You wrote "Though details on the precise specs of the C15 rifle are surprisingly tricky to find."
Rubbish, stop using self censoring UK google and use DuckDuckGop for example. There are loads of websites providing detailed info on tactical rifles etc., not least of which is the manufacturer's own.
And UK British army kit... British Armed Forces AWM 338, designated L115A3
So, why would you write that? ♥