I saw at a gun show some elegant take-down repeating rifles I could not afford. Their appeal was obvious. Instead of carrying a long clumsy gun case you carry a suitcase. Traveling with the traditional gun case is always a nuisance and may draw attention from thieves, officials and busybodies.
Fortunately you don't have to be rich to own a proper traveling rifle. A single shot, break action rifle is already a take-down model. It can be shortened, within reason, to produce a very convenient travel companion. What is reasonably short? In the U.S., at least, it's no shorter than 16" in the barrel and 26" over all for the assembled rifle, unless you want to fool with NFA registration. It is sensible to add half an inch to these dimensions. Should the question arise, you want a rifle that is obviously compliant with the rules, not a marginally compliant one that may be impounded for further study.
Thinking this through, I came up with what I call the Traveler carbine, a specialized single shot made around the idea of convenience when traveling. I define the Traveler as a centerfire break action rifle which, when separated for transport, has neither component longer than the other. The lock and stock are the same length as the barrel. There are due exceptions to the rule, but that is the general idea. One reasonable exception would be if your preferred stock length would take you afoul of the NFA if you shortened the barrel to match. Another would be that you don't like the ballistics of the cartridge you're using if the barrel is shortened way back. Or maybe you just don't like the flash and racket of a stubby rifle barrel. Those are all good reasons for making the lengths unequal.
My reason for saying the lengths should be equal is that is the maximum compactness achievable, when the gun is packed for transport. You gain nothing in compactness by making the barrel shorter than the stock and receiver unit, or vice versa.
What is a Traveler good for? It is sometimes very convenient to have compact luggage that does not shout "Rifle over here," and still be able to function as a rifleman, by virtue of having a rifle. Among other things, it saves you from borrowing a rifle to go hunting, a practice with several pitfalls. This kind of rifle may also appeal to the backpacker, who of course wants the smallest and lightest gear he can get.
Weight versus caliber is something to consider in planning your Traveler. The lighter the gun, for a given power level, the more it kicks. Mr. Newton explained why. It seems to me that you should select the lightest cartridge you feel is suitable for your purposes, for the trimmed down single shot rifle will end up quite light. Not everyone agrees. I once saw an H&R .45-70 pared down to minimal dimensions. It must have been great fun to invite others to shoot it.
Rifles suitable for conversion into Travelers are available in all price ranges. The upper price ranges need not concern us here. For the price of a best grade, hand made single, you can buy a very good take-down repeater, and the idea of sawing off a vintage Purdey or Holland does not appeal to me. H&R, Baikal and a number of others are quite suitable as starting points for building a Traveler. The Thompson Center actions certainly merit consideration but are more expensive.
When armed with a single shot rifle, the procedure for reloading in a hurry is the same as for the single shot shotgun, but easier, because rifle cartridges are not blunt like shotshells. I consider an automatic ejector more desirable on a Traveler than a simple extractor, because it speeds the process of reloading.
It has long been said that rimmed cartridges are preferable for single shot rifles, because the ejector or extractor has more to grasp. It is alleged that break action rifles made for rimless cartridges will always give trouble down the road. I am not sure whether this is a real concern or merely a theoretical one--one of those cracker barrel 'facts' that does not bear out in practice. A quick look at the gun catalogs shows more single shots for rimless than rimmed cartridges, but that may be due only to the great popularity of rimless cartridges in the era of the repeating rifle.
I would think a big part of your choice of caliber would be what cartridges are most available in the places you will be taking your Traveler. Because a rifle is no good without ammunition, the question of cartridge availability is far more important than whether there is a rim or not. That, at least, is how the matter appears to me.
There are so few parts inside a single shot rifle that a comprehensive kit of repair parts is no trouble to carry along. Depending on how far afield you travel with your Traveler, spare parts might be useful to have, or at least comforting. It may be possible to fit the parts into a hollow inside the buttstock, where you will not lose them--or if you do lose them, you no longer need them, because you have lost the rifle too.
What the conversion job entails: If you wish, you may shorten or lighten the stock, or fit a different one, and cut out a recess, or recesses, for the spare parts. Trim the barrel to length, cut a new crown in the end. If there was a front sight replace it farther aft, or forget about it, if you wish, and discard the rear iron sight too. Applying a rust resistant finish might be prudent, in light of the go-anywhere nature of the gun. These jobs are not beyond some home hobbyists but most people will prefer to have a gunsmith do them.
The most sensible way to proceed is first of all to decide what length you want the stock to be, then derive the barrel length from that, measuring stock and receiver as a unit. I recommend a stock that fits you. Some claim a stock shorter than normal for your measurements is quicker in field shooting, but don't say why that would be. It does not seem to bear out in practice, for me at least, but I haven't made a detailed study of the matter.
To recap, a Traveler is a short single-shot rifle, optimized for convenient transport. Its barrel is the same length as the lock and stock assembly--with due exceptions to this rule, as noted above. It fires a readily available cartridge. An automatic ejector is preferable to an extractor mechanism. A rust resistant finish is not a bad idea. Neither is a spare parts kit, which may be stored inside the stock if there is room.
So far this project is just in the conceptual stage. If and when it goes farther, I'll post pictures. Anyone else see this sort of thing as a very useful rifle to have?