Thread: Unexpected POI shift

Originally Posted by Micky Duck

@Rushy isn't this I "thing" used by handgun users??? Heavier load,higher recoil,higher impact???? And the muzzle brake type thing helps to dampen it down. Years back I fired ten different loads into same target through my 270w they made a playing card sized group so about 2"wide and 3"high...not enough to worry about at bush hunting ranges but enough to matter out further....so I keep a few loads that I know are zeroed in handy but not always up the spout so to speak.

MD you are aware from the couple of beers that we have had that I am as technical as the hole in a doughnut. I point rifles annd pistols and I gently massage the triggers and animals and other shit develop holes in them and fall over. Beyond that I like beer and bacon. Life is good. Ha ha ha ha

By hoki...I can relate to that. Have a beer for me bud.

It is quite simple , it's called science or Newtons apple?. Put simply the faster a projectile goes, the less the effect of gravity has time to do its thing. So a heavier projectile needs more elevation of the trajectory to reach the same POI, where as the lighter/faster projectile needs less elevation , cause it gets there quicker,so hence it is discribed as being flatter and there fore a lower POI

Originally Posted by Tankd

It is quite simple , it's called science or Newtons apple?. Put simply the faster a projectile goes, the less the effect of gravity has time to do its thing. So a heavier projectile needs more elevation of the trajectory to reach the same POI, where as the lighter/faster projectile needs less elevation , cause it gets there quicker,so hence it is discribed as being flatter and there fore a lower POI

Tankd, what you are saying is to get the same point of impact, you would need to aim higher with the heavier, slower projectile.

BUT - I'm am using the same point of AIM, so to go with your comment, the slower bullet should impact LOWER right? not higher!

work with the ttsx bullets only. changing nothing but the cartridge o.a.l. try 3 cartridges .10 shorter and 3 .10 longer. you will see changes in p.o.i.. try it and see.

Originally Posted by saljer

work with the ttsx bullets only. changing nothing but the cartridge o.a.l. try 3 cartridges .10 shorter and 3 .10 longer. you will see changes in p.o.i.. try it and see.

I dunno if I'd bother.

Even if you care that there's a POI difference, it's not clear that changing anything about the load will actually change POI at all, let alone predictably.


Here is 8 different COALs fired for 3 shots each with the groups overlaid by measuring distance of impact from a consistent reference point. They're all just part of a larger group. Same powder charge, brass, primer, bullet, rifle, etc.

Originally Posted by Crusty

Tankd, what you are saying is to get the same point of impact, you would need to aim higher with the heavier, slower projectile.

BUT - I'm am using the same point of AIM, so to go with your comment, the slower bullet should impact LOWER right? not higher!

No using the same POI , heavier projectile , the lighter/ faster will print lower because it has a flatter trajectory and flies underneath the heavier projectile .
It is a bit like comparing a 30/30 to a 223 , or you could use something like the Shooterscalculator.com to see it. It seems counterintuitive
but it isn't.

Changing seating depth can make a big difference. https://www.youtube.com/watch?v=_h4iAiGYbwg https://www.youtube.com/watch?v=5Oj9Os8rT8M

Originally Posted by Tankd

No using the same POI , heavier projectile , the lighter/ faster will print lower because it has a flatter trajectory and flies underneath the heavier projectile .
It is a bit like comparing a 30/30 to a 223 , or you could use something like the Shooterscalculator.com to see it. It seems counterintuitive
but it isn't.

NO just NO - you have got it completely wrong. Completely upside down. Using a ballistic calculator, to get the same point of impact you need to AIM lower with the faster bullet.

If you AIM at the same spot, the faster bullet will hit higher than the slower bullet according to the ballistic calculator.

POI is not the same as Point of Aim!

To prove this, use a 30yd zero for both bullets - then you will see that the faster lighter bullet will hit higher at 100yds - but only by .2moa

Try a 180 gn pill and they will print higher still. Likely another 2 inches at 100m.

Gimp and Gundoc are on the money. Recoil and rotational force upwards. Exacerbated by a loose hold and or bipod use on a hard surface possibly.


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Originally Posted by saljer

Changing seating depth can make a big difference. https://www.youtube.com/watch?v=_h4iAiGYbwg

I'm not sure how you can watch this video and come to that conclusion.

1. Demonstrates that he cannot successfully find "better" or "worse" precision from seating depth changes using conventional 3-shot group selection methods (as we all know thanks to discussions here over the past 2 years)
2. Demonstrates that there is no evidence for a statistically significant difference between any of the seating depths
3. Demonstrates that there is no evidence for meaningfully different POI between seating depths at significant group sizes (but some give the misleading appearance of more stability for the meaningless 3-shot subsamples, because of random variance)
4. Concludes that "In the end guys, I don't have any answers, I don't have statistical significance to pronounce a winner, 'this is the way to do it', instead I have generated more questions than answers"



Now, Glasscock is an F-class champion, shooting an F-class rifle, and cannot resolve a difference.

He's not ready to update his belief, despite the evidence he has presented - but he has presented some pretty useful evidence.

Originally Posted by Crusty

NO just NO - you have got it completely wrong. Completely upside down. Using a ballistic calculator, to get the same point of impact you need to AIM lower with the faster bullet.

If you AIM at the same spot, the faster bullet will hit higher than the slower bullet according to the ballistic calculator.

POI is not the same as Point of Aim!

To prove this, use a 30yd zero for both bullets - then you will see that the faster lighter bullet will hit higher at 100yds - but only by .2moa

Yeah there's like 11mm of difference in drop at 100m based on a speed difference between 2850 and 2650 (and the faster bullet hits higher...)

It's worth remembering that even though the "node theory" thinking on accurate loads has been discredited, barrels do have a individual harmonic which has all sorts of effects, especially on poi.

Originally Posted by Tentman

It's worth remembering that even though the "node theory" thinking on accurate loads has been discredited, barrels do have a individual harmonic which has all sorts of effects, especially on poi.

It's more like barrel whip based on bore concentricity and straightness. One component of bullet 'jump' from muzzle to target. Variable in magnitude and direction, more or less so depending on the quality and/or stiffness of your barrel. Or any stress in it (if it has stress that causes any shift it's junk)

All bets are off if your barrel isn't free floated

But never mind the esoteric stuff.

With a factory rifle you'll almost certainly see a POI shift of some size and direction, often quite significant, between 2 different types of ammunition. You will need to understand it sufficiently for your needs and have a way of accounting for it that is sufficient for your needs, if you wish to use both ammunition types.