Future wildcats, real and imagined - The 6mm and .257 WSM

I had a great time putting together the .358-378 RG this past year, and it of course was gratifying to have the cartridge perform to expectations. The project was accomplished with a relatively inexpensive rifle, the Ruger No.1, and because there was no bolt face to machine or mag well to modify, the conversion work was also relatively inexpensive. At a total cost of $1,600 I had a slick custom rifle, cartridge, chamber reamer and a set of quality reloading dies. I also had a gun that could throw 300+ grain bullets just a shade under 3,000 fps, and 250 grain bullets at over 3,200 fps.

The cartridge is easy to form from .378 brass, the rifle required a minimum of work beyond a rebarrel. The bore : case capacity ratio isn’t in the least bit extreme and the case works well with any powder suitable for my short magnums. This year, I’d like to come up with another combination for a bolt action rifle, maybe even a short gun, but without the limitations associated with current SRUM or WSM cartridges.

In the beginning

I do my best thinking when I can’t do anything else. I had just finished changing the Harley’s battery, another whole story, and I was reinstalling the seat. I was on my knees, one arm wrapped around the seat, forearm bending the cushion up and forward to make room for my other hand. The other hand, herewith known as my right hand, was holding the grab strap retaining nut in a two finger chopstick grip. A little too much pressure was applied to the nut, and I watched the nut pop up from between my finger tips and descend, in slow but unrecoverable motion, into the abyss know as the frame. It was 20 minutes into the nut recovery fishing expedition that I began assembling this article. The following represents my stressed mental wandering, verbal ramblings and subsequently skewed conclusions.

Conclusion one: Big cases need heavy bullets and s-l-o-w powder

Cartridge	Case Capacity Grains	Capacity with Bullet	Bullet Weight	Powder Type	Powder Charge	Barrel Length	Pressure PSI	Velocity
7mm RM	82	72.7	175	IMR7828	66.3	26	62156	3005
7mm STW	98	87	175	Re 25	82.2	26	64941	3148
7mm STW	98	87	175	H870	89.5	26	64992	3197
7mm-UM	112	105.5	140	IMR 7828	89.6	26	64784	3505
7mm-UM	112	102.0	175	IMR 7828	83.0	26	64846	3163
7mm-UM	112	102.0	175	Re25	88.5	26	64921	3157
7mm-UM	112	102.0	175	H870	96.3	26	64891	3210
7mm-UM	112	102.0	175	H50BMG	103.2	26	64887	3257
Factory equivalent load			Math Model, not for use as loading data

The primary reason big cases exist for a given bore size, is to propel the heavy end of the available bullet range at higher velocities. The resulting benefit of this increased velocity of heavy bullets should be greater damage to a target, damage sustainable at longer ranges. Light bullets driven from big cases, traveling at exceptionally high velocities, frequently exceed light bullet structural design limitations and either fragment on impact or over penetrate. At an optimal distant ranges where light bullets don’t fragment or over penetrate, they will make a smaller wound channel on expansion than a bullet of same bore and heavier weight.

The easiest way to illustrate the expansion potential of bullets of the same bore, but different weights, is to encase them in an imaginary cylinder, and assign each of these cylinders a volume. In this case, the bullet on the left, a 7mm 175 grain Sierra at 1.394″ in length was assigned a volume of 0.353 cubic inches. The 140 grain version of the Game King at 1.160″ was assigned a volume of 0.294 cubic inches. Under impact, the cylinder (the bullet) would compress and grow shorter, as the sides expanded to contain the original mass. Assuming the cylinder is compressed to a length of .500″ the 175 grain would now be .475″ in diameter and the 140 grain in .432″ diameter . The difference of .043″ may not seem like much, but it is greater than the difference between a 6mm and 7mm bullet, or a bullet from a .338 Winchester and a .375 H&H.

Why did Remington go with a light bullets and a corresponding powder selection ? I think they were initially pushing a 24″ barrel concept that needed light bullets to make a sensational velocity statement; 3,500 fps sounds a lot better than 3,200 in the world of marketing, and the selection of IMR7828 meant 15% – 20% less powder in each case, which sounds much better in the world of accountants.

Conclusion Two: Big cases, heavy bullets and s-l-o-w powder need long barrels

There are three camps in this world when it comes to rifle length; the guys who think they are going to find themselves wedged between a rock and a hard place exactly 40″ away and can only carry snub nose magnums, guys with 30″ barreled guns who take folding shooting tables on hunting trips and, finally, guys who accept reality and realize guns are a system and components must compliment one another. Ultra Mags have Weatherby and Lazzeroni like  capacity, basic laws of physics require equal barrel lengths for equal performance. It doesn’t matter if you squish the case, or stretch it, case volume, not shape generally sets barrel length requirements. Big capacity Ultra Mags – long barrels and high velocity. Stubby lesser capacity SRUM and WSM cartridges, moderate barrel length and decent velocity.

