The Best Rifle Cartridges that are Mostly Ignored: .257 Roberts +P, .338-06 A-Square, 6.5mm-284 Norma and 6.5mm-06 A-Square

By Gary Zinn

In the course of studying the various rifle cartridges that are most popular for hunting, I have noticed three situations in which there are noticeable performance gaps between popular cartridges. There are existing, but mostly ignored, cartridges that would fill these gaps nicely. This article focuses on these ignored cartridges and illustrates how they can fill the performance gaps in question. The cartridges are the .257 Roberts +P, .338-06 (A-Square), 6.5mm-284 Norma and/or 6.5mm-06 (A-Square).


These four cartridges are so overlooked that factory loads for them are very limited, at best. Accordingly, I selected representative loads for each cartridge from reloading data tables, relying primarily on the Hornady Handbook of Cartridge Reloading (10th edition) and the Nosler Load Data website. (Loads using Hornady bullets are from Hornady data and loads using Nosler bullets are from Nosler data.) I also built representative loads for the cartridges with which the subject cartridges are compared.

I built and compared loads with two or more bullet weights for each set of cartridges in question. For brevity, though, I am limiting data presentations to one bullet weight for each set of cartridges being compared. I provide brief notes on other data sets, as appropriate.

The .257 Roberts +P

The first performance gap is in the category of heavy varmint / light game cartridges. The gap is between the .243 Winchester and .25-06 Remington cartridges, and the gap filler is the .257 Roberts +P. The gap between the .243 Win. and .25-06 Rem. is largest for Class 2 game hunting loads for these cartridges.

Here is a comparison of representative loads for the three cartridges, with a 100 grain hunting bullet for the .243 Win., the heaviest popular hunting bullet weight for the .243. This load is compared with 117 grain bullet loads for the .257 Roberts and .25-06 Rem., both of which comfortably handle bullets up to 120 grains.

The first data line identifies the cartridge, bullet weight and type, and muzzle velocity (MV) from 24" barrels (unless otherwise noted). The second line lists the ballistic coefficient (BC) and sectional density (SD) of the bullet, the +/- 3" maximum point blank range (MPBR) of the load (rounded to the nearest 5 yards), the Guns & Shooting Online Killing Power Score (KPS) of the load and its downrange energy (both at MPBR).

.243 Win.: 100 grain Hornady BTSP bullet, MV 3000 fps:
BC .405; SD .242; MPBR 290 yds. with KPS = 13.8, energy 1232 ft. lbs.

.257 Roberts: 117 grain Hornady SST bullet, MV 2900 fps (22" barrel):
BC .390; SD .253; MPBR 280 yds. with KPS = 17.5, energy 1335 ft. lbs.

.25-06 Rem.: 117 grain Hornady SST bullet, MV 3000 fps:
BC .390; SD .253; MPBR 290 yds. with KPS = 18.6, energy 1413 ft.lbs.

The first thing to note is the killing power of the .243 Win. is restricted on larger Class 2 game. Using a KPS of 15 or greater as a guideline killing power value for Class 2 game (up to 300 pounds), the .243 load comes up short. The KPS of the load falls below 15 at 240 yards.

However, it has sufficient power at MPBR distance for use on small to medium sized Class 2 game (e.g. pronghorn, or a deer weighing about 150 pounds); this is based on a KPS value of 12.5 being judged generally adequate for efficient killing shots on such game (assuming a vital area hit).

Meanwhile, the .257 Roberts and .25-06 loads have KPS values comfortably above 15 at MPBR range. The KPS of the .257 Roberts load falls between those of the .243 and .25-06 loads, but closer to the latter. The Roberts load is more capable on Class 2 game in general than the .243 load; it bridges the performance gap between the .243 and the .25-06.

115 to 120 grain .257 diameter bullets have higher sectional densities than 100 grain, .243 bullets. This is a subtle, but important, added advantage of the .257 Roberts over the .243 Win., because (other factors being equal) higher SD bullets penetrate better than those with lower SD.

The .243 Win. holds its own much better with varmint loads. This is partly because .257 bullets weighing less than 70 to 75 grains are necessarily too stubby to have high BCs and therefore do not make for good long range varmint loads, while there are relatively sleek lighter weight bullets of .243 diameter. If one wants to use varmint loads with bullets weighing less than about 75 grains, the .243 is the way to go.

I evaluated loads with 75 and 85-87 grain varmint bullets for the three cartridges. The .257 Roberts and .25-06 Rem. loads achieve MPBRs slightly longer than the .243 Win. load and the flatter trajectories of the .25 caliber loads yield somewhat less bullet drop at extended ranges (beyond MPBR). The differences are not large enough to be worth getting excited about, as all three cartridges perform well with heavy varmint bullets.

The .243 Win. is clearly the mildest recoiling of the three cartridges. The 100 grain .243 load generates 11.1 ft. lbs. of recoil energy (assuming a 7.5 pound rifle weight), while the 117 grain .257 Roberts and .25-06 loads produce 13.3 and 15.7 ft. lbs., respectively. The .25-06 thus generates 41 percent more recoil than the .243, while the .257 Roberts splits the difference, with 20 percent greater recoil than the .243.

