This is a great idea some 3 inch slugs in a 12 gauge would certainly train you for the big boys for a relatively low costWhile some may laugh at the suggestion, do you own a 12 Ga shotgun with sights on it, i.e. a "slug" gun? If so shoot it. Shells are relatively cheap. Once you start cloverleafing groups with the shotgun, take mdwest up on his offer.
I just jumped right in with a .375 and .416. It takes some getting used to but the more you shoot the better you are. Shoot a .416 then the .375 is easy!In preparation for my first trip to Africa (no date set, yet). Currently, I use a 7mm Rem Mag for my hunting rifle, and plan to use that for plains game when I go. However, I also plan to hunt Cape Buffalo, so I'll need something larger. And, of course, part of the dream is that I will come into more money unexpectedly and buy a nice double rifle. I have looked at the dramatic increase in recoil over my current rifle on recoil tables, and want to learn how to shoot big bore rifles with big bore recoil without a big bore flinch.
The methods I can think of are:
a) buy the gun and start shooting! Oh, and try not to develop a flinch
b) buy the gun and use light loads to help the transition
c) find a gun range that rents out big bore rifles, from this easy list maintained on africehunting.com--start with a 375 H&H, rinse and repeat until you get to the caliber you want, and buy that rifle (heck, make it a double if you've found the suitcase full of nonconsecutive bills)
If you had to do it all again, how would you learn?