Most Mosins (every one that I've seen) shoot high, its the way the Russians built the for some reason.
Actually we (the US did the same thing with our M1917s and M1903).
Anyway I'm against "external" changes that would prevent the rifle from being used in CMP GSM as issued matches.
The fix is simple, and is within the rules of the CMP.
The Mosin like most vintage military rifles has a ladder sight, with a slider. As the slider slides up and down the ladder it changes elevation.
You can remove the sight by pushing the sight pin out, (the part that allows the sight to pivot). Take the sight off and mill, file or grind, the bottom of the slider part which will caused the sight to set lower on the base.
To determine how much to remove you set the sights on the 100 mark, and shoot it at 100 yards. Mine shot 8 inches hight.
There are 3600 inches in 100 yards. You measure the sight radius and divide that number by 3600. Sight radius divided by inches to target.
If I remember right, I got .0061 on my Mosin.
What that means is for every change of .0061 of the sight, you change the impact 1 inch at 100 yards.
So I stuck the slider in my miling machine and milled .0488 (8 times .0061) off the bottom of my slider. Put it back on the rifle and shot it.
It was right on at 100, I set the sights at 200, it was on at 200, same for 300 & 400 (thats as far as my range goes).
If you file or grind the bottom of the sight, be careful to keep it straight, you don't want the sight to set cockeyed on the base.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071