A new horse is a good investment when the old one can no longer run. But rifle cartridges don't go lame. They get no less useful with age, which is why the .30-30 and .30-06 are still all you need to know about .30-bores.

But centenarian cartridges get musty in our minds. So we concoct newer, more potent rounds until we reach the limits of recoil tolerance and action length. Then we reconfigure cases to fill the diminishing gaps in the lineup. As recently as the World War II, big game cartridges stateside could be counted on your fingers. From the .25-35 Winchester to the .375 Holland & Holland, they included traditional deer rounds (the .30-30 Winchester and .35 Remington), flat-shooting .25s for the prairie (the .250 Savage and .257 Roberts), all-around .30s (the .300 Savage and .30-06) and long-legged specialists for the Mountain West (the .270 Winchester and .300 H&H).

The field has since become quite crowded, capped by the eruption of short magnums. Hunters hardly need another cartridge, let alone a new short magnum, but Hornady has just announced two: the .300 and .338 Ruger Compact Magnums. At first blush, they seem redundant--thinly veiled copies of the short .300s from Winchester and Remington, and the .325 WSM. But dig deeper and you'll find meaningful differences.

"We set out to give hunters magnum performance from short rifle actions," explains Hornady's Mitch Mittelstaedt, who headed the project. Nothing new there. "Our second goal was to get that muscle from 20-inch barrels." Now that's new.

Even short, efficient magnums typically warrant 24-inch barrels. Velocities drop off noticeably as you chop tubes below 22 inches because the substantial charges of slow powders that push heavy bullets fast need barrel time to burn. Dave Emary, the ballistics wizard who developed Hornady's LeverEvolution rounds, says: "We've used proprietary means to alter propellants in the Ruger Compact Magnums, tightening their pressure curves. The .300 RCM performs about like the .300 WSM in standard 24-inch barrels, but it beats the WSM in 20-inch tubes."

There are other dimensional differences, too. Inspired by the 2.580-inch .375 Ruger case, the .300 and .338 RCMs share its .532 head and base diameter. In contrast, the WSM series has a .535 head (which still works on the .532 bolt face standard for belted magnums). But the WSM case is rebated; body diameter starts at .555.

The .300 RCM has a .532 head and base diameter, which will allow an extra round in the magazine over short mags such as the WSM, and its slim profile should make for smooth feeding.

"WSM magazines feed RCM cartridges just fine," says Mittelstaedt. "With a reconfigured follower, though, the smaller RCM body diameter lets us stuff four rounds in magazines designed for three WSMs."

The relatively slender profile also helps RCMs feed smoothly. The shoulder angle is 30 degrees.

As for length, the new Hornady .300 and .338 rounds measure 2.100 and 2.015, respectively, base to mouth. They're both loaded to an overall length of 2.840, same as the WSM line.

Says Mittelstaedt: "We kept the .338 hull shorter (.308 Winchester length) to accommodate the current crop of cannelured bullets, most of which were designed for the .338 Winchester Magnum." He points out that necks on both RCM rounds are "about .300 inch long. Base-to-shoulder measure on the .338 is thus a tad shorter than on the .300. Case capacities run 68 and 72 grains of water to the mouth. That is, the .338 RCM holds about as much powder as a .338-06."