The .338x57 O'Connor

By Chuck Hawks

Jack O'Connor, the Dean of American gun writers, proposed this wildcat cartridge over 50 years ago. O'Connor suggested necking-up the 7x57 Mauser case to accept .33 caliber bullets. He wanted to drive a 200 grain bullet at a muzzle velocity (MV) of 2400-2450 fps. He felt that this would make an excellent brush cartridge for the deer and black bear hunter.

In 2003 I stumbled across O'Connor's description of his proposed .33 caliber brush cartridge. The more I thought about it, the better and more practical the idea seemed. (Perhaps not surprising, as the late Jack O'Connor was an eminently practical shooter and writer.) I became interested in the cartridge's possibilities, so I began to do a little research on the subject.

I could not find mention of a wildcat .338 on the standard 7x57 case anywhere, and when I asked Bradford O'Connor (Jack's son) about the cartridge he told me that he had no knowledge of it ever having been developed. I did find a few references to the .338x57 Mauser Ackley Improved, which is based on a blown-out 8x57 case. Since for our purposes there is absolutely no need to fire form (blow-out) the 7x57 case, the .338x57 MAI was not what I was looking for. I decided to pursue Jack O'Connor's idea myself, did more research, and ultimately fleshed-out O'Connor's concept.

I named the cartridge the .338x57 O'Connor in Jack O'Connor's honor. I wrote about it in considerable detail in my article "New Woods Cartridge: The .338x57 O'Connor," which you can find on the Rifle Information Page. That article has generated considerable interest among the readers of Guns and Shooting Online and I have received quite a bit of e-mail about the cartridge, so I decided to include the .338x57 on the Rifle Cartridge Page. The article you are reading is the result.

O'Connor's concept was a medium bore cartridge with moderate recoil that the average woods and brush hunter could use for shooting deer, black bear, and possibly elk. Such a cartridge should get through brush quite a bit better than the .30-.32 caliber cartridges (.30-30, .307 Winchester, .300 Savage, .308 Winchester, .30-06, .32 Special, etc.) and hit harder than the .35 Remington or .38-55, making it a better elk cartridge. If the new wildcat's recoil could be kept below 20 ft. lbs. the average deer hunter could take advantage of its benefits.

O'Connor felt that the 7x57 case had the right capacity, overall length, and neck length to be just about ideal as the basis for such a cartridge. The .338x57 O'Connor is therefore based on the 7x57 Mauser cartridge simply necked-expanded to accept .338" diameter bullets. I resisted the temptation to get fancy by "blowing out" the case for greater capacity and a sharper shoulder. Moderate recoil is part of the requirement and the case does not need greater capacity to achieve its performance goals. I decided that simpler is better.

This means that we will retain the basic dimensions of the 7x57 case. The 7x57 is a bottleneck, rimless case that headspaces on its shoulder. It accepts standard large rifle size primers. It has a rim diameter of .474 inch, a rim thickness of .046 inch, a base diameter of .4729 inch, a shoulder diameter of .4294, and a shoulder angle of 20 degrees 45 minutes. Its neck is .3686 inch long. The maximum case length is 2.2350 inches (trim to 2.225 inches).

Modern rifle actions are routinely designed to safely handle cartridges loaded to a maximum average pressure of 52,000 cup (or 62,000 peizo psi). Since there are no weak rifles chambered for the .338x57 O'Connor, I suggest a MAP of 52,000 cup/62,000 peizo psi.

I estimate that a 200 grain bullet could be driven to the target MV of 2400-2450 fps at a MAP of somewhere around 51,000 psi in the .338x57 O'Connor. Which is good; it is always nice to have extra "headroom" available for unforeseen requirements or circumstances. At some point someone may want to load 250 grain bullets at maximum pressure and go moose or grizzly bear hunting.

The overall cartridge length of the 7x57 is 3.065 inches. This length should also be adequate for the .338x57, as the longest bullets ordinarily used in either cartridge (175 grain bullets in 7mm and 250 grain bullets in .338) are about the same length. Thus, bullets as heavy as 250 grains should not protrude into the case below the bottom of the shoulder, a common complaint about most short action (.308 Winchester length) calibers. The .338x57 O'Connor should be able to avoid such complaints. I recommend a COL of 3.065 inches.

When developing loads for the .338x57 O'Connor it should be remembered that the primary goal is an effective woods cartridge that kicks less than the best previous woods cartridges such as the .338-06, .348 Winchester, .35 Whelen, and .358 Winchester. And it must outperform such old standbys as the .30-30 Winchester, .35 Remington, and .300 Savage.

The lack of popularity and commercial success of the previously mentioned medium bore cartridges is directly attributable to their recoil, which is greater than most shooters are willing to tolerate in a lightweight woods rifle. In the .338x57 O'Connor, deer and black bear loads that exceed 2450 fps with a 200 grain bullet defeat one of the primary purposes of the cartridge. To enhance the cartridge's versatility, I suspect that a 225 grain bullet could be driven to a MV of about 2300 fps and would make an excellent elk load.

