The .257 Roberts Improved
By Chuck Hawks
The two most popular "Improved" .257 Roberts cartridges are the Ackley and RCBS versions. They are quite similar except for the shoulder angle, which is 40 degrees for the Ackley version and 28 degrees for the RCBS version. (The standard Roberts case has a 20.75 degree shoulder.) It may be worth noting here that shoulder angle per se does not significantly affect the ballistic performance of any cartridge, although it does affect headspacing, feeding, and perhaps case life.
Gunsmith P. O. Ackley is perhaps the most famous of all wildcatters, and the .257 Roberts Improved is one of his most popular and useful creations. It was probably developed sometime in the late 1940's or early 1950's.
Brass for Improved cartridges is created by the simple expedient of firing standard caliber factory loads in an Improved chamber. The pressure molds the brass to the new chamber, and what is ejected is an Improved case. Usually the Improved version of any case has less body taper and a sharper shoulder. This increases case capacity, making possible somewhat greater performance.
One of the nice things about Improved cartridges in general is that in a pinch regular factory ammunition can be fired in the Improved chamber. One of the drawbacks is that since wildcat cartridges are by definition not standardized by any authority, chamber dimensions can and do vary. So .257 Improved cartridges reloaded for one rifle may not fit properly in another.
No cartridge overall length (C.O.L.) is specified for any wildcat, since they are not standardized cartridges. And there are also no pressure standards. In the Nosler Reloading Guide Number Four it is stated that they used a maximum average pressure (M.A.P) of 58,000 psi when developing their loads for the .257 Ackley Improved.
The .257 Ackley Improved is a blown out .257 Roberts case with very little body taper and a 40 degree shoulder. It measures 1.8" to the bottom of the shoulder, 1.906" to the top of the shoulder, and has a case length of 2.233" (Nosler figures).
The RCBS version of an Improved .257 Roberts is similar, except it has a 28 degree shoulder and a longer neck. It measures 1.529" to the bottom of the shoulder, 1.907" to the top of the shoulder, and has a case length of 2.23" (Hornady figures). The Hornady Handbook, Third Edition used a C.O.L. of 2.99" for the .257 RCBS Improved with a 120 grain Spire Point bullet, which is the same length they used for the standard .257 Roberts with the same bullet.
A .257 Improved should give a velocity advantage of about 100-300 fps, depending on bullet weight, over the .257 Roberts +P. This is due to its greater case capacity and higher pressure. The greatest advantage is generally with lighter bullets. It is a deadly long range varmint cartridge with 70-87 grain bullets, and a fine deer, antelope, sheep, and goat cartridge with 100-120 grain bullets. Its performance is not far behind that of the .25-06, and it can be made to work in short action rifles (depending on the C.O.L.).
The Hornady Handbook and Nosler Reloading Guide show maximum reloads for the .257 Improved (either version) achieving muzzle velocities (MV) of about 3600 fps with a 75 grain bullet, 3400-3500 fps with an 85-87 grain bullet, 3200 fps with a 100 grain bullet, 2900-3000 fps with a 115-117 grain bullet, and 2900 fps with a 120 grain bullet.
The 115-117 grain bullets seem about right for this case capacity to me. The fifth edition of the Noser Reloading Guide shows that 43.5 grains of RL19 powder can drive a 115 grain Nosler Partition bullet (BC .389, SD .249) in an Ackley Improved case to a MV of 2672 fps; 47.5 grains of RL19 will take the same bullet to a MV of 2960 fps.
The trajectory of that bullet at a MV of 2900 fps should look like this: +2.7" at 100 yards, +3" at 135 yards, +1.9" at 200 yards, and -4.6" at 300 yards. The maximum point blank range (+/- 3") is 282 yards. The recoil energy in an 8 pound rifle amounts to 10.3 ft. lbs. with this load.
In terms of killing power, the 115 grain bullet at a MV of 2900 fps can take medium size big game animals beyond its maximum point blank range. What more can be asked of any medium game cartridge? Those who think they need greater point blank range are going to have to go to the .25-06 Remington (which gains only a few yards) or, for a significant advantage, to the .257 Weatherby Magnum.
Standard .257 Roberts factory loads have always been under pressure in good rifles, and the Improved versions eliminate such problems. Any Improved cartridge can be loaded to whatever pressure is reasonable and safe in the rifle for which it is intended, generally around 52,000 cup or 65,000 piezo psi for modern bolt action rifles, without concern that it could be accidentally chambered in a weaker standard caliber rifle. For this reason one of the major factories should adopt the .257 Improved, neatly sidestepping the issue of high pressure loads for the regular .257 Roberts.
Copyright 2002, 2013 by Chuck Hawks. All rights reserved.