The Lee Enfield Rifle MK 1-5 has been used throughout the world, and as you say, modified to take different calibres other than .303; I have even heard of it being bored out and used as a riot control 410 shot gun!
The fact that the SMLE was still being used, through out the world, as a sniper rifle long after the SLR was introduced goes along way in showing its pedigree.
Whilst I was in Patrols Pltn we had regular training and range days with the L96. Although I never did the Skill at Arms or Sniper course I was still quite tasty with it.
Even now I can remember the Sniper's telephone number
The L96 is probably one of 3 weapons that I would rate still used by the British Army. We use to hit targets (fall when hit) over a mile away, the round was spent almost, but it was still semi accurate at that range - it was a fu**ing good piece of kit.
The GPMG in 2nd place, the blokes named it the widow maker and rightly so.
The HK53 was the other (again in Patrol Pltn), typical German made, seriously well engineered weapon. No stoppages with the NATO 5.56 (unlike the SA80 with the same round) could use the American 5.56 with out stoppages too (unlike the SA80). The bolt was an absolute nightmare to strip down and took a lot of practice to master.
I think I would rank the SA80 just above the 84mm Carly G, which should have been issued with its own Joe Crow to carry it. It was still referred to as 'Anti Tank' in my day, but would just about strip the paint work off a T72. We used it for bunker busting and considering the weight of the thing, it was stupid.
(I remember a Gobby Crow be shot in the arse with the 84 on TPTP once, f*cking hilarious!

Those were the days, can you still beast new blokes and Gobby Crows like that and get away with it??)