Remington's new .30 AR cartridge is the same length as the .223 and functions through the company's standard R15, its version of the AR-15.

When Remington got into the AR-15 business in 2007, company officials made it clear that the new R-15 rifle was intended for hunting. At the time, that struck me as rather odd since the only chamberings offered were .204 Ruger and .223 Remington, both excellent varmint cartridges but a bit short on power for use on deer and such.

And since Remington had previously been involved in the development of an AR-15-compatible cartridge of larger caliber called the 6.8mm SPC, which is better suited for use on deer, I wondered why that chambering option was not offered. Little did I know at the time that an even more effective cartridge called the .30 Remington AR was on the drawing board.

When designing the new cartridge, Remington's engineers started with the .450 Bushmaster case. For the benefit of those who are not familiar with the .450, it was a joint-venture development between Hornady and Bushmaster.

A pointed .30 caliber bullet of reasonable weight is longer than a blunt-nosed .45 caliber bullet, so to keep overall cartridge length compatible with the standard AR-15 magazine, the case was shortened to 1.530 inches from the original case length of 1.700 inches.

At its base, the .30 AR case has a diameter of .500 inch. From there it tapers forward to .488 inch at the juncture of the body and shoulder. Neck diameter is .342 inch, and the case has a shoulder angle of 25 degrees. With a length of .305 inch, the neck is capable of exerting plenty of tension on the bullet, a good thing to have in a cartridge designed to survive the rather violent trip it must take from the magazine to the chamber of an autoloading rifle.

An AR-15 rifle in .450 Bushmaster utilizes a standard .223 bolt, which is modified by increasing its bolt face diameter to a nominal .473 inch, which is the same as the .308 Winchester.

When this is done, the counterbore wall or shroud of the bolt becomes rather thin. This is considered a safe modification for that cartridge because it operates at a maximum chamber pressure level of 38,000 psi, which is the same as the .30-30 Winchester. But since the .30 AR is loaded to 55,000 psi (a bit higher than the .308 Winchester), Remington opted for additional case rim support. Engineers accomplished this by modifying the larger-diameter AR-10 bolt to fit the AR-15 upper.

The face of said bolt is commonly sized for the .473-inch rim diameter of the .308 Winchester, but Remington went one step further by opening it up a bit and increasing the rim diameter of the .30 AR case to .492 inch.

This, by the way, is a rather uncommon rim diameter. The only other cartridge I could dig up that shares it is an 1800s number called the .35-40 Maynard. At any rate, by the time the job was done, the only thing the .30 AR case had in common with the .450 Bushmaster case was a base diameter of .500 inch.

In case you are wondering why Remington went with a case rim diameter larger than that of the .450 Bushmaster, it is to prevent the use of a bolt built for that cartridge in an R-15 upper with a .30 AR barrel.

What we ended up with is a short, fat case of rebated rim design with a gross water capacity of 44 grains, about 10 grains less than the .308 Winchester case and approximately the same as the .30-30 Winchester case. But since the .30 AR is loaded to higher chamber pressures than the .30-30, it exceeds the maximum velocity of that cartridge.