7MM Medium Cartridge

[Tequila jake]

Is anybody familiar with the "7MM Medium" cartridge? As far as I know, this is the actual name of the cartridge, not a description. I believe it was intended as a postwar assault weapon cartridge by the British, but was never put into service. If anybody has any info such as case length, bullet weight, muzzle velocity, bc, sd, etc., I would appreciate it.

There is a picture of it, but no info that I could find, on this website:

http://www.quarry.nildram.co.uk/Assault.htm

Tequila Jake

 

[Tony Williams]

Yep, that's my site

The story of the postwar British/Belgian 7mm family is told in Assault Rifle: the Development of the Modern Military Rifle and its Ammunition which I co-authored with Max Popenker of the guns.ru site. This is an extract from the book:

"Next in the field came the British, who in 1945 set up the Small Arms Calibre Panel in order to determine the optimum cartridge for a lightweight rifle. After many calculations and experiments mainly involving rounds of between .25 to .27 inch calibre (6.35-6.8 mm), they reported in 1947 in favour of further development of two alternative designs. One was a .27 inch (6.8 x 46) firing a steel-cored 100 grain bullet at 2,750-2,800 fps (6.5 g at 840-850 m/s), which still retained 81 ft lbs (109 j) of energy at 2,000 yards (1,830 m), a significant figure as the estimated energy required to inflict an injury to an unprotected man is around 60 ft lbs (80 j). The other was a .276 (7 x 43: later redesignated .280 to avoid confusion with earlier cartridges) which was tested with bullets weighing between 8.4 and 9 grams (130-140 grains) at between 747-710 m/s (2,450-2,330 fps). The 130 grain/2,450 fps loading had a retained energy of 100 ft lbs at 2,000 yards (135 joules at 1,830 m). Eventually a loading of a Belgian-designed 9 g bullet at 736 m/s (140 grains at 2,415 fps) was decided on. The .280 calibre (actually 7 mm, with a .276 inch bore and .284 bullet) was a little larger than was thought ideal but it was selected for further development, reportedly in order to try to meet American preferences for good long-range performance. For the same reason, the original case rim diameter was increased slightly to match that of the American .30-06 to enable them to rebarrel existing guns more easily, leading to a change in designation to .280/30."

and:

"Interestingly, American opinion on this subject was far from united. The EM-2 and its .280 cartridge were thoroughly tested in the USA in 1950 alongside the FN FAL in the same calibre (the Belgians being enthusiastic supporters of the British concept) and in comparison with the American T25 prototype rifle. This was chambered for the original "T65" cartridge, a 7.62 x 47 round which was the first stage in the development programme which eventually led to the FA-T1E3 case (and associated T65E3 ball loading), later adopted as the 7.62 x 51 NATO. Testing took place at Fort Benning where the US Army Infantry Board was based, and the tests were expected to result in the choice of a new standard rifle cartridge for NATO.

At Fort Benning, the Trials Board reported on the cartridges as follows:

"That the T65 Cal .30 is not satisfactory because of its excessive recoil, blast, flash and smoke. That the Cal .280 is not satisfactory because of its comparatively high trajectory. That of the two basic types of rounds submitted for test the British calibre .280 is preferred."

The detailed findings from the Fort Benning tests showed that while the T65 had a flatter trajectory and produced more severe wounds at ranges of less than 1,000 yards (900 metres), the British round became more effective at longer ranges because of its superior ballistic coefficient. At 1,000 yards the .280 could penetrate body armour 70% of the time, compared with 60% for the .30. The British cartridge also produced considerably less flash and smoke. Most significantly, while the T25 was found to be the more accurate rifle and achieved more hits per minute when fired from a bipod, the EM-2 was far superior in this respect when fired from the shoulder.

Clearly, the British designers had achieved all that they had aimed for, but the Trials Board recommendation to focus development on the .280 cartridge was rejected by the Chief of Staff of the US Army. This was due to the clear preference of the Ordnance Department and the American senior military, political and industrial establishment in favour of a full-power .30 calibre rifle of US origin.

The British felt that the Aberdeen trials should have settled the matter so didn't give up easily. They set about meeting the American objections by producing more powerful versions of their cartridge, with the support of Belgium and Canada. The first change was to upload the 43 mm case to 2,550 fps (777 m/s) with the 140 grain (9 g) bullet, to meet the criticism of the trajectory and also to address complaints that the low temperatures of Arctic conditions reduced the performance to an unacceptable level. This raised the energy remaining at 2,000 yards to 126 ft lbs (170 joules). However, the British cause was severely damaged by a change of government, which led early in 1952 (reportedly followed a meeting between the US President Truman and Winston Churchill, the new Prime Minister) to a decision to rescind the adoption of the EM-2 and its 7 mm cartridge before any had been issued.

Despite this setback, Britain, Belgium and Canada combined (in the 'BBC Committee') to make one last attempt to develop a new 7 mm round which would be acceptable to NATO. Various lengthened cartridges with such designations as 'Optimum', 'High Velocity', 'Compromise' and 'Second Optimum' were developed, mostly with 49 mm cases although the final attempt was simply the 7.62 x 51 necked-down to 7 mm. Muzzle velocities were in the range 2,750-2,800 fps with the 140 grain bullet (9 g at 840-850 m/s). However, the Americans would not be convinced. In any case, the recoil had by this time increased significantly and the balance of the original EM-2 concept had been lost. At the end of 1953, the BBC Committee reluctantly bowed to American pressure and the 7.62 x 51 was formally adopted as the new NATO cartridge.

The only result of all of this effort was a 7 x 49 cartridge, known as the 7 mm Medium, which saw service in an FN FAL selective-fire rifle which was sold to Venezuela."

Actual ballistics of the 7mm Medium were 140 grains at 2,590 fps.

Tony Williams: Military gun and ammunition website and discussion forum

 

[Tequila jake]

Mr. Williams,

Thank you so much for the information concerning the 7MM cartridge and the .280/30. I have fired quite a few rounds from M1 carbines, AK-47s, AKMs, AK-74s, and M-16s, but I'm still far from an expert on assault weapon cartridges. However, just from the looks of it (if anyone can determine effectiveness from appearance!), the 280/30 would seem to be an optimal assault weapon cartridge. It's too bad that the U.S. was so hidebound and traditional that they didn't accept it. I believe that, if we are going to go with just one cartridge for both an assault weapon and a main battle rifle, something on the order of the 6.5 Grendel, the proposed 6.8, or the 280/30 would be ideal. Using a bullet with good sectional density, a high ballistic coefficient, and a reasonable velocity should fill the bill.

Another question for you: weren't there some experiments with a 6.5MM version of the 6MM SAW with a view toward using it as an assault weapon cartridge?

Tequila Jake

 

[HorseSoldier]

The British .280 cartridges are ballistically very similar to 6.8mm SPC. The .280 round used a heavier bullet a little slower, 6.8mm SPC a lighter bullet a little faster, but muzzle energy is very close. There is not a whole lot of new going on in the world

 

[Tony Williams]

Another question for you: weren't there some experiments with a 6.5MM version of the 6MM SAW with a view toward using it as an assault weapon cartridge?

I've not heard about that one - which doesn't mean it didn't happen!

Tony Williams: Military gun and ammunition website and discussion forum

 

[HorseSoldier]

Like Mr Williams said, I'd think it might have been possible, but I doubt it was very serious if they happened. OAL won't allow that 6mm SAW cartridge to feel through an AR magwell, so it would have meant scrapping M16A1s or A2s (depending on when it was done) by the truck load.