The 6mm Musgrave caused quite a stir in South Africa when Musgrave commercialized it in 1969 as a South African development. Although true from a South African perspective, the wildcat version had by then already come and gone in the rest of the world.
Australians are reported to have been the first to have used the .303 British case as basis for wildcats in a variety of calibres. After the advent of the .243 Winchester it was a logical step to neck the .303 British case down to .243" as well, and so a wildcat known as the .243-303 or 6mm-303 was born.
In Canada, gunsmith Ellwood Epps developed an improved version of the .243-303 by blowing the shoulders out, while Chicago gunsmith, Paul Haberly, did the same in the USA. This all took place during the late fifties. The advantage of the improved version being better ballistics and case life, but they do not function through Lee-Enfield/Metford magazines and magazine capacity is limited to one or two cartridges. Enfield P-14 rifles can however be modified to feed the .243-303 Improved well.
The .243-303 and the 6mm Musgrave are not hundred percent identical as some dimensions vary, but this is not an uncommon phenomenon. Depending on the reamer used for the cutting of the chamber - these two cartridges are at least interchangeable and rumor has it that South Africa’s PMP (Pretoria Metal Pressings) arms factory exported a considerable number of these cartridges to Australia at one stage.
In South Africa the idea to develop the cartridge originated when the late Benjamin Musgrave (1900 -1978), ‘Oom Ben’ (uncle Ben) realized the need to re-commission the large quantities of .303 service rifles in private possession by means of a more modern hunting cartridge. The newfound popularity of the 6mm cartridges at the time prompted the idea of necking the .303 down to 6mm calibre.
Musgrave approached Chris Hafele, the then managing director of Armscor (now Denel) to make unused production facilities at PMP available for the manufacturing of the ammunition. The technical development on the 6mm Musgrave was done by Stephanus (Fanie) Christiaan Luther, who attempted duplicating .243 Winchester ballistics insofar possible. In final configuration, the Musgrave fell somewhat short of .243 performance, but reigned for many years as the only commercial cartridge of African origin.
The rimmed 6mm Musgrave, with its now outdated tapered body, offers about 50% odd more case capacity than six millimeters based on the .222 family, but due to its relative low pressure level of 46,000 psi (for Lee-Enfield rifles), it forfeits some of the capacity advantage. Why 46,000 psi is the pressure limit I do not know, but it may have to do with military specifications to keep pressures sufficiently low to operate in all temperatures and battlefield conditions. The weakest link in any rifle system is the cartridge case. We all know that cases can handle pressures way in excess of this pressure level and the Lee-Enfield action has been proven to handle .308 Winchester pressures. Pressure specs for this cartridge could therefore have been set higher – but it has not.
The cartridge feeds and extracts smoothly in the Lee-Enfield. The most logical modern bolt action for the 6mm Musgrave cartridge is the Enfield P-14 rifle as its extractor, magazine and bolt face are perfectly suited to a cartridge based on the .303 and it can handle modern pressures. But it is massive action for what effectively is an intermediate length cartridge. Enfield Mod-1917 rifles require some modification. By fitting a slanted magazine to a Mauser M-98 action such as those on the so-called Siamese Mausers and those magazines Rigby used and depicted on page 94 of ‘Mauser - Original Oberndorf Sporting Rifles’ by Speed, Schmid and Herrmann, this cartridge can be used in it. The best Mauser version for the 6mm Musgrave being the intermediate length version.
By modern standards the rimmed and tapered case of the 6mm Musgrave is outdated and prone to stretching. The cartridge was never intended to extend the parameters of cartridge design. It was simply intended to re-commission shot out Lee-Enfield rifles through rebarrelling and in that regard it served an excellent purpose.
Performance falls just short of the .243 Winchester. The latter is to the 6mm Musgrave what the 6mm Remington is to the .243 Winchester.
Differences are not sufficient to warrant any other application of the 6mm Musgrave than of the .243 Winchester. Readers are therefore referred to the Performance discussion of the .243 Winchester for this cartridge.
The riflescope recommendations for the 6mm Musgrave differ from those made at the .243 Winchester. The reason for that is not ballistics. The cartridge was envisaged as a cheap conversion of shot-out military rifles. This type of rifle normally serves as a ranch working rifle. It is thrown into a truck and left in the vehicle where it gathers dust and where it is subjected to endless abuse.
The space where holes are drilled for the rear scope base on a number of Lee-Metford models is not a sturdy as one would like to use either. This means that big riflescopes generating lots of inertia should be avoided.
My idea of a riflescope for a Lee-Enfield conversion to 6mm Musgrave is an the fixed power Weaver K4 steel tube scope. It is strong, simple and reasonably small. If however you are building yourself a fancy rifle – use the riflescopes recommended at the .243 Winchester.
The cartridge is not difficult to reload, except that case stretching and growth has to be attended to on a regular basis. Accuracy from Lee-Enfield rifles is not as good as from P-14 Enfield and other bolt action rifles, but most properly assembled 6mm Musgrave Lee-Enfields deliver around MOA accuracy with suitable handloads.
The cartridge prefers extruded propellants such as IMR – 4350 and Rottweil R-904 on the faster side of the slow burning category. Components are easy to come by but reloading dies often are special order items.
Making a 6mm Musgrave case from a .303 British case is a simple operation. Anneal the neck and shoulder area and run through the 6mm Musgrave die. Trim if necessary, check the neck wall thickness and Bob’s your uncle.
The only sources known to us at SAS which are accessible to American handloaders are the Australian Defence Industries (ADI) website. ADI manufactures Hodgdon propellants.
1955 / 1969 Country
Australia/SA Relative Case Capacity
49.9 gr water RCBS Shellholder
7 Groove Diameter
.243” Bore Diameter
.237” Proof Barrel Twist
1:10” Groove Details
6 x 0.068” Max Average Pressure
46,500 psi Case Trim Length