I was shooting a test load last time, gun is a Swedish M96 6.5X55 using Sierra MatchKing 140grn. HPBT and 43.0grn of IMR 4350. Range is 100m

As I shot the load the recoil is remarkably less than when I use a same-load flatbase bullet. The group strikes 3 inches lower than the flat-base group.

When I ask my gunsmith, he said the matchking produce a flatter trajectory so the bullet is still climbing to meet the line of sight. I think not, the muzzle is about 3 inches lower to the scope, the bullet either meets the trajectory and drops very fast or never climb up to meet the line of sight at all.

I know different barrel vibration produced different groups but the obvious low recoil tipped me off that something has to do with pressure of that load.

Anybody can confirm this thought?

Only one round? Hard to come to a conclusion when only one round was fired...

But...it more than likely was a light load...

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IMHO

I think a major contributer is the lesser recoil felt with the MatchKing bullet fired in this example. If the rifle recoils significantly upward the round with the greater recoil would have the higher Point Of Impact (for similar barrel time). What were the velocities? If the MatchKing is faster than the other round this also effects recoil induced elevation.

The lower "pressure" between the boattail and the flatbase may just be the difference in bearing surface. You might also want to measure the bullet diameter with a micrometer, this could be revealing.