Yuo can’t go wrong with any of the Holy Trinity:
FN-FAL
M14
HK93
Honorable mention - M1 Garand.
I’m a FAL and Garand rifleman. Even at my age and condition, I can still shoot in the low to nid 240s on the AQT. Good enough to knock a blue helmet or a turban off an enemy head at 500 yards.
HK-93 was 5.56mm. HK-91 (G-3) was 7.62.
They are all piston rifles and therefore less accurate than the standard M16/AR15 independent of caliber of ammo used.