Drastic POI Windage shifts with different loadings...

[5whiskey]

So I've worked up dozens of loads over the years in good number of rifle calibers. From 300blk to 7.7 Jap. I've seen this phenomena exactly twice and I am baffled at how to explain it.

While working up a new 7.5x55 (7.5 Swiss) load, I loaded 15 rounds each of 3 charge weights. For safety, I actually started at the starting load and fired a few rounds to work up to where I want to start accuracy testing. I did this to look for any pressure issues and to check that velocity was where it should be. The loading behaved as it should based on published loads/velocities, so I moved on to the 3 test loads for accuracy. I found a definitive accuracy node at about max published load -2%. Chrono data told me velocity is right where it should be. The load was 41 grains of IMR 4064 under a 180gn Hornady SP. While the other loads (38 gns and 39.5) shot high but center of windage, my most accurate load held a nice tight group about 5 inches left of center. And by nice tight group I mean all 15 rounds were under 3", but this shooting 3 separate groups. The largest individual group was a smidge over 2", the best being a bit under 1.5".

The main question is, have you ever found a load during your workups that inexplicably had a significantly different windage POI than the other loads? I can understand elevation changes in POI during workups, but not windage changes. This is the second time I've had this happen, the other time was finding a 300blk load that my son's rifle liked. In both cases that I've observed this, the most accurate load showed a significant difference in windage POI than all other loads. Every other workup I've done proceeded like normal. Minor shifts in POI, but nothing that raised eyebrows. Shifts of an inch or two at most. Not a pronounced 5" shift. What could cause this?

 

[hounddawg]

What I would do is

Run the same test again and see if it is it repeatable except instead of going up in charge start at the top and go down or do it in a round robin fashion

 

[NoSecondBest]

Barrel harmonics can do some strange things. I just got a new .358 Win bbl and if I shoot it with a muzzle brake on it, it shoots 9.5-11.0” high. Shoots very, very good groups with or without the brake, but wow, does it ever change. I think it has to do with this problem. Have you checked to see what difference there is on each side of the barrel for clearance? Barrels don’t just “whip” in one direction.

 

[Shadow9mm]

When working up loads I have seen shifts. 308 at 100 shifter 3in from left to right going from start to max

 

[lugerstew]

Just curious, is that a thin hunting barrel, or a heavy bull barrel, also, what is the length of the barrel?

 

[44 AMP]

What could cause this?

The specific combination of that load, and that rifle. And, maybe you. On that day.

You can call it barrel harmonics, or you can involve the rest of the gun as well, and you can go bughouse nuts trying to ID the exact cause sometimes.

Every gun and ammo combination has the potential to act slightly differently. Sometimes, the stars line up so one combination acts radically differently. Sometimes the cause is reasonably obvious, sometimes it's an unsolvable mystery. The results are obvious, but the exact reason WHY may be known only to the gods, who aren't telling...

I didn't see it mentioned, but since you're talking 7.5mm Swiss I assume you're using one of the Swiss straight pull military rifle?? Even if that's not the case, questions about bedding, free float, barrel contour etc, and not relevant. They can be, if the gun was doing it with everything, and it's not. It's just one specific combination.

Do it again. See if it was a complete fluke, or if it is consistent, and repeatable. 5" left of POA (at 100yds?) is a lot of change when the only a grain or two of powder seems to be the only difference. But guns do what they do, and sometimes, things don't follow the usual pattern.

There is always a "bell curve" and though rare, there are always examples at each end of the curve.

The most radical impact shift I can remember was a few decades ago with a clone of a Colt SAA in .357 mag. (I no longer remember who made it). Spot on and sweet with 158gr .38 special at 25yds. EVERY .357 load put in it shot 2 FEET LOW AND RIGHT. Every load, and in the hands of 3 different shooters. NO idea WHY that much, or what was the cause, only that it did it.

See if that "accuracy node" is repeatable, and if the windage shift is consistent. Then its up to you. Zero the gun for that load, or for something else.

Good Luck
and please let us know how it goes.

 

[Bart B.]

It's common to have bench tested charge weights shoot further right as they get heavier for right handed folks; more left for southpaws. Rifle recoil movement starts when the bullet leaves the case.

My zeros for off-hand, sitting and prone are about a MOA or more left of a bagged rifle atop a bench. Do your own tests.

I've shot 4 man team matches with the same rifle and ammo observing a 2 MOA spread in sight zeros. We all hold the rifle different so it doesn't move the same in recoil during barrel time

 

[stinkeypete]

Experimenting with a Contender Carbine from sandbags, in 30-30, I built up 3 rounds incrementing in half grain increments. I got a nice spiral pattern around the bullseye.

It’s a light whippy barrel, very sensitive to how it’s held, and I reckon the barrel rings and where the end has vibrated to, or my grip position applied forces to, or both threw the groups about 6” apart.

