One of the greatest things about the AR platform is just how incredibly versatile it can be. Since the only part of the rifle that is legally considered a gun is the lower receiver (which contains the fire control group) and since it is so simple to swap out upper receivers (which contains the bolt and barrel), the possibilities are nearly limitless. You can convert your AR lower into a black powder muzzle loader, into a crossbow and into a bolt action .50 Cal.
Pistol versus Rifle
You can purchase an AR lower as a pistol or as a rifle but take heed. If you purchase your AR lower as a rifle, you cannot use an upper receiver with a total barrel length of less than 16 inches even if you remove the stock and install a pistol buffer tube. If you do you must first register it as a short barreled rifle with the Feds, and this is not a quick, simple or cheap process. There is just no other way of converting an AR rifle into and AR pistol.
AR Pistol Lower with Arm Brace
If however you purchase your lower receiver as a pistol, you can convert it into a rifle and back as much as you please, just as long as you never have a combination of a stock and a short barrel. If you purchase a completed lower receiver, it will be identified in the paperwork as a pistol or rifle. If you purchase a stripped lower receiver, it will be neither until you complete your build as one or the other.
If you are looking for a good quality and inexpensive stripped lower, try the Anderson Manufacturing forged aluminum lower receiver. This is a standard Mil-Spec lower made from 7075 T6 aluminum forging and hard coat anodized black with a smooth even finish. I built an AR pistol with this lower receiver and was extremely pleased.
If you are looking for something a bit different there is also the OMNI Hybrid polymer AR lower receiver from American Tactical Imports (ATI). It weight about half an ounce less than a standard lower but it also includes an extended trigger guard built in. This is a very nice lower receiver that features a billet look to it and includes metal inserts at key spots for durability and strength. Polymer is corrosion proof and scratch resistant and it looks great. It is also very economical.
A good example of an almost complete lower is the Del-Ton LR101. This is a standard Mil-Spec aluminum lower receiver that comes with the grip, safety, fire control mechanism, magazine release and bolt catch already installed. It does not include any stock or buffer tube so it could conceivably be assembled into a rifle or a pistol. It is much easier and faster to purchase a completed lower receiver and buffer tube installation is a fairly simple and straightforward process.
It is possible to fire nearly 30 different calibers from a standard 5.56m NATO AR lower receiver by simply changing the upper receiver and, depending on the caliber, changing the magazine or using a mag well adapter. I won’t list all the choices, but they can range from the usual 5.56/.223 to .300 Blackout, and 7.62×39. Other options include pistol ammunition like 9mm, .40 Cal and .45 ACP as well as rimfire ammo like .22 LR.
Most lower receivers will come with a caliber designation stamped on the side. Usually this is 5.56 or .223 or both. Some will simply be marked “Multi” indicating that the manufacturer wants to remind the customer of the versatility. There is no difference for practical purposes since you can put any caliber upper you want on your lower receiver regardless of how it is marked. Some prefer the “Multi” designation because it helps remind then that they are not using a standard 5.56/.223 caliber and helps avoid mix ups with ammo.
Complete AR Rifle Lower
When you are swapping different caliber upper, especially if the new caliber uses standard AR magazines, you must be extra careful to use the proper ammunition for the new upper. Otherwise bad things will happen. It may be a good idea to use dedicated magazines which can be identified with a piece of colored tape or paint for the specific caliber use. Also the caliber designation will appear on the barrel (although not always in the easiest place to see) so you can double check.
For maximum versatility stick with Mil-Spec sized lower receivers as the plethora of upper receivers out there are designed to fit these most often. This is almost never an issue except with some models of Colt rifles which use different sized pivot pins. It was a bad idea and even Colt doesn’t make these anymore.
For shooting .22 LR rimfire ammunition you have the options of just swapping out the bolt carrier group and magazine. WMD Guns makes a good conversion kit that offers drop in versatility and a dedicated magazine. There was a military issue drop in magazine kit that allowed you to use .22 LR in your standard magazine but I don’t see those around commercially. You can also purchase a dedicated .22 LR upper receiver. These will produce better accuracy as the barrel has the correct twist rate and dimensions for .22 LR. Be aware that the bolt catch may not function properly on these .22 upper receivers or conversion kits.
There are about half a dozen calibers (although this includes some oddballs) that will fit in standard AR magazines and (with the correct upper) will run fine. The most popular of these is the 300 Blackout. For the rest you will need to use a different magazine and upper.
Having one rifle that can shoot different calibers is a great benefit to everyone. Your same lower receiver can be used for long range target shooting or varmint hunting, it can be used for backyard plinking, home defense or large game hunting with a simple swap of the upper receiver and magazine.