Point of impact has me scratching my head!

[gamestalker]

I was at the range yesterday shooting some 7mm RM from two different rifles.
From 100 yds. my 140 gr. Sierra #1910 MV of 3050 fps average were shooting 4 1/2" high. No problem, I was expecting them to shoot about this high being that they were zeroed at 300 yds.
However, I ran some 100 gr. HP's and 110 HP's @ 3450 fps and they were shooting 2 1/2" high. If anything I would have expected the much faster & lighter HP's to have a bit higher point of impact if anything. Both of my rifles produced the same identical POI. Groups were one ragged hole at this distance with both rifles, so I'm certain there is nothing going on with the rifles.

 

[rcmodel]

The 140's kick harder and move out of the barrel slower.

The light fast ones get out of the barrel without as much recoil, fuss, muss, and bother.

It can also just be a barrel vibration thing with different bullet weights.

rc

 

[Naterater]

Agreed on barrel harmonics (vibrations). That's about all I have to offer for an explanation. I have experienced the same situations. My groups will even vary left to right.

 

[gamestalker]

I think I figured it out. I thought harmonics too at first, but my groups were tight .520" - .740" and vertically true. The rifles were both sighted in at 300 yds. with the 140 gr. bullets. So it made sense that at 100 yds. they will shoot much higher at 100 yds. than the 100 and 110 gr. bullets that are moving 400 fps faster, and produce a flatter trajectory at such close range. It just caught me off guard because I don't usually load 140's unless for deer hunting, which these were left over from.

 

[Jasper1573]

Yep, I agree with your last assessment...flatter trajectory keeps the bullet closer to the target center at ranges closer than your zeroed distance.

 

[helotaxi]

Actually your assessment makes no sense unless the rifles miraculously shot to the same POI at 300yds with both the 140gn and lighter loads. The sights don't somehow know that the ammo being fired is lighter and faster and adjust to compensate for that.

rcmodel hit the answer on the head. The slower round prints higher on the target because the muzzle has had more time to rise from recoil before the bullet leaves the muzzle. This phenomena is well documented and accepted by guys shooting handguns with fixed sights. They know that their sights are regulated to a particular bullet weight and velocity and that going with a lighter bullet at a faster velocity will print lower and a heavier bullet at a lower velocity will print higher.

 

[Jasper1573]

Actually your assessment makes no sense unless the rifles miraculously shot to the same POI at 300yds with both the 140gn and lighter loads.

From the original posting:

Both of my rifles produced the same identical POI

I am assuming that both rifles were zeroed at 300 yds and so impacted the target in the same place.

Additionally, I have to ask how much ballistic comparison should we make between a 2-3 pound handgun with a 6 inch barrel and a 6-10 pound rifle with a 26 inch barrel? I tend to believe not too much.

And as far as barrel jump, I am sure that this is true and has some effect on the end outcome of the POI, but do the ballistic charts in the back of my reloading manual take that variable into account for every round at every powder weight and every range? I don't know, but I am relatively sure that they do not as every rifle and every shooter is different, so every repetition of a barrel jumping must also vary from shooter to shooter and rifle to rifle. What remains consistent is the muzzle velocity and ballistic coefficient of the bullet and how they affect the trajectory of the bullet and its POI.

 

[murf]

i have to agree with rc's assessment.

the faster bullet will strike lower at every point in the flight paths of the two bullets, assuming identical aim point and recoil.

the rifles recoil will be greater with the heavier bullet, especially a heavy recoiling round like the 7mm mag.

if the rifle weighed 20 pounds, the lighter bullet would probably impact higher at 100 yards.

fwiw,

murf

 

[rcmodel]

Try holding it down harder on the bench and see what happens.

I know for a fact you can shoot a 30-30 carbine off a bench and let it jump, and it will print several inches higher then it will if you hold it down firmly.

Your 7mm Mag is no different.

rc

 

[murf]

yup, shooting a 7mm mag "benchrest style" (off hand under the rear bag) increases muzzle jump substantially. if i don't hold down my 300 weatherby mag, it'll catch air on the front rest. bad for accuracy.

murf

 

[R.W.Dale]

This is why there's no replacement for actually shooting a given load at range. Up down heck even left or right poi shifts can result from even just changing one component of a load.

There's no figuring it out. You just shoot and rezero accordingly

posted via tapatalk using android.

 

[Jasper1573]

Try holding it down harder on the bench and see what happens.

Not to be confrontational, but is this the way you would normally shoot the rifle? If not, it makes no sense to me to shoot it one way off of a bench and another way when hunting or target shooting. When I zero my rifles, I shoot them in the same manner as I will when hunting or target shooting, etc. By holding the rifle down with additional force, wouldn't you be inducing artificial stress on your rifle and your own body? Not that the rifle would be affected that much, but having that much stress and pressure on your body, I believe, would make the end result less accurate and less consistent since the pressure you apply each shot will vary.

 

[rcmodel]

Standing on your hind legs and shooting it off-hand will provide more downward force then shooting off sand bags with both hands behind the trigger guard and letting it jump.

I was not trying to change the way the OP shoots off a bench, or off-hand while hunting.
Only trying to explain why he is getting the results he is getting with heavier bullets off the bench.

rc

 

[Jasper1573]

Understand...

 

[helotaxi]

I am assuming that both rifles were zeroed at 300 yds and so impacted the target in the same place.

Yes, that's what he said. But what you took that to mean and what he said are two different things. What he said is that each rifle shot to the same POI with a given load, not that all loads shot to the same POI. IOW, both rifles shot the 140gn 4-whatever inches high and both shot the lighter loads less high than that. That is a great deal different from saying that the 140gn and the light loads were both shooting right to POA at 300yds; something that he certainly neither said nor implied.

Additionally, I have to ask how much ballistic comparison should we make between a 2-3 pound handgun with a 6 inch barrel and a 6-10 pound rifle with a 26 inch barrel? I tend to believe not too much.

The physics are the same. If it helps, consider that the differences with the handgun show up at 10-15 yards while the differences with rifles is usually not very obvious until a good bit more distance has elapsed and the magnitude of the shift is usually smaller with the rifle. But the other thing to remember is that recoil is usually significantly higher with the rifle.

I don't know, but I am relatively sure that they do not as every rifle and every shooter is different, so every repetition of a barrel jumping must also vary from shooter to shooter and rifle to rifle.

Correct. The charts assume no effects from recoil. The rifle is completely stationary.

What remains consistent is the muzzle velocity and ballistic coefficient of the bullet and how they affect the trajectory of the bullet and its POI.

Correct. What ballistics tables tell you is how a given bullet will fly assuming that the rifle is zeroed at the reference range. You have to adjust the zero for each load to match the reference zero. If you didn't, and rather zeroed for a certain load, shooting the same bullet faster or a bullet with a higher BC at the same velocity, it would be effectively zeroed at a longer range and would strike higher on the target (assuming the rifle was clamped down or recoil was zero). Likewise, if the same bullet were fired at a slower velocity or a bullet with a lower BC was fired at the same velocity, the effective zero range would be at a shorter range and the bullet would strike lower on the target. The only way that a bullet that exhibits a more arcing trajectory impacts higher on a target inside the zero range than a faster, flatter shooting bullet, without recoil being a factor is if the zeros are adjusted and the bore angle is higher for the arcing shot. The bullet never rises above the bore line. It begins to drop immediately.

 

[popper]

Test your theory with any ballistic calc SW. Put in velocity, BC, sight info, etc. and see what is supposed to happen. If it doesn't match the result, investigate to find the problem.