[Born on the Storm King]

What is the caliber and maximum effective range of this rifle?

[kipj]

It's 5.56mm which is the same caliber as our military uses in our AR-16's.

[SevenDaysInMay]

Effective range of the rifle is a function of the restrictive gear the average soldier is wearing at the time. Compounding the analysis of the effective range is the potential damage done to the Brit by the AK-47/74 rounds hitting said soldier:

Maximum effective range is 15 meters - horizontal, unless the throwing arm is shot to pieces before hand. Or vice versa.