Above is the M1, a fully powered rifle just like the ones America used to win both world wars. Below is an AR-15, an intermediate cartridge rifle used in America's infamous proxy conflicts.
The intermediate cartridge was used in WWII for smaller, carbine rifles for officers and rear echelon troops. It wasn't considered suitable for front line infantry. Rifle technology has improved a great deal since WWII, but calibers for standard issue rifles became smaller.
The modern rifleman can be far more accurate and deadly with a modern fully powered rifle and optics. The M1 Garand weighs 11.5 lbs fully loaded. A fully loaded AR-10 is about the same, but is far more deadly with modern manufacturing standards, optics, and attachments.
Even the new M7 chambers an intermediate cartridge. A modern rifleman could be so much more, technology has improved to allow it. But instead we have people running around with carbines fighting at 1940s ranges dumping ammo.
To this day, the guerrilla sniper remains the bread and butter of guerrilla armies as well as the bane of conventional forces in these costly proxy conflicts. If the rifleman evolved to a greater range and armor piercing power, then the entire force becomes a far more formidible threat.
What constitutes a "marskmen" is subjective when fully powered rifles are now relegated to their own niche roles. The standard can be raised.