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Before the 223/5,56mm cartridge
Attachment 122362Attachment 122363The US Military had been testing cartridges using 224 diameter bullets during the mid 1950's.
These included the M1 carbine cartridge necked down to 224 diameter and others including the 22/7,62x51 cartridge
Around 1957, designers lengthened the 222 Remington case with a new 55 gr bullet at 3200 fps and a redesigned AR-10 rifle into a small scale called the AR-15 chambering this cartridge.
The new 1958 cartridge was called the 222 REMINGTON SPECIAL (later changed to 223 Rem)
Winchester developed a cartridge close to the 222 Rem Special and used a breech design and gas system very close to the M1 carbine. The 224E2 Winchester in 1957 fired a bullet at 3300 fps and a steel bullet at 3600 fps.
Both the Armalite AR-15 and Winchester Lightweight rifles were tested by the US Army. Final section was the AR-15 and the new 223 or later 5,56mm military
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The 222 Remington magnum still lives but with a 5mm bullet
The Ruger 204 cartridge is nothing more than the 222 Remington magnum necked down to 204 cal (5mm) with the shoulder moved forward, case length is 47mm. It has been reported to be very accurate and the AR-15 series are offered in 204 Ruger by some custom builders.
I should also include the 4,85X49mm British militaryAttachment 120682 cartridge too, also a 5mm bullet on a longer 223 case.
Photo shows 4,85 tracer, 224 Winchester and 222 Rem Special
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photos of 224E2 Winchester and 222 REMINGTON SPECIAL
enclosed are photos of both the Winchester and Remington 22 cal cartridges. After testing the Remington was selected and became the 223 Remington with the military version called the 5,56mmAttachment 122364Attachment 122365
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posting photos that were lost of 222 Rem-Spcial and 224 Winchester
The 222 Remington Special was a Stoner design and tested during 1954-56, this became the Rem 223 and military 5,56mm (there is a slight difference between these two cartridges).
The 224 E2 Winchester was tested during the 1957 trials and loaded to 3300 fps.
Also shown is the British 4,85x49 mm (tracer) for comparisonAttachment 123325Attachment 123326Attachment 123327