The .300 Winchester Magnum: A Reliable Performer

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It’s interesting watching how the tastes of long range shooters and hunters evolve as technology changes and new cartridges are introduced. Advancements in bullet design, case construction, and propellants all continue to deliver improved performance and exterior ballistics. New cartridges like Federal’s 7mm Backcountry, Hornady’s 7mm PRC, SIG Sauer’s 277 SIG Fury, and Winchester’s 6.8 Western all compete to catch the attention and favor of shooters and hunters, with the intent to replace existing popular cartridges. It is, after all, the circle of life. Cartridges are developed, become popular and then over time are edged out by newer designs. Some older cartridges may fall to the wayside due to more modern cartridges having an edge in performance or simply better marketing.

The "Win Mag" Beginning

There are certain rifle cartridges though which stubbornly refuse to fade away. One of these is the popular old .300 “Win Mag.” A .30-caliber belted magnum, the .300 Winchester Magnum was introduced by Winchester Repeating Arms Company some 62 years ago way back in 1963. Winchester had previously introduced three new magnum rifle cartridges in 1958: the flat shooting .264 Win Mag, the hard hitting .338 Win Mag and the big-bore .458 Win Mag. While all three of these new offerings would be successful, the .338 Win Mag would become the most popular of the trio.

Winchester based their new magnum rifles cartridges on a modified .375 H&H Magnum case which was both blown out and shortened to 2.500 inches. Why the 2.500 length? This was important as it allowed the new cartridges to function through a standard length rifle action. This was a lesson gleaned from Roy 
Weatherby. Fifteen years earlier, in 1943, Weatherby had introduced his .270 Weatherby Magnum and popularized this length. Winchester astutely followed his lead when they developed their own line of magnum cartridges. It was the right decision and all three designs grabbed the attention of riflemen and hunters.

These three magnum rifle cartridges provided a good performance spread, but Winchester recognized they were missing something. They needed a design to cover the hugely popular .30-caliber bore size. Americans in the 1950s and 1960s loved their .30 calibers, and a properly designed cartridge in this bore size would be both an effective hunting cartridge and useful for long range target shooting. So, they put their engineers back to work.

While the new cartridge was based on the same parent case, they decided to move the shoulder forward 0.156 inch to further increase powder capacity. Doing so gave the new cartridge a distinctive feature it is known for, a relatively short neck. The belted-magnum case has a length of 2.62 inches. Rim diameter is .532 inch while base diameter is .513 inch. Shoulder diameter is .489 inch, and overall cartridge length is 3.34 inches. Maximum SAAMI recommended pressure is 64,000 PSI.

Winchester’s new .300 Winchester Magnum cartridge not only proved to be an instant hit, but it would eclipse all the other various magnum rifle cartridges in the .30 caliber bore size. Why did it become so popular?  Winchester’s new design was embraced by hunters and shooters for a variety of reasons. Firstly, it offers a practical brute performance advantage over standard rifle cartridges like the classic .30-’06, but without being too hard kicking. It will take any game animal in North America; yet will still fit in a standard length action. On the rifle range, it gained a reputation for both its accuracy in 1,000-yard competitions and its exterior ballistics, but, perhaps most importantly Winchester designed it with the working man in mind. Unlike certain other magnum cartridges, loaded ammunition and reloading components are relatively affordable.

Military Sniper Adoption

The US military also took note of the virtues of Winchester’s magnum. It soon became popular with the various rifle teams for use in competition. In addition, the Navy in particular took an early interest in the Win Mag as a means to extend the range of their snipers. They originally began using a 185-grain Lapua FMJ-BT, but eventually switched to a 190-grain Sierra MatchKing. Target velocity was 2,950 fps. During development of what came to be known as the A191 load (type classified as the Mk 248 Mod 0), much work was done on the chamber design (headspacing off the shoulder, not the belt) as well as maintaining a consistent velocity over a wide temperature range.

The result of this work was a sniper cartridge with extended reach compared to the 7.62x51mm M118. Combat performance of the .300 Win Mag has been very good. The US Army eventually decided to retain their M24 rifles by converting them to .300 Win Mag. With the US military embracing the .300 Win Mag, work was undertaken to further improve it. The A191/Mk 248 Mod 0 was refined and a new Mk 248 Mod 1 load introduced. This new load was developed to stretch the .300 Win Mag’s reach even further. The traditional 190-grain Sierra MatchKing was replaced by a 220-grain MatchKing. Doing so bumped the G1 ballistic coefficient (BC) from 0.533 to 0.629. Muzzle velocity remained very similar despite the increase in projectile weight. This was accomplished through a very heavy charge of H1000 powder which is pushing pressures slightly above SAAMI levels. As the Mk 248 Mod 1 is loaded to a longer OAL than SAAMI specifies it may not be safe in some commercial rifles. The end result is an effective long range sniper load well-suited to the US military’s particular needs effective to 1,500+ yards.

Reloading For The 300 Win Mag

If you are interested in the .300 Win Mag, loaded ammunition, dies, and components are all readily available. Ammunition, even match loads, is significantly less expensive than its Finnish rival, the .338 Lapua Magnum. Brass is plentiful from a variety of domestic and foreign manufacturers. Federal, Remington, Nosler, Hornady, Norma and Winchester all offer brass. Winchester brass is relatively inexpensive while Norma brass has earned a reputation for excellent quality. Just keep in mind that brass will vary in weight and volume from manufacturer to manufacturer. It can also vary depending upon the lot and date of manufacture.

When it comes to powder, there are a number to choose from. IMR 4350 is a traditional favorite while Hodgdon’s H1000 and Retumbo both have their followers. If you are a fan of VihtaVuori powders, one I recommend trying is N560. RL-22 is also an excellent powder to consider. Just remember, the faster the load the better the exterior ballistics, but the harder it will be on your barrel.

Seeing as the Win Mag utilizes standard 0.308-inch diameter projectiles, there are a host of fine choices available. Sierra’s 190-grain MatchKing (0.533 BC) is the traditional choice, but it’s hardly the only choice. Sierra’s 210 (0.670 BC) and 220-grain (0.629 BC) MatchKings also deserve consideration. Hornady’s 208-grain ELD Match (0.690 BC) in particular has earned an enviable reputation. While on the heavy side, Hornady’s 225-grain ELD Match (0.777 BC) has a very impressive claimed BC. Berger Bullets has a wide assortment of Match projectiles to consider and Lapua should not be forgotten. In my experience, Sierra’s 190-grain MatchKing is well-known for being relatively easy to get to shoot well. Many of the VLDs can be a bit touchy regarding OAL, and can take a bit more effort to dial in.

Is the .300 Win Mag perfect? No, the belt is a hold-over from times long past and the neck is indeed short. Plus, if you want to go fast you’ve got to pay the piper. Do not expect to stuff copious quantities of slow burning powder down a .30-caliber hole and have Methuselah-like barrel life. Even with all its faults, the old “Win Mag” remains very popular and is certainly one to consider for big game hunting and long-range shooting.

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