4.85x49mm British red tip experimental tracer, Model XL 2 E1, headstamp RG/ 77/ 4.85
Submitted to the NATO trials (1976-80), this caliber was not selected in favor of the 5.56.  
Based on a lengthened, necked-down .223.  Load for ball round is 1.54gr nitrocellulose, weight 3.58gr and has muzzle velocity of 950 mps.
For a early SA80 rifle (XL64 E5) with the magazine feed behind the trigger/ pistol grip.
wolfganggross

Interesting. The neck seems awfully long for a modern cartridge.

Now that's a 17 that could do some damage. It just has to turn sideways.