you have to realize that you don't have a .30-30, which is a true 200 yard deer rifle and much more powerful.
Best Regards,
Mannyrock