Still, it never hurts to try again. To eliminate variables, shoot load A, load B, load A, load B, Load A, Load B

That will isolate scope shift, barrel heat, and other progressive changes and concentrate on the loads alone

 

[Unclenick]

There are all sorts of modes of whipping and vibration. If there weren't, barrel tuners wouldn't do anything. Varmint Al has movies of different harmonic modes of oscillation if you scroll down here. Keep in mind the up and down modes can also happen horizontally if you have a strong recoil moment with a vertical axis.

 

[5whiskey]

Thanks for the replies. To respond to some…

-44amp yes this is in a k31 with iron sights. 2” groups (even 15 rounds in 3”) is not what I would consider “nice and tight” in an accurate modern scoped rifle. 15 round, in 3 separate 5 round groups, in 3” was actually very consistent shooting with the post and blade K31 sights. To others who mentioned bedding, etc, yes this is irrelevant as I’m not altering the K31. It is a shooter if you find what it likes. I do tend to vertical string with it a little and I chalk that up to my eyes, ability, combined with post and blade sights with a rather large U in the rear blade.

-hounddawg, I actually like your idea of an effort to repeat the test. Unfortunately, the reason why I’m so please with the group is this was a short work up. I was gifted about a box and a half of these projectiles, so 150. I just used 50 of them for this work up. I want to turn the other 100 into a good standard load. If I’m still happy with it, I’ll buy more, refine the load a little, and carry on. I do have about 30 rounds of another load left. 168gn Sierra game kings on top of h414 iirc. I regulated the sights to this load, and the other groups in my work up shot roughly center (windage wise). I will compare groups from this new load to my old game kings on top of H414 and see if there is a Windage shift. If there still is, I know it isn’t a fluke. If not, definitely need to test more.

-to everyone who suggested my hold/sight picture/firm is in play. I do not disagree. My eyes are getting older. I can still shoot iron sights pretty well. On good days, with a good aperture sight and a good fsp, I can shoot around 1moa groups with irons. With that being said, achieving consistent sight picture and focusing on the FSP is more difficult these days. I often have to look down and rest my eyes during a group as it’s fatiguing to focus more with age. Certainly, I often do have minor poi shifts between groups, as keeping an exact same sight picture from group to group, range day to range day, etc is not easy. Especially as I get older. But any group shifts that have comes from form previously have been no where near this drastic, and are much more common in elevation. Still I understand that this could have magnified what happened, but I don’t for a second believe it fully explains it.

 

[Unclenick]

Recoil doesn't drive the muzzle perfectly straight up and down. It has lateral components. The late Harold Vaughn actually measured that things as small as whether or not chamber safety vents were drilled symmetrically on both the top and bottom of a receiver would introduce a recoil moment that affected POI a little. Major asymmetries, like uneven stock contact, will obviously do the same. Additionally, when you change bullets and their weights, you change the barrel time which changes the phase of wiggling muzzles deflection at the bullet exit.

I don't see this as particularly mysterious. A recoil moment is causing lateral muzzle deflection. It may be exacerbated by stock contact with the receiver or the barrel. You could play around with your stock and paper or cardboard shims between the stock and action and barrel to see what effect they have on this phenomenon. You could try using a sling position rather than the bench bags to see if that shifts it.

One other thought: I don't know how you set up to shoot from the bench, but with guns that whip the barrel or have possible stock contact issues, I've found I often get more consistent grouping by bringing the front bag in close to support the rifle under the magazine rather than further out on the stock. Something to try, if you haven't already.

 

[5whiskey]

^^^unclenick I will try bringing my bag closer in and see if that had an effect. The other day (when I noticed this) I rested about halfway the length of the barrel. And yes ultimately I prefer to shoot from field positions. I do shoot from a bench a lot, but I actually prefer field positions and try to shoot them as often as I can. I do use a sling to aid in support, and I do need to try shooting with a sling to see what that looks like. I’ve actually had pretty large poi shifts between bench shooting from a bag and using a sling with a few of my full stocked milsurp rifles. And then there is my Mauser, which shoots much better without the cleaning rod inserted into the stock. I guess maybe it shouldn’t be that much of a mystery now that I think of it. Milsurp rifles and full length barrel shrouding stocks do have some finicky ways about them. If a cleaning rod can open up groups and shift poi, I guess it’s not that strange that a grain of powder and 50fps can as well. Still… it was literally a 5” shift at 100 yards.

 

[Bart B.]

Another thing to consider when using iron sights. Visual acuity.

After hyperventilating (deep breathing a few times) then holding our breath to aim, best vision lasts for about 15 seconds. Then we get start over.

Dealing with the heated up powder charge shooting bullets faster is another issue.

 

[RC20]

5Whisley:

There is a clamp on side mount scope adapter for the K31. Totally non intrusive, leaves no marks and is seriously solid. I have one on my K31 and it works well. Its a bit odd being off to the side but works fine.

the only oddity of it is it has the CZ type Dovetails (forget what the number is, same as my CZ452). You have to get 22 rings for it. I have had no issues with scope movement or anything coming loose.