interesting little fact: No bullet can go supersonic until AFTER it leaves the barrel. How? The speed of sound when the round is in the barrel is higher due to the pressure behind the round (speed of gas is speed of gas) when the round hits the lower pressure right as it leaves, then its supersonic... just a little fun with math there.
So gravity will pull a round down ~30 ft/ sec^2 period. So the faster the round the more distance it covers before it falls whatever distance, thus flatter.
You can change the powder load of a round so that it goes whatever speed you want (within limits of course). When a round is going at transonic speed, ~90% the speed of sound to Mach 1, it experiences turbulence and effects the accuracy of a round. Thats why for long range shooting you want a very fast round so it doesn't slow below mach 1 before it hits the target. Also the faster it goes, the less wind can effect the trajectory just like with gravity.