What the hell are you talking about?
Modern rifles use smaller calibers than the guns they used in World War 2.
The only sniper rifles you're going to find on the Eastern Front (all of which can be found in RO Ost Front) are the Kar98k, the G43 (both of which use 7.92x54mm which achieves a high velocity of over 2700 FPS), the Mosin Nagant and the SVT-40 (both of which use 7.62x54mmR cartridges and achieve a velocity of 2600-2800 FPS). These cartridges travel quickly and achieve stable trajectories due to their high velocities and their high projectile weight means they aren't subject to much variation due to wind compared to lighter projectiles.
Two of those sniper rifles, the Mosin Nagant, and the Kar98k, are highly accurized bolt action rifles. No modern semi-automatic rifle achieves the same level of accuracy of a Mauser or Lee-Enfield style bolt action rifle. They have too many moving parts, granted, we've done a very good job of making them closer to the performance of a bolt action, but even the best semis fall short. The moving parts shift the weight of the rifle as the projectile is leaving the barrel, affecting accuracy.
Now, modern semi-automatics have MUCH better accuracy than the SVT-40 and the G43 would have, this much is absolutely true, but modern bolt action rifles are only marginal improvements on old-school bolt action rifles. In fact, modern sniper rifles used in the US military like the R700 are only modifications of the original Mauser design of 1898 used in World War 1 in the Gewehr 98.
To drive the point home, here's some asshole hitting a target with a Mosin Nagant at 1000 ****ing yards:
YouTube - 1,000 yards 1942 91/30 Russian Mosin-Nagant (P/U) sniper rifle
And here's the same guy with a Kar98k at 900 yards:
YouTube - WWII German 98K rifle at 900 yards
A M4 Carbine would be lucky to score a hit at a third of that distance, even with quality optics. An M-16 would be able to regularly function at 300 yards but with a shadow of the accuracy.
Even an R700 would only match the performance of these bolt action sniper rifles which are only really variations of their original turn of the century design.