New Woods Cartridge: The .338x57 O'Connor

By Chuck Hawks

In my article "Woods and Brush Rifles" I reported the results of brush penetration tests conducted by the late Jack O'Connor, the Dean of American gun writers. O'Connor described these tests in the chapter of his Gun Book devoted to "Rifles for Woods Hunting."

Briefly, O'Connor spent several days testing a variety of rifle calibers and bullet weights by firing them through a heavy screen of brush at a 3 foot by 4 foot target bearing the outline of a deer. O'Connor summarized his results this way: "I found that the higher the bullet velocity, the sharper the point, the thinner the jacket, the lighter the weight, the greater the deflection."

He concluded the chapter by proposing a new wildcat woods cartridge intended for hunting deer and black bear. Its purpose was to provide the best possible penetration of intervening brush with less recoil than the .348 Winchester. The .348 had given the best brush-bucking performance of the rifle calibers tested by O'Connor, but he felt that it had "pretty husky recoil for the once-a-year hunter."

I came across O'Connor's proposed cartridge in the course of researching my article on woods rifles, and it made a lot of sense to me. That is the cartridge I am calling the ".338x57 O'Connor," and the inspiration for this article.

Jack O'Connor proposed necking-up the 7x57 Mauser case to accept .338 inch bullets. He wanted to drive a 200 grain flat point (FP) bullet at a muzzle velocity (MV) of 2400-2450 fps. Flat point bullets at medium velocity had proven most successful in his brush-bucking tests. O'Connor suggested a flat nose bullet design with plenty of lead exposed for good expansion.

As I related in my article "Woods and Brush Rifles," O'Connor theorized that such a cartridge should be able to drive its bullet through the brush well, open up fast, and would have a lot of shocking power. A wounded animal hit with it should leave a substantial blood trail for easy tracking even if the bullet did not go all the way through. Recoil would not be bad, and the trajectory should be flat enough to allow a point blank range in excess of 200 yards.

Evidently nothing has been done with Jack O'Connor's proposed cartridge, as no .338 based on the standard 7x57 case is listed in the 9th Edition of Cartridges of the World by Frank C Barnes/Edited by M.L. McPherson. The only similar cartridge I found with an internet search was the .338x57 Mauser Ackley Improved, based on a blown-out 8x57 case. Since for our purposes there is absolutely no need to fire form the 7x57 case, the .338x57 MAI was not what I was looking for.

Hence this article, intended to flesh out the .338x57 O'Connor cartridge. Be advised that I do not have a rifle chambered for the .338x57 O'Connor (yet, anyway) with which to conduct testing, so this is a theoretical exercise.

Measurements and specifications

O'Connor, in his brief proposal, suggested no changes to the basic configuration of the 7x57 case, and I don't see much point in such changes either. We could "improve" the case, of course, for greater powder capacity, but what would be the point? Our target velocity is 2400-2450 fps, and that should be attainable by simply necking-up the 7x57 case with no other changes. Simpler is better. If we wanted more velocity, we could always buy a .338-06.

So let's start with a 7x57 Mauser case simply neck-expanded to accept standard .338 inch bullets. 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).

The barrel of a .338x57 O'Connor rifle should have a bore diameter of .330 inch and a groove diameter of .338 inch. It should be rifled with 6 grooves, right hand twist, as is customary for modern .338 barrels. The rifling twist for almost all modern .33 caliber cartridges is 1 turn in 10 inches, which should also be suitable for the .338x57 and should probably be adopted as the standard twist. Sometimes it's best to go along in order to get along.

The old .33 Winchester drove a 200 grain bullet at a MV of 2200 fps and used a 1 in 12 twist, which I suspect would also be perfectly adequate for the .338x57. The 1 in 12 twist might even give slightly better accuracy and brush bucking ability with the 200-225 grain bullets most suitable for the cartridge to beyond 300 yards, which is about the maximum effective range of the .338x57 O'Connor.

The SAAMI maximum average pressure (MAP) for the 7x57 is 46,000 cup due to the relatively weak Mauser Model 93 and 95 rifles once chambered for the cartridge. Modern rifle actions are routinely designed to safely handle cartridges loaded to a MAP 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 well under the MAP of 62,000 psi in the .338x57 O'Connor (probably around 51,000 psi). Which is good; it is always nice to have extra "headroom" available for unforeseen requirements or circumstances.