Obviously, to develop loads we must select appropriate bullets and powders. Due to its generous neck and moderate overall length, the .338x57 O'Connor can use practically any sort of bullet. Let's start with 200-210 grain bullets (SD .250-.263). In those weights Barnes offers X-Bullets, Hornady offers flat point and Spire Point Interlocks, Nosler offers a Ballistic Tip and a Partition, and Speer offers a Hot-Cor.

If a heavier bullet is desired for elk hunting, a bullet weighing 225 grains (SD .281) could be substituted. Hornady, Nosler, Speer, Swift, and Woodleigh offer 225 grain bullets. These are widely recognized as suitable bullets for large game.

Powder must be carefully chosen for safety's sake. The 8x57JS Mauser has somewhat greater powder capacity and a slightly smaller diameter bullet than the .338x57 O'Connor, but it is in the same ballpark and can handle similar bullet weights. The .356 Winchester and .358 Winchester have case capacities similar to that of the .338x57 but somewhat fatter bullets. They also use bullets of similar weight, and they share the same 62,000 psi MAP as the .338x57. One would think that powders recommended for all three of these calibers might also be reasonably suitable for the .338x57 O'Connor.

Popular powders for 200 and 220 grain bullets in the 8x57JS, .356 and .358 include (from fastest to slowest in approximate burning rate) H4895, RL-15, IMR 4064, IMR 4320, W748, and VIHT N140. If I were developing loads for 200-225 grain bullets in the .338x57 O'Connor, I would start with one of those powders.

Let's take a look at the ballistics bullets weighing 200 and 225 grains could provide when fired at our target MV's from a .338x57 O'Connor rifle. I chose the Hornady 200 grain Interlock flat point and the Nosler 225 grain Partition spitzer as examples.

The 200 grain Hornady FP at a MV of 2400 fps has muzzle energy (ME) of 2558 ft. lbs. At 100 yards the velocity is 1996 fps and the energy is 1770 ft. lbs. At 200 yards the velocity is 1638 fps and the energy is 1191 ft. lbs. At 250 yards the velocity is 1507 fps and the energy is 1008 ft. lbs. And at 300 yards the velocity is 1368 fps and the energy is down to 831 fps, about the minimum recommended for deer hunting. So, in terms of killing power, the .338x57 O'Connor is a 300 yard deer cartridge with the 200 grain flat point bullet.

The trajectory of the 200 grain FP bullet, assuming a scope mounted 1.5 inches over the bore, should look like this: +1.6 inches at 50 yards, +2.9 inches at 100 yards, +2.1 inches at 150 yards, 0 at 185 yards, -3 inches at 214 yards, and -7.9 inches at 250 yards. So zeroed, the maximum point blank range (MPBR) of this load is 214 yards (+/- 3 inches), just about what O'Connor predicted.

The 225 grain Nosler Partition spitzer bullet at a MV of 2300 fps would have ME of 2640 ft. lbs. At 100 yards the velocity would be 2128 fps and the energy 2262 ft. lbs. At 200 yards the velocity is 1963 fps and the energy is 1925 ft. lbs. At 250 yards the velocity is 1883 fps and the energy is 1771 ft. lbs. And at 300 yards the velocity would be 1806 fps and the energy 1629 ft. lbs.

The trajectory of the 225 grain Nosler bullet should look like this: +0.3 inch at 25 yards, +2.9 inches at 100 yards, +2.4 inches at 150 yards, 0 at 196 yards, -3 inches at 230 yards, and -12.6 inches at 300 yards. So zeroed, the MPBR of this load is 230 yards.

With either bullet the MPBR exceeds 200 yards, plenty for a woods and brush cartridge. The .338x57 is ultimately limited by its trajectory, not its killing power.

The theoretical killing power of the .338x57 looks pretty good. The optimal game weight for the 200 grain Hornady flat point bullet at a MV of 2400 fps is 253 pounds at 215 yards, 368 pounds at 150 yards, and 487 pounds at 100 yards. As anticipated, that should be a very effective deer and black bear load.

The optimal game weight for the 225 grain Nosler Partition bullet at a MV of 2300 fps is 533 pounds at 230 yards, 649 pounds at 150 yards, and 732 pounds at 100 yards. That looks like a pretty good elk load for the woods hunter.

Okay, the trajectory is adequate and the killing power is excellent, but what about recoil? I ran the numbers and found that a .338x57 rifle weighing 8 pounds and shooting a 200 grain bullet at a MV of 2400 fps should generate about 19.2 ft. lbs. of free recoil energy. A 225 grain bullet at a MV of 2300 fps delivers about 19.8 ft. lbs. of free recoil from the same rifle.

These recoil figures demonstrate that Jack O'Connor's original concept was valid, as they are slightly below the 20 ft. lb. figure often quoted as the maximum free recoil energy the average shooter can tolerate. They are, in fact, a little less than the 20.4 ft. lbs. of recoil attributed to an 8 pound .308 Winchester rifle shooting a 200 grain bullet. At last, a practical medium bore caliber that kicks like a popular small bore!

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Copyright 2004, 2014 by Chuck Hawks. All rights reserved.