mini

What if, I could invent a bullet, and it'd be the same size as the 7.62x51 we use now, but instead of effective range of 800 yards, it would have an effective range of two miles, for the average skilled user?

That would be a game changer as soon as we were able to see that far.  

But that's the game changer of the Minie Ball, which came out just before the Civil War.  An order of magnitude better than the standard spherical ball.    

I first learned about these in Civil War History, in college, of course, and it's stayed with me ever since.  Able to be loosely packed in a rifled barrel, the looseness defeated fouling from residue.  But it could still be muzzle loaded as quickly as a non-rifled musket patch/ball.  The cupped base made the round expand on firing pressures, to engage the lands and grooves and thus spin from the rifling.  Hence the performance jump for standard issue small arm.  Not small, Minie is a French guy's name.  The French were great military technologists with poor timing.  They never seemed to get their advances in and deployed late enough to be of use to only them, and not improved upon by the Germans.  Not after 1815.