Caliber conversion kits sound great. A marketing department sees them as a great selling point and a lot of people think they have to get a gun with a caliber conversion kit or it's not dynamic enough of a weapon or platform. The truth is somewhat different.
Getting a kit to convert your AR from 5.56 to 6.8 SPC allows you to use multiple types of ammo and can be as simple as swapping uppers. In other guns, such as the Tavor or Glocks, these conversions are even easier - a quick and easy barrel swap and a spring change and you're running 9mm instead of 5.56 or 9mm instead of .40 S&W. But, is it worth the extra money to be able to convert the caliber of the gun?
Well, first off, the money you spend on a conversion kit normally isn't cheap. And, that kit isn't a gun that you can hand to a friend who needs a rifle to defend himself or to shoot at the range with you. If you are building a specialized upper for 6.8 SPC for example, why not just buy an extra lower? You can get complete AR lowers for as little as $140 on sale from Palmetto State Armory. As for some other weapons, it's part of the gun straight from the factory. But, even these are still faced with another challenge - stockpiling ammo and magazines for the conversion caliber. Maybe you're like me and you shoot 5.56 and 9mm and stockpile both, so a 9mm conversion kit may have some practical use. But, how many magazines for the conversion kit do you have? In the case of more exotic calibers, such as 6.8 SPC or 300 AAC Blackout, are you really going to stockpile those and 5.56? Again, price matters - stockpiling those exotic calibers isn't cheap. Plus, swapping to a 9mm round hurts the range/lethality of the weapon.
Basically, the only time you'd need a conversion kit is if you ever found yourself in a Without Rule of Law (WROL) situation where you were scavenging for ammo and you need to be versatile so you can use a greater variety of the ammo you find. But, if you're going to do that, having another gun chambered in that caliber instead of a conversion kit makes a lot of sense. (You may need to arm your family in such a situation and you may be able to use an additional rifle instead of just a conversion kit.) If you already have handguns that use the 9mm you've stockpiled, why do you need a 9mm conversion kit for your rifle? One could argue that having more options can't hurt. The thing is that we have limited resources - what I choose to buy prevents me from buying other things because I have limited money. Instead of spending hundreds on conversion kits, what if you just bought more ammo for the caliber/s you do shoot? Nevermind the possibility that you can't find that specialty ammo in a WROL situation, so both the specially chambered rifle and that conversion kit becomes worthless. And, if you find some place that has the ammo around, they probably have a gun that shoots it, so you may not need to have the extra gun or conversion kit at all.
The conclusion I've come to is that the most important thing to do is to get rifles and ammo in one particular caliber and stockpile it. If you have extra rifles chambered in a different caliber for special purpose, like 300 AAC Blackout for suppressed fire, that's fine - just make sure you keep your primary round stocked. And, seriously consider the fact that any military supported calibers are going to be the easiest to stockpile and to find in a WROL situation. I believe, in any case, it's better to buy more of your chosen ammo than conversion kits.