The .243 Win. is a very good choice for young or beginning shooters, or those who are very recoil adverse. If I were guiding a neophyte deer hunter armed with a .243 Win., I would restrict him/her to moderate range shots, to facilitate a vital area hit with plenty of energy and killing power.

The .240 Weatherby Mag. might serve as a bridge between the .243 Win. and the .25-06 Rem. The Weatherby would get 3100 fps MV with a 100 grain bullet (per the Hornady manual), giving a MPBR of 300 yards and a KPS of 14.6 at that distance; KPS would equal 15 at 285 yards.

The .240 has a 20 yard longer MPBR than the .257 Roberts, but the Roberts can fire a larger diameter, higher SD bullet, which carries more energy and killing power out to its MPBR distance. Everything considered, I think the .257 Roberts fills the gap between the .243 Win. and the .25-06 better than the .240 Weatherby.

The .338-06 A-Square

The .338-06 wildcat cartridge has been around for a long time. The A-Square munitions company commercialized it circa 1998, but then went out of business in 2012, which left the .338-06 and 6.5mm-06 (also commercialized by A-Square) orphaned. Currently there is no regular production of rifles in either caliber, very limited production of ammunition for the .338-06 and none for the 6.5mm-06.

The limbo status of the .338-06 is unfortunate, for it could fill a void in the .338 caliber slot. The .338 Winchester Magnum so dominates the medium bore category that other cartridges struggle to be recognized. Currently, the only notable non-magnum cartridge that shares the .33 bore slot with the .338 Win. Mag. is the .338 Federal. There is a large performance gap between the two cartridges, which the .338-06 would fill nicely. The data on representative loads illustrates this.

.338 Federal: 200 grain Hornady SST bullet, MV 2600 fps (26" barrel):
BC .455; SD .250; MPBR 255 yds. with KPS = 45.2, energy 2014 ft. lbs.

.338-06: 200 grain Hornady SST bullet, MV 2800 fps (23.5" barrel):
BC .455; SD .250; MPBR 275 yds. with KPS = 51.5, energy 2294 ft. lbs.

.338 Win. Mag.: 200 grain Hornady SST bullet, MV 3000 fps:
BC .455; SD .250; MPBR 295 yds. with KPS = 58.0, energy 2584 ft. lbs.

The .338-06 load pretty much splits the difference in performance between the .338 Federal and .338 Win. Mag. loads in MPBR, KPS and energy. All three loads have power at MPBR that is fully adequate to be effective on Class 3 game (KPS values above 30 at MPBR).

The .338-06 also fits neatly between the other two cartridges in performance when all are loaded with 225 grain bullets. 225 grain loads generate more downrange energy and killing power, with the .338-06 load getting a KPS of 63.7 at 270 yards (24% more than the 200 grain load).

The .338 Federal case does not have enough powder capacity for the cartridge to use 250 grain bullets effectively, but the other two cartridges are very much in play with heavy bullets. The .338 Win. Mag., stoked with 250 grain bullet loads, is a beast (2543 ft. lbs. of energy and KPS = 71.4 at 260 yards) and a .338-06 load is no wimp (2297 ft. lbs. of energy, KPS = 64.5 at 245 yards). Given the power these loads generate, along with the very high SD of 250 grain .338 spitzer bullets (.313), both are fully effective against all Class 3 game and also Class 4 critters.

Any high intensity medium bore cartridge is going to kick hard, but the .338 Win. Mag. could be called abusive. Assuming an 8.5 pound rifle, the 200 grain .338 Win. Mag. load would generate about 34.5 ft. lbs. of recoil energy.

By comparison, the same bullet weight loads in the .338-06 and the .338 Federal would generate 25.2 and 19.9 ft.lbs. of recoil, respectively. My shoulder would certainly tolerate the recoil of a .338 Federal better than that of a .338 Win. Mag. (42% less recoil). The 27% decrease in .338-06 recoil, compared with that of the .338 Win. Mag., would make shooting less traumatic, if not truly comfortable.

6.5mm-284 Norma and 6.5mm-06

The performance gap in the 6.5mm bore slot is between the most used 6.5s (the very similar 6.5x55mm SE, .260 Remington and 6.5 Creedmoor) and the seldom seen magnums (6.5 Remington Mag., .264 Winchester Mag., .26 Nosler). The 6.5mm-284 Norma or 6.5mm-06 can fill this gap nicely. I use the 6.5 Creedmoor and 6.5 Rem. Mag. with 140 grain bullet loads to illustrate the gap, and show both 6.5-284 and 6.5-06 loads as gap fillers.

6.5 Creedmoor: 140 grain Nosler AccuBond bullet, MV 2650 fps:
BC .509; SD .287; MPBR 265 yds. with KPS = 23.8, energy 1514 ft. lbs.

