For me, space is limited and I keep my logistics simple. So, for a centerfire longarm, I tend to focus primarily upon the 12ga shotgun. I have had many military style weapons in the past, including many AK’s (yes, I do like the AK very much), HK’s, AR’s, etc, but I have multiple reasons for the choice of a shotgun. It’s one of the best weapons for defense in a marine environment where everything is often moving to some extent or another. It’s also the most practical hunting weapon possible, especially for a coastal estuary.
When you have a weapon that you will rely upon long term, you essentially adopt an entire weapon system if you want long term sustainability. So, it’s a logistical nightmare and a waste of resources to have too many weapon types. Also, some weapon types, such as military style rifles, have a very narrow band of utility for civilians facing full spectrum survival, and they can represent a significant tie-up of resources. When you travel, military style rifles can become a legal white elephant.
One exception to this could be handguns. They are small, most of them are seldom fired, and they can be stashed in all sorts of interesting places.
If you want the ballistics of a .45-70, you can get similar ballistics to that with a 12ga shotgun, a rifled barrel, and .50 cal sabot ammo. Unlike other types of shotgun slugs, you have to have some form of rifling for sabots. Modern rifled shotgun barrels and sabots have gotten pretty accurate in the last couple of decades.
Most modern pump or semi-auto shotguns can quickly change their barrels. The Mossberg 500 series is one of the easiest to do this with. You jack the pump back, unscrew the nut at the end of the magazine, and the barrel pops off. Putting on another barrel is just as easy.
For a modern pump or semi-auto you generally have three options on using a rifled barrel:
1) A regular rifled barrel with iron sights basically turns your shotgun into a .73 caliber rifle. If you want an optic, you need to use a receiver mount.
2) A ‘cantilever’ barrel with no iron sights has a dog-leg scope mount that that extends out over the top of the receiver. It keeps the scope on the barrel and zeroed when the barrel is removed from the shotgun.
3) A rifled choke tube (works surprisingly well), is simply a choke that screws into the end of a smoothbore barrel that is threaded for chokes. It is cylinder bore with rifling, and it tends to extend out past the muzzle a ways. It will give about 3″ or more of rifling to impart spin on a slug.
For hunting larger game in the USA with a shotgun, sabot ammo is some of the most popular. There are many commercial loads, but you can load your own. Shotgun shells are easy and simple to reload, and they operate at about 1/2 the chamber pressure of a .22LR. You can buy sabot jackets that hold .50 cal bullets ($15 for a pack of 50). Sabots are loaded into the shell with a gas seal wad (no cup).
The commercial sabot loads prefer to use jacketed bullets like the Hornady ballistic tip. I’ve seen handloaders use cast .50 cal pistol bullets and .50 cal conical muzzleloader bullets. Muzzle velocity is generally around 1900 fps.
A 12ga is .73 caliber, and you can shoot conventional 12ga slugs out of either a smoothbore or a rifled barrel. Out of a smoothbore, they are normally considered a 100 yard or less item, but I have seen decent hits well out beyond that. A conventional shotgun slug has a hollow base and operates much like a badminton birdie or an airgun pellet.
My favorite homemade slug, for which I have a mold, is the Lee 1oz (437.5 grain) ‘keyed’ slug. They are easy and cheap to make. The Lee slugs are kind of ‘wad slug’ that uses a standard plastic shotgun wad as a sabot jacket. They can be used with either a rifled or a smoothbore barrel.
With a common 2-3/4″ 12ga shotgun shell, all of the payloads tend to weigh about the same, whether it’s birdshot, buckshot, or a slug. Birdshot is very common in that gauge and is sold in bulk, making it inexpensive. If you have on hand a slug mold and a buckshot mold, you can ‘re-purpose’ birdshot loads into buckshot or slugs by opening the end of the shells up (or gutting the tips off), melt and cast the birsdhot into something else.You can then refold it, or if you cut it, use an overshot card (for buckshot) and a roll crimp.