Took my 30-06 to the range today and at 100 yards, Federal Premium 165 gr TSX's hit the paper at 1.5 inches high. Stated MV is 2,800 fps. Three shots formed a perfect, 3/4 " triangle.

I then shot Federal Premium 180 gr TSX's and they hit 3" high. Stated MV is 2,700 fps. Three shots, all touching.

I would have expected the opposite as far as impact heights.

I'm curious if anyone has an explanation as to why a heavier bullet weight with a slower MV hits the paper 1.5" higher?

I simply can't answer that one..

I may be wrong, but I think the slower bullet spends more time in the barrel which is rising due to recoil. Thus, the heavier bullet impacts higher at 100 yards.

Please set me straight if I am off base.

I've found that to be true in handguns but not usually in rifles.

Let me make a stab at this...and I'll defer to the engineers & physicists....

The 165 gr. bullet probably gets to the 100 yard target quicker than the 180. As the flight time to target is less, the 165 hasn't reached the top of its trajectory arc (which may be at 150 yards or so).

At 100 yards the 180, the other hand, may be approaching the high point of its trajectory (or already there).

One way to know for sure is to shoot both bullets at intervals of 100, 150, 200, etc.

Which is what make it both fun & so very maddening!

Good luck,

friar

exit time difference and variance in vibration from one bullet and load to another. roger

I'm surprised there's that much difference as well. I agree it's probably more about the trajectory but wouldn't expect it to be that much.Ballistics calc

It is true that whether from rifle or pistol, the longer time the projectile is in the barrel will increase the departure angle due to the rising muzzle caused by recoil.

Whether that makes a heavier bullet impact higher on the target depends on at least a couple of other things...

1. How far away the target is...a long time of flight means gravity has more time to pull the bullet down toward the earth.

2. The muzzle velocity of the bullet...a lower velocity again means gravity has more time to pull the bullet downward before it reaches the target.

There are a number of other factors involved as well, but those two are the significant ones for most of us.

Basically, if the time of flight is not long enough for gravity to offset the higher muzzle angle, the bullet will hit higher on the target. And if the time of flight is more than enough for gravity to offset the higher muzzle angle, then the bullet will strike lower.

Thanks to everyone for your responses.

I wasn't losing sleep over it, but a lot of head scratching going on.

quote:

exit time difference and variance in vibration from one bullet and load to another.

+1

Without chronographing both rounds you probably can't assume that as velocity, particularly in factory ammo is seldom correct.

But more time in the barrel will raise POI it has been reported by Whites Lab and others. barrel vibes can also make a difference..

The old addage that a barrel is an enity unto itself makes the most since as I have seen two barrels of the same caliber vari as much as a 131 FPS on one ocassion and many times as much as 50 to 75 FPS.

Harmonics

As others have said it could be the recoil, barrel vibration.

Was only a total of 6 rounds fired? IMO, that proves nothing. No matter how they clustered. And taking the velocity posted on the box as anything other than a very vague guessimate isn't a good idea.

I would also add that each bullet is an entity unto itself. If it was not, why would type A 150 grain bullet at the same velocity as type B 150 grain bullet impact 6" or more from each other? I found this to be true for Partitions and cup and core bullets in some cases. I can't give the physics, but it is.

Larry,
I might add to your post that a lot of than can be about bedding of the rifle in many cases, and stresses on the barrel in others, and yes its not at all uncommon.

Some guns will shoot all bullets to the same POI regardless of shape, weight or velocity within certain and reasonable perimeters, those are the keepers.

Barrel Harmonics (and where each load leaves the barrel in it's movement) causes this.....I'd shoot over a chronograph and take what your rifle likes best. Then shoot at longer ranges.....

Cheers,

Dan

All are correct.
And, don't forget to add Spin Drift. The bullet is spinning so fast,as in Thousands of revelations per second, the heavier bullets gyroscopic forces tend to "throw" the bullet up and too the right
(depending on left/right rifling twist).
The longer the bullet, the more you will see this at close range. There is actually a lift caused by this.
Also the slower bullet will have a longer time to be affected by wind or air currents/thermals.
Until any rifle bullet has traveled 200 yards and settled down in flight,really strange things can be witnessed as it travels down range.
Col.Hatcher and Dr.Mann went into great details on this phenomena in their books.
It is called Procession.
Here is a short vid on just what this causes. Look closely at the bullets path:
http://www.larrywillis.com/bullet%20path.html

Barrel whip and exit time. Trying to predict POI with different loads and bullet weights will drive you crazy. Wait til you get a barrel that starts shooting lower as you work up your loads, or moves the POI left to right as you progressively load hotter.

More than likely the higher POI is due to higher recoil. It doesn't take much to lift the muzzle enough to move POI by 1.5".

quote:

Originally posted by joe25/06:
I may be wrong, but I think the slower bullet spends more time in the barrel which is rising due to recoil. Thus, the heavier bullet impacts higher at 100 yards.

Please set me straight if I am off base.

My exact experience as well.

recoil

quote:

Originally posted by jorge:

quote:

Originally posted by joe25/06:
I may be wrong, but I think the slower bullet spends more time in the barrel which is rising due to recoil. Thus, the heavier bullet impacts higher at 100 yards.

Please set me straight if I am off base.

My exact experience as well.

Same here and that has been my "theory" as well.

quote:

One way to know for sure is to shoot both bullets at intervals of 100, 150, 200, etc.

+1

A long time ago, I remember Jack O'Connor describing that same issue with the '06. He said either 150's or 180's would shoot well, but POI was significantly different so could not be interchanged with re-sighting. He also said that .270 loads, both 130 & 150, shot pretty close together.

It's definitely a barrel harmonics thing. The .270 could show less effect of differing bullet weights because there is more metal in the barrel for the same profile.

In BPCR rifles where the bullets are going much slower, that effect is sometimes seen if the rifle is not held tightly enough to the shoulder. Heavier bullets, being slower, can sometimes hit higher on the target because the recoil has moved the rifle before the bullet leaves the barrel.

Clarence

Nothing new about this phenomenon It has been written about and observed for decades. It isn't barrel harmonics. If so, heavy bullets would be just as likely to shoot below light bullets as to shoot above - it would be a matter of chance in each particular barrel/bullet combination. It is simply, as stated above, the extra time spent in the barrel plus the effect of recoil. (No, I'm not an engineer but this has been written about in PO Ackley's books, Hatcher, and many other leading rifle books over many years)

This is simply the effect of recoil. Keep in mind that you are talking about 1/40 of one degree difference in the exit angle of the bullet from the barrel. I have fired 150 and 200 grain bullets in the same sitting with my '06 and the 200 grain bullets hit just over 4 inches higher on the 100 yard target. That is 1/15 of a degree of barrel rise. It is pretty easy to understand when you can even feel the difference in recoil. Look at a protractor and imagine how little 1/40 of a degree is.... It isn't rocket science - just simple physics.

Don't believe any of it! I have it on good authority that the phase of the moon along with a some help from my little friends are what makes these bullets land in different places.


It must be true, after all I read it on the web!

quote:

Originally posted by Cougarz:
Don't believe any of it! I have it on good authority that the phase of the moon along with a some help from my little friends are what makes these bullets land in different places.


It must be true, after all I read it on the web!

You must of attended the Shootaway school of ballistics.