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?

 

[M. Sage]

Did you try the other ammo at the same time? It might not be the ammo change throwing you off, it could be something in your rig itself.

 

[Medic218]

Yeah, I loaded up 20 more 55gr and was in about a 3" group on the cross hairs.

 

[mikeofcontex]

You can search the Inet and find much more eloquent explanations... bullet delivery is not a simple event. From the moment you apply pressure to the trigger until the bullet impacts the target, many things occur. The issues pertaining to your situation are the swollen ring of steel that chases your projectile down the barrel and the straightening of the barrel that occurs due to the same pressure.

The barrel is drooping slightly due to gravity (at rest). When pressure is applied, it straightens... my best mental picture is like the party toy that unwinds when you blow through it. There is actually an oscillation up and down and probably side to side (it's complex movement). Depending where the muzzle is when the bullet exits has everything to do with POI. There is also a sound impulse that "bounces" through the steel of the barrel. I'd love to point you to a more detailed explanation, but I can't remember where I once found it.

Bottom line... POI has everything to do with all of the components of the cartridge AND the rifle in which it is shot.

 

[40Arpent]

In addition to the previous posts, the Hornady ammo has a lower muzzle velocity, so when it exits the barrel, muzzle rise is greater than it was with the Federal ammo.

 

[scap99]

Harmonics at play...
http://www.varmintal.com/atune.htm

Sent from my DROIDX using Tapatalk

 

[Charlie]

Yeah, I loaded up 20 more 55gr and was in about a 3" group on the cross hairs.

With handloaded ammo (or even good factory ammo) you should be getting better groups than 3" at 100 yds. with a scope. That's not to say you are doing your job shooting only to say that your particular rifle may not like that particualr ammo or handload. It's been my experience that a decent .223 with a 55 grain pointed bullet should shoot at least a 1 1/2" group, expecially after handloading. Like Mike said, there are a lot of things going on when you shoot and harmonics has a lot to do with the barrel/bullet/load and your group sizes. Just my two bits.

 

[Medic218]

With handloaded ammo (or even good factory ammo) you should be getting better groups than 3" at 100 yds. with a scope. That's not to say you are doing your job shooting only to say that your particular rifle may not like that particualr ammo or handload. It's been my experience that a decent .223 with a 55 grain pointed bullet should shoot at least a 1 1/2" group, expecially after handloading. Like Mike said, there are a lot of things going on when you shoot and harmonics has a lot to do with the barrel/bullet/load and your group sizes. Just my two bits.

Someone suggested that the twist might be too fast for the lighter round and I'm thinking he may be on to something seeing as how I had about a 1" group with the TAP rounds.

 

[mikeofcontex]

If the bullet is "too soft," it can begin to misform or even disintegrate on the way to the target. 1 in 7" is faster than necessary for a 55g projectile.

 

[scap99]

I've never had any issues shooting 55 grainers in my 1:7 twist AR's, but every rifle is different.


You could be experiencing some accuracy issues from a .223 case being fired in a 5.56 chamber.
5.56 VS .223 Ammunition and Chamber Dimensions - AR15 Armory.com
The first post on the link above explains this better than I ever could.

 

[tmd11111]

Heavier bullet and slower muzzle velocity causes the muzzle to rise more before the bullet leaves the barrel. This is why POI is different with the heavier bullet.

 

[Vaquero]

^ and harmonics. As stated above in numerous replies.

 

[M. Sage]

Gotta be more harmonics. That kind of difference is very extreme for such a short range, and the velocities aren't all that far apart...

That kind of shift still seems like a lot to me, though.

 

[woolleyworm]

good info in previous posts. I will add that whenever I'm doing a sight-in; bring LARGE targets or create a large target area with smaller ones if at all possible. Alot depends on the size of your backer boards, but go as large as you can.

1:7 is very fast for a 55gn and 50 yards, the ballistics for each bullet differ. Easier to link to some graphics and charts so that you'll see what I mean,

Rifle Trajectory Table compare same calibers vs different wts. and you'll see.

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FREE Remington 2009 Catalogs and Ballistics Sheet « Daily Bulletin click on the ballistic link

 

[woolleyworm]

Gotta be more harmonics. That kind of difference is very extreme for such a short range, and the velocities aren't all that far apart...

That kind of shift still seems like a lot to me, though.

I'll agree, Barrel harmonics are very real and that's why I use the Audette ladder test method to initially start my load work-ups. You'd be amazed at the little things that can make a difference.