Higher MV = Lower POI?

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I noticed something unusaul, or at least different than I had expected yesterday while doing some load testing. Using 9 sets of 3 rounds shot at 100 yards with charge weights varying from 40.0 to 43.4 (40.0, 40.5, 41.0, 41.5, 42.0, 42.5, 43.0. 43.2, 43.4) my point of impact actually got lower as the charge weight went up.

I would have thought that a higher MV would mean higher POI, but one of the other guys at the range said this was due to the higher charge weight rounds shooting flatter.

Can anyone provide any insight?
 
Lee Enfield #4s would do this. It was called "compensation", and it actually help the #4s get better scores at long range (900y and 1000y) with service ammo than a stiffer, "better" action would usually get.

The target rifles I have tested (Win M70, Savage #10 solid bottom .223, and Musgrave) both showed a rise in 100y POI with an increase in muzzle velocity... this "negative compensation" would _amplify_ the long range POI spread due to shot to shot speed variations.
 
I read an explanation for this somewhere, that claimed it was due to barrel time, i.e. with a heavier charge, or a lighter bullet, the POI was lower because the bullet would spend less time in the barrel. The higher velocity was supposedly causing the bullet to exit the barrel sooner, before the muzzle had risen as high. I know that I've observed the same effect myself, when switching from a lighter to a heavier bullet.

I'm not sure I buy this...I don't really see why the muzzle would move at all before the bullet exits. I would think that it is the "jet effect" of gases exiting the muzzle that would cause the muzzle to rise? I dunno...I'm just throwing the idea out there.
 
the higher the MV the faster the bullet turns out of the barrel. The faster it turns, the quicker gravity takes over,thus pulling the impact point lower.
 
Jfarrel:
How does the "gravity constant" (9.81 meters/second squared) change with the rotational rpm?
Maybe I misunderstood your post? I have been known to do that on occasion.
 
In handguns the faster bullet has less barrel time. As soon as the cartridge fires, the recoil starts the gun rising. The quicker the bullet leaves the barrel the less rise of the barrel and the lower the POI.
I am not sure however is this also applies to rifles.
 
colinjw said:
In handguns the faster bullet has less barrel time. As soon as the cartridge fires, the recoil starts the gun rising. The quicker the bullet leaves the barrel the less rise of the barrel and the lower the POI.
I am not sure however is this also applies to rifles.
Click to expand...

I'll be the first to conceed that I know nothing of this.

After thinking about this for a bit I think it may be real but meaningless in the real world. Recoil is made up of several actions and to say it will have a meangful effect on POI seems unlikely to me unless your talking really big cartridges (or really slow ones) and a radical differance in barrel length.

Can you or anyone else elaborate?
 
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jfarrell said:
the higher the MV the faster the bullet turns out of the barrel. The faster it turns, the quicker gravity takes over,thus pulling the impact point lower.
Click to expand...

I assume you're either a nuclear physicist of some sort or joking.
 
colinjw said:
In handguns the faster bullet has less barrel time. As soon as the cartridge fires, the recoil starts the gun rising. The quicker the bullet leaves the barrel the less rise of the barrel and the lower the POI.
I am not sure however is this also applies to rifles.
Click to expand...

How and why does recoil start the gun rising before the bullet exits? The primer ingites the charge, the powder burns and creates expanding gases which push in all directions, this pressure then propels the bullet down the barrel. An equal pressure applied to the rear of the chamber moves the gun backwards, but the effect is much less than on the bullet due to the gun's vastly higher mass. You feel this as recoil on your shoulder...more for heavier bullets (closer to the weight of the gun) and less for lighter bullets. I think (?) that explains why the gun moves backwards before the bullet exits. This pressure is also applied in all other directions within the chamber as well...up, down, sideways, all the same. So what causes the vertical component of recoil?

I'm not arguing or saying you're wrong...I don't know the answer either. Could it be that the gun is pivoting on the point of contact with the shoulder, which is typically below the line of bore?

I really wish someone who actually knows would chime in here. I've observed this phenomenon many times, and always assumed it was directly related to the mass of the bullet...heavier ones print higher than lighter ones...I wasn't thinking of velocity.

jfarrell, sorry, but your explanation makes no sense whatsoever.
 
I posted the same question on AccurateShooter and the response I got from Laurie Holland seems pretty well thought out (he expanded on some of the points Daniel had mentioned; thanks BTW):

"It's most unlikely that it's down to a flatter trajectory at only 100yd, although that affects POI at longer ranges. It's normally a result of the combination of bullet 'barrel-time' and the way the action and barrel flex as the bullet travels down it. Most actions see the muzzle move vertically as the bullet travels along it. If the muzzle is moving upwards during this process and the MV is higher, the bullet takes less time to travel from chamber to muzzle and so exits at a marginally lower muzzle position than with a lower load, lower velocities and longer 'barrel-time'.

I've usually found front-locking sporting ('hunting') rifles with skinny whippy barrels produce the opposite effect giving a markedly higher impact point with each grain increase in powder charge, while heavy barrel target rifles on stiff actions can often, but not always, see the bullet impact drop as MVs rise.

It's also not unusual to see the POI move laterally as pressures / velocities increase.

Incidentally, this 'barrel-time' related effect which is often called 'compensation' produced very marked and beneficial long-range results in British Lee-Enfield rifles. The rear-locking and not very stiff Lee action produced very marked 'positive compensation', that is the muzzle was rising as the bullet went through. As a result, a cartridge that produced a high MV exited earlier at a lower muzzle position and hit low, a slow bullet hit higher. At 600yd and beyond the slow bullet which would normally hit low on the target hit at the same elevation as a fast bullet thanks to the trajectory and muzzle position effects cancelling - or compensating for - each other."
 
jjohnwm said:
How and why does recoil start the gun rising before the bullet exits? The primer ingites the charge, the powder burns and creates expanding gases which push in all directions, this pressure then propels the bullet down the barrel. An equal pressure applied to the rear of the chamber moves the gun backwards, but the effect is much less than on the bullet due to the gun's vastly higher mass. You feel this as recoil on your shoulder...more for heavier bullets (closer to the weight of the gun) and less for lighter bullets. I think (?) that explains why the gun moves backwards before the bullet exits. This pressure is also applied in all other directions within the chamber as well...up, down, sideways, all the same. So what causes the vertical component of recoil?

I'm not arguing or saying you're wrong...I don't know the answer either. Could it be that the gun is pivoting on the point of contact with the shoulder, which is typically below the line of bore?

I really wish someone who actually knows would chime in here. I've observed this phenomenon many times, and always assumed it was directly related to the mass of the bullet...heavier ones print higher than lighter ones...I wasn't thinking of velocity.

jfarrell, sorry, but your explanation makes no sense whatsoever.
Click to expand...


Recoil does start the moment the bullet and propelling gas start down the barrel. The heavier bullet is slower to exit, and as in a pistol, the rifle barrel muzzle is pointing a little higher, as the recoil has had longer to act and is more than with the lighter bullet. You will also note the heavier bullet will impact to the left of the lighter bullet. More opposite twist is applied to the rifle by spinning the heavier bullet. Barrel harmonics could also be a play here as well.

So colinjw is correct

NormB
 
KDX said:
I assume you're either a nuclear physicist of some sort or joking.
Click to expand...


for some reason, i have a terrible feeling that neither is true,



on that not ill agree that the higher charge was exiting the bore at a lower point in the ocilation cycle.

but it doesnt matter, find the node for your rifle (at the highest charge u can) and figure the rest out with your scope
 
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