Development
Why neck down a .30 cal. cartridge to 7mm? This quote from a review of the 7-08 Rem. (a .308 Win. case necked down to 7mm), provides the answer.
| “ | Anything a 7mm can do, a .30 caliber of comparable sectional density and ballistic coefficient can also do. The catch is, in order to send a .30-caliber slug over a trajectory as flat as that 7mm bullet, about 20 percent more recoil is going to be generated. . . . 7mm produces clearly superior downrange performance in terms of delivered energy and trajectory at any given recoil level . | ” |
There are two primary reasons a 7mm recoils less than an comparably effective .30 cal. cartridge: (1) to match the 7mm's ballistic coefficient requires a significantly heavier .30 cal bullet; and (2) to drive that heavier .30 cal bullet at similar velocities (for kinetic energy and wind resistance ("time-to-target")), requires more powder. This combination of heavier bullets with heavier powder charges significantly increases the recoil of the .30 caliber.
The .30-30 Winchester is typically limited to short ranges, primarily because of the relatively small case capacity and the 150 grain and 170 grain bullet weights. To compensate for this, Waters necked the cartridge down to use a 7mm bullet (.284 inches), rather than the original .308 caliber (7.62 mm) bullet. Because it was designed to function in lever action rifles, the 7-30 maintained the same low working pressure, but was able to fire bullets of lighter weight at higher velocity. A typical .30-30 factory load fires a 150 grain bullet at 2390 f/s, while the current 7-30 factory load fires a 120 grain bullet at 2700 f/s. Muzzle energy is just over 1900 ft-lbs for both loads, but the lighter weight 7mm bullet has a higher velocity and flatter trajectory.
Read more about this topic: 7-30 Waters
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