Riddle me this...223 55gr vs 75gr POI way off

[Medic218]

Ok, so I was at the range today to sight in a new scope and I ran in to something interesting that I would like a little education on.
Here is what we had going on.

Rifle: Sig556 16" 1:7 twist barrel
Ammo: Federal BP223BL 55g FMJ
Hornady TAP 75gr
Zero: 50 Yards


Ok, so I got my new scope zeroed in with the 55gr rounds and then I decided to try the TAP round to see where the POI(point of impact) would be since I hope to do some coyote hunting soon. I figured it would be different simply because its a different, heavier round.
I send the first one and I have no idea where it hit.
I adjusted lower just to see if I could get on paper and see where I'm hitting. Once I figured it out I ended up having to put my cross hairs about 4" low and 2" left to hit my target. Once I did this I was able to put four shots in about a half dollar sized group with good 'ol Kentucky windage.
My question is why?
I knew there would be a difference in the POI at a greater distance between the two, due to bullet weight and drop, but I wasn't expecting such a significant difference in POI in the two rounds at such a close range.
Also, why does the heavier round hit so much higher than the lighter round?
Why were the two so far off from one another?

[Medic218]

Heavier bullets typically hit higher because they're slower - they stay in the barrel a fraction longer than lighter bullets, so the muzzle has risen that tiny bit more under recoil by the time the bullet exits the muzzle.

[Heartland Patriot]

1/7 twist is pretty fast and better-suited for the longer bearing-length of the heavier bullets. For lighter bullets with shorter bearing-length - a slower twist is better to stabilize them.

Heavier bullets typically hit higher because they're slower - they stay in the barrel a fraction longer than lighter bullets, so the muzzle has risen that tiny bit more under recoil by the time the bullet exits the muzzle.

I had my .357 Taurus revolver at the range one day, but was shooting .38 Sp through it (saves money). Anyway, lighter bullets hitting lower, despite only being 15 yards...now I know, thank you very much.

[MoJo]

Encyclopedia Andy strikes again!