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.

The maximum cartridge length could be set at 3.34 inches. This is the same as for the .30-06, and since most 7x57 rifles are built on a standard (.30-06) length action, no real harm would be done. This would allow seating exceptionally long bullets in the .338x57 O'Connor. On the other hand, Mauser did make a special intermediate length Model 98 action specifically for the 7x57, and these actions could not be re-barreled to .338x57 if the COL was set a 3.34 inches. And, remember, the target bullet weight is 200 grains. If a hunter wanted a .338 bullet heavier than 250 grains he would probably be better off with a .338-06 or a .338 Magnum cartridge anyway. Lord knows there are plenty of those.

Suitable rifles

The .338x57 O'Connor is intended for use in modern brush and woods rifles. Jack O'Connor himself suggested that the Remington Model 760 pump gun would be a good rifle for his .338.

Other rifles adaptable to the .338x57 include all commercial bolt action rifles with standard length actions that are now chambered for the 7x57, .270 Winchester, .30-06, .338-06, .35 Whelen, and so forth. These would include (but are not limited to) the popular Browning A-Bolt, Mauser 98, 1917 Enfield, Remington Model 700, Ruger Model 77, Sako Model 75, Savage 110 series, Steyr-Mannlicher, Weatherby Mark V (6 lug), Weatherby/Howa Vanguard, and Winchester Model 70. The lighter bolt actions with 20-22 inch barrels generally make the best woods rifles.

Any strong single shot rifle, such as the Blaser K95, Browning/Winchester High Wall, Dakota 10, Mossberg SSI-One, and Ruger No. 1 could easily be re-barreled for the .338x57 O'Connor. The Browning BLR lever action rifle would seem to be a natural home for the .338x57, as would the Remington Model 7600 pump action (successor to the 760). Popular autoloaders such as the Browning BAR and Remington Model 7400 could also be adapted to the .338x57. Fast handling repeaters with lever, pump, and autoloading actions make especially good woods and brush country rifles.

Minimum barrel length for the .338x57, given its powder capacity, should be about 20 inches, and I cannot see much point to a woods rifle with a barrel over 22 inches in length. Loads for the .338x57 O'Connor should therefore be developed for, and chronographed in, barrels between 20 and 22 inches in length.

Load development

Before load development can begin, of course, a set of reloading and case forming dies in .338x57 O'Connor caliber will have to be special ordered. Pacific Precision can supply wildcat chamber reamers and RCBS has traditionally been the most popular source of custom-made dies for wildcat calibers.

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 .348 Winchester and .358 Winchester, and outperforms such old standbys as the .30-30 Winchester, .35 Remington, and .300 Savage. The lack of popularity and ultimate commercial failure of the .348 and .358 Winchester cartridges is directly attributable to their recoil, which is greater than most shooters are willing to tolerate in a lightweight woods rifle.

I think it may prove possible to drive a 200 grain bullet faster than 2450 fps in the .338x57 with full pressure loads, but the tendency to do so should be avoided for normal use (i.e. hunting deer and black bear at woods ranges). The flat point bullet would have to be replaced by a spitzer bullet to take full advantage of the flatter trajectory made possible by higher velocity. But higher velocity will not only increase recoil, it would probably decrease the bullet's ability to penetrate brush and still reach its target. The spitzer shape would further degrade the bullet's performance as a brush-bucking woods cartridge. And, no matter what, the .338x57 cannot equal the MV possible with the same weight bullet in the .338-06. In the .338x57 O'Connor, loads that exceed 2450 fps with the 200 grain FP bullet defeat the purpose of the cartridge.

Maximum pressure (62,000 psi) loads using 210, 215, or 225 grain bullets for hunting larger game might be considered in order to increase the versatility of the .338x57. I suspect that a 225 grain bullet could be driven to a MV of about 2300 fps and would make an excellent elk load. But remember that the primary purpose of the .338 O'Connor is to be a superior woods cartridge of moderate recoil for hunting deer and black bear. It is not an attempt to re-invent the .348 Winchester or the .338-06 A-Square.

Powder Selection

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 these powders.

