. 280 British The . 280 British was an experimental rimless bottlenecked intermediate rifle cartridge . It was later designated 7 mm MK1Z , and has also been known as 7 mm NATO , . 280 / 30 , . 280 Enfield , . 280 NATO , 7 mm FN Short , and 7×43mm . It was designed by the British Army in the late 1940s , with subsequent help from Fabrique Nationale in Belgium and the Canadian Army . The . 280 British was tested in a variety of rifles and machine gun s including the EM - 2 , Lee - Enfield , FN FAL , Bren , M1 Garand and Taden gun . Despite its success as an intermediate cartridge , the . 280 British was not considered powerful enough by the U . S . Army and several variants of the . 280 British were created in an attempt to appease the U . S . Army . However , the U . S . Army continued to reject these variants , ultimately adopting the 7 . 62×51mm NATO . History . After the Second World War the British , having encountered the new 7 . 92 " Kurz " cartridge on the battlefield and noted its effectiveness , began a programme to replace the venerable . 303 cartridge which had been marked for replacement but had survived as a consequence of wartime pressures on British small arms development . The goal of the British designers was to create a cartridge that would replace all small arms in . 303 calibre including the Bren , the No . 4 Rifle and the Vickers medium machine gun with a cartridge suitable for a " light rifle " . Thus the cartridge had to demonstrate ballistic performance equal to that of a full powered rifle round and yet exhibit as little recoil and blast as possible , so that it was controllable during rapid or automatic fire . A shorter cartridge producing lower recoil also enabled the weapon to be shorter and lighter , and hence easier to use . After extensive tests by the " Ideal Cartridge Panel " in 1945 , the British decided upon two 7 mm cartridges – the . 270 and the . 276 . Both designations reflected the measurement of the distance between the rifling lands in the cartridges ' respective barrel s ; the . 276 bullet ' s actual diameter was . In order to focus their efforts , the British ceased research on the . 270 and concentrated their efforts on the . 276 . The . 276 was later renamed the . 280 even though no dimensions were changed . Recoil of the . 280 cartridge was calculated to be a little under half of the . 303 . Long range performance actually surpassed that of the . 303 , and shooters reported that it was much more comfortable to fire with the reduced recoil and reduced blast . It seemed that the British designers had accomplished their goals , and proceeded to introduce the cartridge to their NATO allies . Despite interest from the Belgians ( FN would later produce the . 280 in quantity and help improve it ) and the Canadians , the Americans were not at all interested , claiming they would not adopt a calibre under . 30 inch , or with ballistics inferior to the then - standard . 30 - 06 round . The British attempted to appease the Americans , first with small changes such as changing the rim diameter of the . 280 to the size of the . 30 - 06 ( resulting in the . 280 / 30 cartridge which was produced in large numbers and is the basis of the dimensions listed to the right ) . Later , when the . 280 / 30 was rejected by the Americans as being too weak with too great a drop in trajectory , the British and Belgians made large changes to the cartridge design . These resulted in several different variations ; one was just a . 280 / 30 with the bullet seated less deeply so more powder could be put in the case , another was a T65 cartridge case necked down to 7 mm . The different cartridges that the British and Belgians eventually came up with fired bullets at around , but with a much greater blast and recoil than the . 280 / 30 , which defeated the design parameters of the initial . 280 venture . Unsatisfied with the U . S . Army ' s response on the issue , the British adopted the EM - 2 and the . 280 / 30 as their primary rifle and ammunition in 1951 with the . 280 / 30 being re - designated as the " 7 mm MK1Z " . This effort was to be all in vain , as the Americans adopted the T65 ( later to be designated the 7 . 62×51mm NATO ) . A change of government meant that the 7 mm , EM - 2 and Taden gun projects were abandoned soon afterwards by Winston Churchill , who returned as the prime minister and desired commonality between the NATO countries . Small amounts of . 280 ammunition were later produced during the 1960s for various small arms trials . The . 280 British concept would later prove to have been far ahead of its time , as the U . S . itself adopted an intermediate cartridge — 5 . 56×45mm NATO — by the end of the following decade . Soon after America ' s large - scale involvement in Vietnam commenced in 1965 the 5 . 56 mm ArmaLite AR - 15 rifle , later standardised as the M16 , was purchased in ever increasing numbers and by the late 1960s had displaced the 7 . 62 mm M14 in combat units . After insisting on a . 30 calibre round with full - power ballistics almost identical to those of the existing . 30 - 06 , the U . S . then adopted the 5 . 56 mm intermediate cartridge , which demonstrated the emergence and dominance of intermediate cartridges on the battlefield ( the other notable one being the 7 . 62×39mm AK - 47 round ) . The adoption of the 7 . 62×51mm NATO round and the adaptation of the intermediate cartridge CETME ( later developed into the G3 ) and FN FAL designs to fire it , produced rifles that were relatively longer and heavier and had greater recoil . The result was weapons that performed well as longer - range semi - automatic rifles , but were more cumbersome and only marginally controllable in automatic fire . These guns also had a higher training burden and were not well suited to soldier s of smaller stature , again due to the recoil . Coincidentally , in 2002 the Americans developed a military calibre intended for the M4 version of the M16 family called the 6 . 8 mm Remington SPC — with similar ballistic properties to the . 280 British cartridge — which was intended to provide better ballistics than the 5 . 56×45mm . During the late 1960s , experiments took place in the U . K . to design an ideal military small - arms round . It started with calculations of the bullet energy required to inflict a disabling wound on soldiers with various levels of protection . Large calibers were found to need more energy to push through armor . A range of " optimum solutions " were produced ranging from 4 . 5 mm to 7 mm . The preferred solution was a 6 . 25 mm bullet housed in a necked - down . 280 case . While the 7 . 62 NATO required of force on impact to penetrate helmets and heavy body armor , the 6 . 25 mm needed only of impact force . It had a bullet with a muzzle velocity of and of muzzle energy . Tests found that the 6 . 25 mm cartridge matched the 7 . 62 NATO in penetration out to 600 m and remained effective for a longer distance , while producing recoil close to that of the 5 . 56 mm . However , it was not designed for very long range and its bullet was relatively light . Testing of the 6 . 25×43mm was conducted from 1969 to 1971 , when development ceased in favor of the smaller 4 . 85x49mm . Specifications . Types of bullets and colours of tips : Note : Most cartridges have been observed with a purple annulus . Several experimental cartridge cases were made out of aluminium , in various colors including orange . Known manufacturers : Performance . The following comparisons are excerpts from a manual published by the " Small Arms Group Armament Design Establishment " from the Ministry of Supply : Comparable cartridges . For . 280 : For 7 mm HV , 7 mm Compromise , 7 mm Second Optimum :