From Nick on Patreon:
"What small arm of the 20th century do you think got the most updates and changes from first production model to last? Was the effort worth it? Or should this country/company have adopted an entirely new design at some point before that last production?"

After initially jumping to the Mosin Nagant and then the Enfield, I realized that I think this really comes down to a question of AR vs AK. Both of those two rifles have been adapted and redesigned (updated and changed) for use in a wide variety of different roles. Ultimately I think the AK takes the prize on this question, with these variations:

Original AK-47, intended to be used as a submachine gun.
AKM, completely redesigned receiver and intended to be infantry rifle.
RPK, Heavier barrel, new trunnion and stock to be a squad automatic weapon/LMG.
PSL/Yugo M76, larger caliber, serve as designated marksman rifles.
AKS-74U, new top cover and sights, much shorter for use as PDW/SMG.
And plenty more less significant examples...

The shirt, BTW, is from Otte Gear...