Conclusion Three: New short magnums are less equal than old short magnums

Short magnums are missing one thing older short magnums had – room. Unfortunately,  as cases got much shorter, bullets did not, and long bullets intrude far into the new short magnum cases and rob them of already marginal powder capacity. The net capacity of a 7mm Remington Magnum is 72.7 grains after a 1.372″ 175 grain bullet is seated. The net capacity of the 7mm SRUM with the same bullet seated is 62.6 grains. The capacity of the mundane 280 Remington is 58.2 grains with the same 175 grain wonder in place. In short, the 7mm SRUM is much closer to the .280 Remington then it is to the 7mm Remington Magnum.

Why is Remington offering a 160 grain bullet in its first factory 7mm SRUM loads? My guess is the use of the 160 grain bullet and W760 powder is to avoid the comparison with the 30 year old Remington Magnum. The Original Remington Magnum would beat the 7mm SRUM round, even if the 7mm Remington magnum were saddled with a short 22″ barrel. In the software industry these cartridges changes and introductions would not be revs, they would be more akin to Microsoft introducing Windows 95, then 98, then ME, then XP – none of them do a damn thing better, the change just causes customers to buy new software.

Conclusion Four: Handloaders can save the UM, not the SRUM & WSM

The Ultra Mags, with some quality handloads, can marginally out perform earlier versions of these cartridges; .375 H&H, .338 Winchester Mag, .300 Winchester Mag, and 7mm Remington Magnum. Eventually there may be a .416 Ultra Mag to put the current .416 Short to rest. The SRUM versions will go the way of the 6.5mm and .350 Remington Magnums, and the carbines they came wrapped in. But there is salvation, it’s just a couple of bore sizes away. I believe the WSM & SRUM should have been released in .257″ and .243″ bores. My plan is to build one of these gems in the coming year.

The 6mm WSM

I went with the slightly longer WSM case, which should still clear the Model 7 Remington short action. I took the liberty of removing some of the case taper and changing the shoulder angle to about 40 degrees. So far I need a sizing die and fire forming for the shoulder. The case has 78 grains of gross capacity and 73 grains of net capacity with a 100 grain boat seated at .310″.

And of course, not to leave well enough alone….

The .257 WSM

I load for the .257 Weatherby Magnum, which has greater capacity, so I don’t think this is particularly overbore capacity for the .257. I like the quarter bores, and this would be a lot of fun to work with.

Cartridge	Capacity Grains	Capacity W/Bullet	Bullet Weight	Powder Type	Powder Charge	Barrel Length”	Pressure PSI	Velocity
.257 WSM	78	75.8	75	Re19	70.4	22	63773	3830
.257 WSM	78	75.3	87	Re22	70.7	22	63795	3662
.257 WSM	78	74.2	100	Re22	68.0	22	63978	3472
.257 WSM	78	71.6	120	IMR 7828	64.2	22	64000	3240

What I believe I have here is something comparable to Weatherby cartridge performance, with about 4″ less barrel length and with a short action. With the 2″ increase in barrel length over the short guns, a Model 7 or Winchester Compact would come in at 41.5″ overall length, compared to 46 5/8″ for the Weatherby. At the moment I like the 6mm version, but I still haven’t made a decision yet. I’ll report when I’ve actually made some tangible progress on this project.

Summary response to testy e-mail regarding new short magnums

The term “Ejecta mass” appeared in several articles promoting the new short magnums, as the magical key to understanding why these cartridges have lighter recoil, at same velocity and bullet weight as their larger cousins. “Ejecta mass” means the gunk that follows the bullet down the bore, like powder that is burned, and powder waiting to be burned. The difference between the .300 WSM to the .300 Winchester Magnum referenced in the article is  21 ft/lbs vs. 24 ft/lbs, not exactly earth shaking, and a different selection of powder could cut that little difference to virtually nothing. Yes, I still don’t care for short actions and barrels as a firearm objective. No, I don’t think they cycle faster, and I don’t believe that 2″ – 4″ more of barrel length means anything, anywhere. Yes, I still think the released short magnums are a waste of time. yes, I really do think people from Texas take themselves too seriously – fortunately, I don’t.

Notes regarding data appearing in this article

The data presented in this article is mostly the product of math models and my imagination; theoretical loads are for use in theoretical guns and are not for use in real guns – although I’m sure I will hear from someone who shot an Arkansas Yak with a similar load in his 7mm STW – those 7mm STW guys are all a little…well, curious.

Thanks
Joe