Custom Wildcat Cartridges With Standard Reamers

As a sort of unconventional project, I set out to develop a special wildcat cartridge based on a commonly available parent case. I knew precisely what I wanted but didn’t want to go through blueprinting a special custom reamer in getting the job done. The cartridge I had in mind was based on the .243 Winchester Super Short Magnum case, necked up to a caliber .308 bore. I had a Broughton stainless target barrel with a 1:16 twist chambered for the much smaller .30BR which I would re-chamber.

I also had a Weatherby .300 WM barrel with a 1:10 twist for the other chambering which I would cut down and rethread. The project has had a most favorable result and I named my new creation the .30BR Magnum. The smaller .30BR version is one of today’s most successful benchrest cartridges. The barrels normally have a slow twist around 1:15 to 1:18 for use with lighter bullets ranging from 110 through 135 grains. The Weatherby barrel would have a standard 1:10 twist for use with a wider range of bullets.

The Winchester Super Short Magnum series of cartridges was 30BR using existing standard reamers to form the new chamber from the .243 WSSM case. From left, .30BR and .30BR Magnum. developed by Winchester in 2003 to fit their Super Short rifle actions. It was introduced in .223, .243, and .257 calibers. The case is non-belted with a rim diameter of 0.535 inch, the same as standard Magnum cartridges.

The short-fat cases have little body taper, resulting in a 28 degree shoulder with a diameter at the shoulder of 0.5444 inch. I vied this short-fat case as ideal in the accuracy arena and was not disappointed. I had worked extensively with all three WSSM cartridges and previously had a special reamer made, building a 6.5mm wildcat on the .243 WSSM case a few years ago named the 6.5×42 NEJ Super Grouper. It performed amazingly well but I wanted to see what a larger caliber would do in this short, fat case.

The Winchester Super Short series failed in popularity and is no longer available through Winchester or Browning in these three original calibers. Reaming .30 BR Magnum Forming the first stage of the new .30BR Magnum chamber began with a .243 Winchester Super Short Magnum reamer, in cutting the new .30BR Magnum chamber only, with no alteration to the case neck at this stage of development.

Here I had carefully made up a .30 caliber brass pilot to fit over a .243 Winchester Super Short Magnum pilot to fit precisely inside the .30 caliber lands. The new chamber was then headspaced exactly for the .243 WSSM cartridge body. The neck and throat (leade) part of the chamber was later formed with a different reamer. Reamer selection here was very critical.

The neck of the slave reamer must be longer than the existing .243 WSSM case neck and the throat/leade section must conform to the bullets being used in the new .30BR Magnum case. The shoulder of the neck reamer must also have a more shallow angle than the .243 super-short WSSM reamer

30BR cartridge to the new .30BR Magnum wildcat. From top, .243 WSSM case, Clymer .243 WSSM reamer with special pilot sleeve from .243 to .308, .243 WSSM headspace gauge, Manson .308 Winchester reamer used to form neck and leade, Broughton caliber .308 barrel chambered for the .30BR used in the conversion threaded for a Remington Model 40X rifle. 300 Weatherby Magnum barrel to the new .30BR Magnum wildcat. Note the cut off portion of the Weatherby barrel.

This barrel is fitted to Savage M112 rifle. to avoid any contact with the existing shoulder as the new neck and leade are cut. This reamer pilot must also fit the lands of the barrel. Here I used a special neck diameter and throat built into a 308 Winchester reamer by Manson Precision Reamers (mansonreamers.com, This .308 Winchester special reamer has a tighter 0.342 inch neck and 0.3085"x0x1-1/2 degree throat which would fit my needs for the new .30BR Magnum perfectly.

The neck was then formed on the new barrel with neck length a few thousandths longer than the .243 WSSM neck. I used a special stop on the lathe to precisely control neck length conforming to the new .30 BR Magnum cartridge. Things went very well in completing the Br oug hton barrel. The Weatherby barrel was shortened and rethreaded for a Savage Model 112 single-shot action and rechambered in the same fashion.

Testing The New Chamberings The .243 Winchester Short Magnum cartridge is a very sturdy case with 243 WSSM reamer was fitted with a special sleeve to convert it from .243 to .308 used in reaming only the chamber on the new .30BR Magnum wildcat. The chamber neck and leade were cut with a special .308 Winchester reamer. The more shallow shoulder slope prevents contact at the .243 WSSM shoulder while neck reaming. 243 WSSM reamer used to form new .30 BR Magnum chamber headspaced up to the chamber shoulder only.

The neck on this reamer is a smaller .243 requiring a special reamer to form the neck and leade. neck walls approximately 0.020 inch. I had to anneal and turn the necks to 0.015 inch wall thickness for the 0.342 inch diameter chamber neck. Redding Reloading made up a perfect set of bushing dies for me based on worked-over .243 WSSM dies necked-up to .30 caliber. I also had a body die made up. The dies worked flawlessly on these super strong, short Magnum cases.

Both barrels were completed together and ready for testing. The heavier Broughton barrel was fitted and headspaced to a 40X Remington rifle of known, fine accuracy. The Weatherby barrel was chambered for a Savage Model 112 single-shot rifle. Both were switch-barrel rifles

308 caliber barrel clearly shows the chamber was formed for the new .30BR Magnum cartridge, designed and made by the author, using two different standard reamers. Cartridge shown is the new wildcat necked up from a .243 WSSM case. 30BR Magnum. Accuracy average was around 0.250 MOA using Berger 135 grain bullets in the Broughton 1-16 barrel. The new cartridge has excellent potential. with other chamberings. Both had extra bolts as well just beckoning for anew cartridge.

Cartridge case capacity for the new necked-up Winchester Super Short case to .308 bore was very close to the same as the standard .308 Winchester cartridge using similar brand cases. Starting reloading data was essentially the same for both cartridges. Standard primers work out well as I had learned from my work with my 6.5x42mm cartridge made from the .243 WSSM. This case was capable of withstanding greater pressures than the .308 Winchester family of cartridges.

Accuracy performance delivered by the newly created .30BR MAG was little short of amazing with some bullets grouping as small as quarter MOA with the Broughton barrel. The modified Weatherby barrel as shot exceptionally well, with some groups well under half MOA. Following is an example of such accuracy from this standard weight 24 inch, fluted barrel. Berger 30BR Magnum average 0.311 MOA. A Remington Model 40X rifle was used along with a Broughton 1-16 twist and Berger 135 grain match bullets. 420 MOA.

A Model 112 long action, single-shot rifle was used, along with a shortened and rechambered .300 Weatherby barrel with a 1-10 twist. Berger’s 168 grain VLD bullets at around 2400 fps fared the best thus far with several others very close. 168 grain VLD bullet ahead of 42 grains of Norma 203B powder and CCI primer averaged 0.410 MOA five shot groups. The two 300 yard groups fired with this load were 0.358 MOA and 0.503 MOA. The cartridge would be pure poison on varmints with 110-125 grain bullets.

I loaded the case to near 3200 fps with Hornady’s 110 grain V-Max bullets and accuracy was excellent. There are other unique wildcat chambering possibilities that can be made from smaller cases to larger bores using a standard cartridge reamer and headspace gauge, along with different slave reamers of correct caliber to form the new neck and leade. It just could add a whole new dimension to your shooting. AG