Choice is a good thing, for the most part. It is good to have multiple options to choose from, instead of a one-size-fits-all situation where your only choice is to take it or leave it.
The AR15 rifle platform is one of the most versatile firearms ever created. Designed from the ground up (as the AR10) to take advantage of space-age manufacturing processes to reduce weight (and to simplify mass production by not requiring the hard-to-learn skills and extensive hand-fitting needed to make traditional guns at that time), the AR15 menu today would bewilder and amaze someone who was issued an M16 back in the late 50’s/early 6o’s.
Standard, short and long barrels with different rifling twists; some stainless, some free-floating, some fluted, or even all 3 options. Long, short and collapsible stocks, some with adjustable cheekweld. Flat-tops & removable carry handles. Match triggers. Tactical charging handles. Single-point sling adapters. Piston options. Rails everywhere. Scopes & flip-up/back-up iron sights, some mounted off the 12 o’clock axis. C-mags. The concept of multiple uppers for a single gun. Even left-handed ARs!
And the last and perhaps most daunting option: caliber. An AR15 buyer has options for the chambering of their gun. A lot of options. Enough calibers to make T/C Contender fans start to feel a little threatened.
How many calibers are we talking about?
Now, not every one of those chamberings is common. Most are not, with a great number of them being designed specifically to modify the AR15 platform for a single special purpose. Some I only found references to, but no proof of an actual gun in that chambering (.224 BoZ for instance). And some I heard about but couldn’t find any information about (7.62x24mm).
Some of those other chamberings work with the standard bolt & magazines. Others require a special magazine and/or a different bolt to function. 9mm AR15s, for example, come in 2 flavors: ones that use a special magazine that is adapted to fit the stock magwell (and use the standard mag release, like Olympic Arms) and others that use an adapter block for SMG magazines inside the magwell and require a different mag release to be used (because the adapter is held in place by the standard mag release, as on Colt/Rock River Arms guns).
Some of these options are clearly aimed at wringing the most range & accuracy out of a .223 platform (6.5PPC and .204 Ruger for example). Others are aimed at improving the lack of stopping power of the 5.56mm cartridge (the 6.8SPC and the plethora of 6mm/6.5mm/7mm/7.62mm proprietary rounds, most recently the 7.62×40 WT from Wilson). Others are designed to make a suppressed AR15 as quiet as possible while maximizing its lethality (the long, heavy subsonic bullets of the .300AAC Blackout).
And some enterprising engineers, God bless them, are trying to put the largest possible bullet through the magwell of an AR15. The big bore options can be frustrating for users because they often hold very few rounds per magazine. But my what a big hole those bullets will make! The Coast Guard supposedly evaluated the big bore AR15 cartridges because they found the .50 BMG sniper rifles they were using to disable boat engines were too lethal.
Is a non-5.56mm AR15 the right gun for you? That depends on many factors. What are you intending to use it for, and is the 5.56mm absolutely not appropriate for that task? What compromises must you make to use an AR15 in that chambering? For the proprietary cartridges, how available is the ammo and parts, and for how long? No one wants to be the gunnie equivalent of the guy who bought a HD-DVD player right before BluRay won the format wars (for you older folks: “…Betamax purchaser when VHS won the format wars”). Reloaders might find this less of a drawback.
Personally, I am a KISS Principle advocate, and the cheap & available nature of 5.56mm guns & ammo make that the best long-term choice for me. The 6.8SPC looks promising, with Ruger and Remington both offering factory guns in that chambering, but I am not even close to pulling the trigger on that round yet.
If anyone has an off-caliber AR15, we would love to hear about your experiences with the gun/cartridge combination. Why that caliber? How accurate is it? How reliable is it? What magazines do you need to use?