why such a big difference in point of impact?

[btmj]

I would like some thoughts and ideas to explain something that has bothered me for some time now.

My 243 was originally sighted in with 100 gr. Remington core-lokt’s. I don’t own a chronograph, but Remington claims 2960 fps muzzle velocity. The weapon is a weatherby vanguard with 24” barrel. I sighted it at 1 inch high at 100 yards, which works pretty well for white tail in dense Missouri woodlands.

I recently wanted to try some alternate ammo... I tried Hornady 100 gr. Interlock, which just like the Remington’s, has a 2960 fps muzzle velocity. As expected, the point of impact was virtually the same..... but not quite. Averaging over 16 rounds fired, the Remington’s averaged 0.75 inch higher at 100 yards than the Hornady. If I plug the data for both bullets into a ballistics calculator, I get the same results for both the Hornady and the Remington at 100 yards.... now way out at 400 yards plus, the difference in BC result in different predicted hold-over, but the calculators say that at 100 yards these two should be the same.

Then I tried the Hornady super performance 95 grain rounds. These have a muzzle velocity of 3185 fps. The ballistics calculators say that, at 100 yards, these rounds should be ¼ MOA higher than the slower 100 grain bullets. But I consistently shot 2.5 inches higher than the 100 grain bullets. I had nice groupings, they were just way high. This was averaged over 12 rounds.

I am trying to understand the physics here... Given the same inclination, the 2960 fps bullet has about ¼ inch more drop than the 3185 fps bullet at 100 yards. So what is causing a 2.5 inch difference?

 

[bamaranger]

each to its own

Why rifles and loads do what they do is one of those arcane things of riflery.
Each rifle and load often has its own personality. Which is why we bench and experiment before we shoot at game or for the record.

Sounds like you have done your homework.

 

[Mike38]

Rifle barrels vibrate as a bullet passes down the bore. Vibrations can be different from one ammo brand to the next, even if the bullet weight is the same.

 

[DPris]

Ballistic calculators & programs should be taken as rough guidelines only. There is no program ever devised that can take the place of actually shooting a given cartridge/poeder charge/bullet through a given gun.
Denis

 

[Abel]

Ballistic calculators & programs should be taken as rough guidelines only.

This. The rifle acts differently with the different loads. You should be grateful that your rifle's deviation is within such a small margin. Some rifles sling different pills all over the paper, creating a need for drastic scope adjustments. Pick a load and zero for it. Buy plenty of that ammo or learn to reload to that ammo's spec.

 

[jmr40]

The ballistics calculators are only accurate if you plug in the "EXACT" velocity you are getting, and if you zero at the range you plug in the program. I know of no program that works with a "1" high at 100 yard zero." To be accurate, you have to determine the EXACT range your rifle is zeroed. It is much easier to just zero at either 100 yards or 200 yards.

Without a chronogrph you have no idea what velocity you are actually getting in YOUR gun. I've seen velocity vary by as much as 100 fps with 2 different guns with equal barrel lengths firing ammo from the same box.

The Superformance ammo is hitting higher at 100 for seveal reasons. #1 it is moving faster. This means more recoil and the muzzle is rising slightly more before the bullet exits.

All rifles have the muzzle about 1.5" lower than the scopes crosshairs. The muzzle is angled slightly upward in relation to the butt. This means the bullet leaves the barrel angled slightly upward. With most guns somewhere around 50 yards or so the bullet will intersect with the line of sight and rise slightly above it. Once it reaches it's peak it will drop back down and cross the line of sight a 2nd time. This is the range at which your rifle is zeroed. Because of more muzzle rise and faster velocities the Superformance ammo will cross the line of sight the 2nd time much farther away, unless you re-zero for that ammo.

You will have to zero with the ammo you plan to use. Using different ammo is OK as long as you actually shoot it to determine how much different it will be from your primary ammo.

 

[DPris]

Calculators also don't allow for elevation (atmospheric density) or humidity effects, among other variables such as bullet jacket thickness & bearing surface and how the bullet engages rifling between brands, etc.

Also, you can never count on a maker's published velocities. Some use vented pressure barrels in obtaining them, others use more realistic non-vented test barrels.
Their test barrel may have been longer or shorter than yours, heavier or lighter.
They may be testing at different elevations than where you live.

In my experience, factory figures seldom match with what I get through my own guns.

As JM said, you need to know for sure what velocities really are, and even then you still can't depend entirely on a calculator or program.
Denis

 

[btmj]

Thanks to all...

JMR40, your analysis makes sense to me. Thanks for taking the time.

Of course I have noticed this same phenomena with every rifle I have ever shot. But it wasn't until recently that I began to think about real world results compared to ballistics calculators and think... what the hell?

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So now another question... Let's say I sighted in on a 75 degree day, with a warm barrel because I have been shooting 1 round every 3 minutes for an hour. Now I take aim on a 22 degree morning with a cold barrell. I am assuming my point of impact will be different, but how much ? 1/2 MOA? 2 MOA? (God I hope not)... any experience from anyone ?