POI vs. bullet types

[TexasShooter59]

Recently, I tested some handloads and had a POI change with a different bullet type and wondered what this is telling me.

The background: I was testing some AR .223 loads at 50 yards. Same powder (Win 748), different headstamp cases, same weight bullets (55 gr.). I was shootiing different charge weights for each bullet type.

Shot a few foulers with FMJBTs - POI right around center.

Shot a number of Hornady spire points across 5 charge weights - POI generally around center.

Shot a number of Hornady Z-MAX across the same 5 charge weights - all POI groups were consistently below the center about an inch or two (don't have targets in front of me).


I am curious if this is tellimg me anything about veloctiy between the SP and Z-MAX bullets. Thanks in advance!

 

[gamestalker]

I would doubt 50 yds. would show any true variance in velocities effecting POI. Every bullet shoots different because it comes out of the barrel differently. Some bullets have thicker/thinner jackets, more or less bearing surface to lands/groves, and then you have mixed head stamps which is another variable.

I keep a log of each bullets zero at a given distance in relation to elevation and windage points of adjustment on the turrets. By doing this, I can refrence my log and make the adjustments to the turrets to zero that bullet. This saves a lot of time and wasted ammo. And if for some reason I have changed the powder charge or seating depth for a particular load, I can still be pretty sure any further adjustments will usually be minimal.

 

[rcmodel]

No, it has nothing to do with velocity difference.

The two different bullet shapes set up different vibrations in the barrel while passing through it due to frictional differences.
IE: Barrel harmonics.

Where the muzzle is pointing in the vibration cycle when the bullet exits the barrel is where the POI will be.

http://www.shootingsoftware.com/barrel.htm

rc

 

[Otto]

Change any variable and you will change the poi.
I'd be more concerned how the Z-Max groups at 100yds. BTW, a Z-Max is just a V max with a green plastic tip.

 

[TexasShooter59]

Thanks for the replies!

The thing that was bothering me about this was hearing one of those statements you hear at the range, "if it's lower on the paper, it's moving faster" (for sighting in at close range). I've never been able to reason that out in my head, but I've heard it more than once from different people. So I had to ask myself if that was the case here. I did not mention it to begin with to prevent any preconceived notions!

Barrel vibrations, bearing surfaces and so forth make much more sense.

This is just the initial testing; I'll be taking the SP and Z-MAX out to 200 eventually. The Z-MAX did show a group less than an inch at 50 yards and this was through a regular M4 barrel.

Thanks again for the responses!

 

[x_wrench]

what i have learned is that in order to determine actual change of impact differences, is you have to do things scientifically. first, you need to do something with the largest variable of all. you. the rifle needs to go into a rest. then, change only one thing at a time. if you are interested in knowing the poi of a set of bullets, use the exact same cases, charge, powder, primers etc. now, if you are interested in which gives you the smallest groups, then you will have to work things up accordingly. pick a bullet, and again, change one thing at a time. it takes a lot of time, shooting, and money to find the absolute best load for a particular rifle. and every time they introduce a new powder, there is another full set to experiment with. the search for the most accurate load is almost endless. there is always one more thing to try. most of us find a really good load, that we are happy with, and stick to it. me, i search for sub moa loads in every rifle. when i find it, i buy a couple of boxes of those bullets, and keep them set aside for that load. i also weigh each case, and every charge of powder.

 

[popper]

"if it's lower on the paper, it's moving faster"

true for pistol, due to bbl flip. Reverse is true for rifle at long range. Free floating AR - bbl whip doesn't sound right - why didn't it go down and to the right, or left? I would suspect the bullet or a loading error. Try canting the rifle 20 deg and see where the bullets go, you have to compare both.

 

[rcmodel]

bbl whip doesn't sound right - why didn't it go down and to the right, or left?

Because barrel harmonics are not linear.

The muzzle actually probably makes a 360 rotation around the bore line.
Where it is at at the specific point in time the bullet clears the crown is where the group will land on the target. It could be high, low, left, or right. or a combination of two of them.

Developing an accurate handload is really done to tame or time that bullet release in the exact same harmonic mode each shot.

rc