6.5-284 Norma: 140 grain Nosler AccuBond bullet, MV 2850 fps (26\'94 barrel):
BC .509; SD .287; MPBR 285 yds. with KPS = 27.1, energy 1723 ft. lbs.

6.5-06: 140 grain Nosler AccuBond bullet, MV 2850 fps:
BC .509; SD .287; MPBR 285 yds. with KPS = 27.1, energy 1723 ft. lbs.

6.5mm Rem. Mag.: 140 grain Nosler AccuBond bullet, MV 2950 fps:
BC .509; SD .287; MPBR 295 yds. with KPS = 28.8, energy 1831 ft. lbs.

These representative loads suggest that the 6.5-284 and 6.5-06 give identical ballistic performance, though the 6.5-284 data is from a two inch longer barrel. The close correspondence of 6.5-284 and 6.5-06 performance holds for loads with other bullet weights (120, 129-130 and 156-160 grains), too. These two cartridges clearly fit in the performance gap between the Creedmoor and the 6.5 Rem. Mag., with a tilt toward the latter.

A subtle capability of heavier (140 - 160 grain) 6.5mm bullets is that they have sectional densities that yield deep penetration on heavy animals. This means that they have the potential to be effective on Class 3 game at appropriate distances.

A 140 grain 6.5mm bullet will generally be effective on Class 3 animals out to the range where it has 1910 ft. lbs. of energy (KPS = 30). Using this criterion, the 6.5 Creedmoor load, above, would be effective on Class 3 game out to 95 yards, while the 6.5-284 and 6.5-06 loads would be effective to 210 yards and the 6.5mm Rem. Mag. load to 260 yards. The point is, a 6.5-284 or 6.5-06 would be a much better Class 3 game cartridge than the 6.5 Creedmoor, .260 Rem., or 6.5x55 SE.

Recoil of the representative 6.5-284 or 6.5-06 load would be about 16.5 ft. lbs. in an eight pound rifle. This level of recoil would be much the same as a .308 Winchester firing a 165 grain bullet at 2600 fps in a same-weight rifle. It kicks some 20 percent more than the 6.5 Creedmoor and 15 percent less than the 6.5mm Rem. Mag.

Rifles, ammunition and reloading

(Information on available rifles, ammunition, load dies and brass are as gleaned from various Internet sources, Spring 2018.)

Rifles: The only production rifles currently available for the calibers featured are in .257 Roberts and 6.5mm-284 Norma. Kimber catalogs their 84M rifle in .257 Roberts, as does Ruger in the M77 Hawkeye. Some Savage rifle models are listed in 6.5mm-284.

The .338-06 and 6.5mm-06 have reverted to wildcat cartridge status, since the closing of the A-Square business. All is not lost, though, for it is possible to have a rifle built in either of these calibers (or, indeed, in .257 Roberts or 6.5mm-284). If I wanted a rifle in .338-06 or 6.5mm-06 caliber, I would have E.R. Shaw build one of their Mk. VII custom rifles for me. See Shaw Precision Guns Mk-VII .338 Federal Rifle for a review of a Shaw rifle; visit for build options, pricing and ordering information.

Shaw quotes the base price of a Mk. VII rifle at $990, which can run up to $1800 if you add enough bells and whistles. You can spec out a rifle on-line and get a prompt price quote for whatever rifle package you specify.

Ammunition: There are limited options in factory ammunition in .257 Roberts (100 to 120 grain bullets), .338-06 (180 to 250 grain bullets) and 6.5mm-284 (129 to 156 grain bullets). I could find no current listings of commercial ammo for the 6.5mm-06.

Reloading: With the paucity of factory ammo options for these cartridges, it is likely that anyone who owns and shoots one of them is going to hand load ammunition. There is a large selection of bullets and powders suitable for loading all four cartridges, so the critical considerations are availability of reloading dies and brass.

Dies are not much of a problem. RCBS and Redding offer die sets for all four cartridges, while Hornady makes dies for three, Lee and Forster for two. Dies are expensive, given that they are limited market items, but they are a one-time investment for the reloader.

There is adequate availability of .257 Roberts, .338-06 and 6.5mm-284 brass, with three to five brands of each. I could identify only one brand of 6.5mm-06 brass, though. As noted for dies, brass for these calibers is generally expensive, due to limited demand and production.

Anyone loading much 6.5-06 is likely to end-up necking down .270 brass to 6.5mm. Not a big deal, just something of a hassle.


Perhaps I am the only one who cares about the performance gaps I have identified. If so, the time I spent researching and writing this article would have been better used tilting at windmills. However, the gaps are real and I have identified the cartridges that can fill them. Whether anything comes of this is up to shooters and hunters. If I help someone find the ideal cartridge for their needs and preferences, then this project will not have been in vain.

Note: Detailed articles on the four cartridges featured here can be found on the Rifle Cartridges index page.

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Copyright 2018 by Gary Zinn and/or All rights reserved.