Change of POI between different bullets.

[TStone]

This has gotten longer than I anticipated but I wanted to give as much information as possible.

My .375 H&H is hunting accurate with any 300gr bullet I have tried in it but it doesn't shoot lighter bullets accurately. This is not a problem for me, however there is a disturbing difference between poi with different 300gr bullets. As an example, while the 300gr Peregrine VRG3 and the 307gr Peregrine VRG2 impact close to each other at 100m, the 300gr SBC bullet impacts 150mm to the left at 100m. Most .375 H&H rifles I've worked with shoot bullets from 230gr to 300gr fairly close to each other but this rifle doesn't.
The rifle is a Parker Hale .375 H&H with a sporter weight barrel, fully glass bedded in a walnut stock. Before having the barreled action bedded accuracy was abysmal. The rifle has a barrel mounted recoil-lug and it is magnaported.

On the other hand my 9.3x62, a Truvello barrel on a Howa 1500 action with a completely free floated barrel in a standard Hogue stock, shoots bullets from 230gr @ 2600fps, 250gr bullets @ 2500fps and 272 gr bullets @ 2400fps to the same poi at 100m and shoot 285gr bullets @ 2250fps 3 moa lower at 100m but exactly in line with the other bullets.

My .30-'06 a stock standard Howa 1500 with a light, fluted barrel free floated in a wooden stock shoot 167gr Peregrine VLR4's, 150gr Fox monolithic bullets, 150gr SBC and 180gr SBC bullets to the same poi out to 200m with good accuracy.

My wife's 7x57 built on a VZ24 Brno action with a walnut stock with a pressure point between the barrel and the tip of the fore-end shoot bullets ranging from 130gr to 173gr to the same point of impact at 100m with good accuracy.

My 6.5 Creedmoor, a Howa 1500 bull barrel free floated in a laminated stock shoot bullets from 118gr to 143gr from different manufacturers to the same point of impact although accuracy varies between different bullets (0.5moa to 1.5moa).

My 6mm Creedmoor, a Howa 1500 with a semi bull barrel, free floated in a standard Hogue stock shoot both 87gr Vmax bullets and 90gr CX bullets with good accuracy and to the same vertical poi but the CX impacts 4 moa to the left of the Vmax bullets.

So, why do some rifles (and I do not believe this to be caliber related) shoot different bullets to the same poi while other rifles have extreme variations between different bullets, even bullets of the same or at least very similar weights and velocities?

[Heath Robinson]

I don't know but i have experienced similar, although most of my rifles do shoot different weight bullets to different heights.

In a similar vein, three rifles with two suppressors. Both suppressors are the same basic design, just different sizes.
First one is a BSA Martini in .22 LR. Thus two-piece stock with the fore-end bolted to the barrel. Barrel is very heavy for a .22. POI between suppressor on and off moves vertically.

Second and third are both .308's, both fairly heavy barrels fully floated in Aluminium chassis. Same suppressor.
The one's POI moves vertically by about 1 mil.
The other moves about the same distance, but mostly sideways...

So the easy theory is that it has something to do with barrel vibrations but what causes that I am not sure. .308 no.2 has an identified problem: the chamber runs out with the bore some. It also has a habbit of shooting 4/5 shots in a lovely little group (usually cutting) but the 5th opens that group to somewhere around 3/4MoA. This could be something to do with the nut behind the bolt, but that same nut doesn't seem to have the same problem when placed behind other bolts. I don't k ow whether the known issue has a bearing on both, either or neither of these symptoms.

[Againstthegrains]

I think there are very good explanations and diagrams (p20-21) in the Somchem manual that show both the vertical and horizonal displacement of the barrel when firing. If you use different bullets, they engage the lands at different time points, and because they have different bearing surface shape, length and hardness, they will exit at different times. If the barrel is moving in both a vertical and horizontal path due to shock waves from firing, and the bullets engagement with the rifling, then the point of release of different bullets will be different and result in different POI's. These are not necessarily lower for heavier bullets.

To explain why a rifle does this more or less than another. There are two possibilities of which both can be true:

1. Your sample size is small with respect to the number of permutations/measurements for each rifle.
2. Different barrels are made differently i.e. different shape rifling, different barrel profile, different metallurgical properties.

I would imagine that if one does enough work on this, one may find that certain barrel manufacturing processes and designs may be less susceptible to this kind of POI shifts than others. Perhaps we should add another survey

[ady]

I would imagine that if one does enough work on this, one may find that certain barrel manufacturing processes and designs may be less susceptible to this kind of POI shifts than others. Perhaps we should add another survey

You touched on what i was thinking about after reading your post. Mass production and the rifles being different brands. So different manufacturing practices and metals and rifling.

