The 7.62x39 vs. 7.62x51 debate is uncommon. Shooters familiar enough with the rounds to compare them already know they are extremely dissimilar (despite sharing the same caliber projectile).
It’s not an apples-to-apples comparison, but “7.62x39 vs. 7.62x51” is comparable to “223 Rem vs. 308 Win.” In both cases, the former is an intermediate battle cartridge, whereas the latter is intended for sniping over longer distances.
We’re not comparing these cartridges because they’re similarly powerful, or share similar applications. We’re comparing them because they’re so similarly named. People confuse the two, or suppose they must be more similar than they actually are due to 39’s nearness to 51.
Here’s why the 7.62x51 and 7.62x39 are really different from one another.
The Differences Between 7.62x39 and 7.62x51
In this article, we’ll be discussing the differences between the 7.62x39mm and 7.62x51mm NATO in terms of real-world performance: recoil, trajectory, stopping power, and more. We’ll conclude by declaring a “winner.”
Before we dig into performance, let’s review the cartridges’ physical differences. Just placing the two side by side illustrates a whole lot.
Cartridge Specs
The 7.62x39 and 7.62x51 are both rimless bottleneck cartridges. The 7.62x39 is a unique design originating in the Soviet Union in 1943. The 7.62x51’s parent case is the T-65 experimental cartridge, which came from the .300 Savage and .30-06 Springfield.
The 7.62x39 is odd for having a range of accepted bullet diameters. The Sporting Arms and Ammunition Manufacturers Institute (SAAMI) sets its bullet diameter ranging from 0.309-0.311”, whereas the Permanent International Commission for the Proof of Small Arms (C.I.P.) sets it at 0.312”. The 7.62x51 keeps it simple with a true bullet diameter of 0.308”.
The 7.62x51 has a larger neck diameter (0.345”) and base diameter (0.470”) than the 7.62x39 (0.339” neck diameter and 0.447” base diameter).
The 7.62x39’s case length is 1.524”, whereas the 7.62x51’s is almost half an inch longer, at 2.015”. This difference accounts for the two rounds’ different names, as “39mm” and “51mm” correspond to their case lengths (albeit inexactly). Its longer case extends the 7.62x51’s overall length to 2.800”, compared to 2.205” for the 7.62x39. This results in a case capacity of 52.0 grains H2O for the 7.62x51 and 35.6 grains H2O for the 7.62x39.
These rounds have very different maximum pressures. The 7.62x39 is 45,010 psi (according to SAAMI); the 7.62x51, 60,191 psi (according to NATO).
Now that we’ve covered each cartridge’s basic specs, let’s see how they affect real-world performance.
Recoil: 7.62x39mm vs. 7.62x51mm
I always compare recoil because it’s one of the first things a shooter asks about. We all want to know how much it kicks – not so much because of physical pain (though this can be an issue), but more so for initial accuracy and follow-up shots. A cartridge with more felt recoil kicks the muzzle off the target more with each shot, thus slowing follow-up shots and possibly causing the shooter to flinch.
The less recoil you feel, the more accurately you can fire. Less is always better (unless you’re a macho man who has to prove how tough you are to the world). “Felt” recoil is subjective, as it varies from shooter to shooter. That’s why we base our comparison on free recoil energy. It’s an objective measure of recoil determined by four variables: projectile weight, propellant weight, muzzle velocity, and firearm weight.
Knowing this, we can predict that the 7.62x51’s heavier and higher-velocity projectile will give it substantially more recoil – but how much? Let’s find out.
When firing an 8-pound rifle, the 7.62x51 generates approximately 22 ft-lbs of recoil – roughly 2.5X as much recoil as the 7.62x39 (8.7 ft-lbs).
The 7.62x39 wins this section. It has less recoil, it’s easier to control, and it’s altogether more comfortable to fire.
Trajectory
A bullet’s trajectory is the path it takes upon exiting the muzzle. We measure trajectory in terms of bullet drop and rise. When graphed out, drop and rise give a trajectory its profile.
A flat trajectory is preferred for firing across longer distances, as it requires the shooter to make fewer adjustments to remain accurate.
A cartridge’s trajectory varies from load to load, so the following scenarios are generalizations.
A 7.62x39mm 123 grain bullet with a muzzle velocity of 2,075 fps sighted in at 100 yards will drop -23.8” at 300 yards.
A 7.62x51mm 149 grain bullet with a muzzle velocity of 2,790 fps sighted in at 100 yards will drop -13.4” at 300 yards.
The 7.62x51 has a much flatter trajectory. It’s only evident over longer distances, but it’s still prominent enough to easily win this category.
Continue reading 7.62x39 vs. 7.62x51: What’s the Difference Between These Same Caliber Cartridges? on Ammo.com