Yaaaa check your screws, make them snug, pull the rearsight slide all the way to the rear and make your hold at the 6'0clock position of your target.
It does sound like your shooting high.
Light ball , 147gr. Silvertip or no color on the tip is "The " Mosin rifle round.
Heavy Ball, with yellow tips is machine gun ammo, made for firing over advancing troops as support during attack.
At 25 yards there will be no difference, but at 600+ the light ball will drop off much quicker than the heavy stuff.
Machine gun ammo isnt always the best stuff, it was made for a "Beaten Zone" type of fire rather than aimed fire.
When the Communist Block issed semiautos to designated marksmen, the light ball was already 50 years old, and the heavy ball 40 something.
Fact is, the Mosin can shoot both types, though a PSL or a Dragonov should only use light ball.
Maxims worked better with heavy ball, but the PK series did well with light ball, so when the Soviets phased out the watercooled machine guns, they dropped heavy ball as well and quit coloring the tips of issue 7.62X54r
My Mosin work very well with Czeck light ball, laquerd cases and silver tips.
A couple dont mind any ammo I put through 'em....