I have had a similar issue with my 243, it just doesn't group nicely with a 100gr bullet, but loves a 87gr bullet. My father has used 100gr bullets for hunting all his life. I started reloading for the rifle and wanted a better grouping and ended on the 87gr bullet (Peregrine VRG 4/5) and changed from lead core to mono.

I have also sad a similar issue with my CZ Shadows. One CZ loves a 147gr bullet and won't group with a 115gr bullet and my other Shadow is the exact opposite. I tried to get them as close to each other as i could. Same springs, same work done to them etc and they are just different. I shot 5 different Shadows in 1 day and they were all different.

[Againstthegrains]

On the subject of barrel harmonics, I did some You tubing, as there are obviously a lot of guys with the same questions. The bottom line, was that you can't see the harmonics on a super slow motion of 950 frames per second before the bullet leaves the barrel, but you can see movement after it leaves the barrel.

That does not mean it does not exist, it just can't be detected that particular piece of equipment. The reason is that a harmonic that moves the POI by 1 MOA is a barrel movement of "0.0067" its just too small.

There was another really boring video of a guy who set up proper sensors that can detect the slightest movement, including a slow-mo and he proved that the movement is both horizontal and vertical.

So to answer the question, yes, barrel harmonics are real, and do affect the POI if there are difference in the bullet release position.

[TStone]

So to answer the question, yes, barrel harmonics are real, and do affect the POI if there are difference in the bullet release position.

Thanks, I understand this. What I do not understand is why there are such huge differences between different rifles, even rifles made by the same manufacturer. Would a barrel tuner even out these harmonics or does it need to be adjusted for specific bullets?

[Messor]

Remember now, there is only two things that requires the rifle to shoot to the same POI.
Firstly the one most computer models are based on, OBT, it just requires a period of stable muzzle distortion, aka a time when the rate of muzzle distortion change is stable based on the shock wave that travels up and down the barrel.
This is a constant, regardless of rifle make or type, it’s based on the length of barrel and how long a shock wave travels in said material.
Now here it’s important to note most hunters does not load top node, they use a cushy comfy load on a lower (read slower) node. So when they load another type of bullet for the same rifle they most often use the same node for that bullet as well, if that makes sense to those that does not know the science.

Secondly the barrel must point in the same direction for each shot, now this is nothing more than barrel harmonics. To be blunt, if the barrel is free floated and the action packed the barrel is pointing in a single direction, once fired the forces exerted twists the metal in sort of silly ways which makes the barrel go in all sorts of directions, as mentioned in the thread from the Somchem book. We just try and find that spot where the rifle spits out the bullet in the same direction.

So now you combine these two things, firstly find a spot where the muzzle is quiet, then find a spot where the bullet exit direction is always the same at muzzle exit.

Now science talk is fun and all, but to measure is to know, so then the ONLY thing any rifle shooters needs to do is load a ladder charge, aka from slow to fast, and then shoot them on the same target on the same day at let’s say 200m or 300m. Then you document each shot, mark it as such, and then you sit with a physical graph of what your rifle does on paper for each load. Then you merely select a load in-between a place where two or three loads shoot to the same place, job done, accuracy node found with a built in margin of error.

[janfred]

When people say they are shooting a ladder, this 👆 is how they should be doing it.

[TStone]

Now science talk is fun and all, but to measure is to know, so then the ONLY thing any rifle shooters needs to do is load a ladder charge, aka from slow to fast, and then shoot them on the same target on the same day at let’s say 200m or 300m. Then you document each shot, mark it as such, and then you sit with a physical graph of what your rifle does on paper for each load. Then you merely select a load in-between a place where two or three loads shoot to the same place, job done, accuracy node found with a built in margin of error.

This too I understand. But I do not have a problem finding an accurate load. The problem I have is why two accurate loads, in the same rifle, loaded with different bullets of the same weight shoot to the same point of impact in some rifles and to wildly different points of impact in another rifle.

And I'm wondering if there is a fix for the rifles that do not shoot two different (both very accurate) loads to the same poi.

[Messor]

And I'm wondering if there is a fix for the rifles that do not shoot two different (both very accurate) loads to the same poi.

Many people will say a barrel tuner, and I would disagree.
You see you can try and tune the load to shoot at a different place, however then you will alter the POI of the other load again, the one against which you are trying to bring it to.
With rifles like these it's good old fashion pen and paper, 10 clicks left and 12 clicks right for X bullet, zero for default load.