Bullet selection

Which 200 grain bullets would be most suitable? Due to the influence of the popular .338 Winchester Magnum, most .338 bullets these days are spitzer (pointed) in form. Barnes, for example, offers an X-Bullet, Hornady offers an Interlock, Nosler offers a Ballistic Tip and a (210 grain) Partition; Speer offers a Hot-Cor.

The only widely distributed flat point bullet that I found in a cursory search is the Hornady 200 grain bullet designed for the old .33 Winchester cartridge. It has a sectional density (SD) of .250 and a ballistic coefficient of .200. Hornady describes this bullet as being suitable for medium and large game. It is designed for a MV range of 1700-2200 fps. (The obsolete .33 Winchester factory load launched a 200 grain bullet at a MV of 2200 fps.) I think this is the bullet with which to start load development; it should be deadly on deer and black bear at .338-57 O'Connor velocities. Anyone who has trouble finding the Hornady 200 grain FP or who is wedded to spitzer bullets could substitute the Hornady 200 grain Spire Point.

If a heavier bullet is desired for elk hunting, a bullet such as the the 225 grain Nosler Partition spitzer (SD.281, BC .454) could be substituted. 225 grain bullets from Hornady, Speer, Swift, and Woodleigh are also worth considering. These are widely recognized as suitable bullets for large game.


Let's take a look at the ballistics and performance these two bullets would provide if fired from a .338x57 O'Connor rifle. The 200 grain Hornady FP at a MV of 2400 fps has muzzle energy (ME) of 2558 ft. lbs. At 50 yards the velocity is 2193 fps and the energy is 2135 ft. lbs. At 100 yards the velocity is 1996 fps and the energy is 1770 ft. lbs. At 150 yards the velocity is 1830 fps and the energy is 1487 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, fired from a rifle with 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, -7.9 inches at 250 yards, and -18.4 inches at 300 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. Clearly, the .338x57 is limited by its trajectory, not its killing power.


In a rifle weighing 8 pounds including scope, sling, and a full magazine the .338x57 O'Connor, shooting a 200 grain bullet at a MV of 2400 fps, should generate about 19.2 ft. lbs. of free recoil energy and a recoil velocity of 12.4 fps. A 225 grain bullet at a MV of 2300 fps delivers about 19.8 ft. lbs. of free recoil and a recoil velocity of 12.6 fps.

These recoil figures are encouraging, as they are below the 20 ft. lb. figure often quoted as the maximum free recoil energy the average shooter can tolerate. Both are a little less than the 20.4 ft. lbs. of recoil attributed to an 8 pound .308 Winchester rifle shooting a 200 grain bullet. As a woods rifle, the .338x57 should surpass the .308 in effectiveness.

Killing power

The theoretical killing power of the .338x57 also looks pretty good. The optimal game weight for the 200 grain Hornady flat point bullet at a MV of 2400 fps is 154 pounds at 300 yards, 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 447 pounds at 300 yards, 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.


Without a .338x57 O'Connor rifle to test, that is about as far as I can take the concept. Frankly, I find the .338x57 an exciting and worthwhile idea. For easy comparison, I have included it in my "Rifle Recoil," "Rifle Trajectory," and "Optimal Ranges for Big Game" tables. Jack O'Connor was one of the most knowledgeable and experienced hunters and gun writers to ever lift a pen, so it should come as no surprise that the .338 O'Connor makes a lot of practical sense.

I am going to be looking into the feasibility (mostly cost!) of having a custom rifle made for the .338x57 O'Connor cartridge. Perhaps the easiest and least expensive way to go would be to purchase a new CZ 550 or Ruger M77R Mk. II bolt action rifle in 7x57 and rebarrel it for .338x57. The Ruger No. 1A Light Sporter should make for an easy and very classy conversion for aficionados of the single shot rifle.

If you, loyal reader, are inspired to have a .338x57 O'Connor rifle built, I would be very interested in your experiences and the outcome of the project so that I could incorporate the information into this article (with full recognition to the source, of course). Good Hunting!

Note: Articles covering the .338x57 O'Connor as well as comparing it to other cartridges can be found on the Wildcat Cartridge Page.

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Copyright 2003, 2013 by Chuck Hawks. All rights reserved.