What causes this?

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gamestalker

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So I build a 22-250 load shooting a 55 gr. Sierra at roughly 3600 fps and some change. At 100, 150, and 200 yards POI remains consistent. In other words, I'm not seeing any indication of spin drift. And at 250 and 300 yards it's still consistently center POI, though elevation is expectantly lower POI, but within ballistic projections.

Then I load a Sierra 100 gr. HP in 7mm RM, about the same velocity and I get a ridiculous amount of what appears to be spin drift. Sighted in dead center at 100 yds, then at 150 it drifts up and to the right like 4", both in elevation and windage. At 200 yds it's about twice as much, barely on the paper. But at 250 it starts to return to center line, and at 300 yds. POI is almost completely center line.

Is this an effect of spin drift?

And if so, why don't I see a similar effect with a .224" projectile at like velocity?

And I can push a .277" 90 gr. Sierra as hard as I want, like just under 3700 fps, and it maintains a predictable center line trajectory, but not the light weight .284" projectile, I'm stumped.

GS
 
Scope is not level. Or gun is not level as you are shooting. Re level the scope and make sure you are level when your shooting. You get no detectable spin drift inside at least 800 yards.
 
I am a novice, but since no one has offered any ideas I will venture to say that it could be an issue with stability. The length of the bullet, the twist rate of the barrel, and the velocity are some of the factors. If a bullet is on the edge of being stable then as it slows, or as it pushes the wind, etc, it might drop out of stability. Just a thought.
 
BTW, this is nothing new, I've dealt with this effect for many years when working up to maximum velocities with various cartridges / calibers / and bullet weights. I've been unsuccessful in trying to nail down the key factor at play. In this respect, one caliber will stabilize at X velocity, while a different caliber using an obscure bullet weight at a similar velocity doesn't.

It doesn't happen with a common weight or heavier weight bullet. For example, if I'm shooting a 145 gr. .284" bullet at 3200 fps, I don't experience any drift effect at any velocity or distance. But I also don't shoot 800 yards very often, but I do shoot regularly out to 550 yds. and haven't had any obvious issues. But if I load a light weight bullet, and in this case, anything from 100 grs. to under 120 grs., and I start to encounter a velocity limit due to this effect. I have been successful in managing the effect simply by managing velocity, so it's not a problem per say, just a grey area I deal with when working with larger caliber light weight varmint bullets.

Up until recently, I just worked around it by determining the velocity limits for those varmint bullets. But a while back I was working with a Speer 110 gr. TNT with 7mm RM, and oddly that bullet remained stable at nearly 3700 fps. I tested it out to 500 yards, and it seemed to handle the high end velocity pretty well, at least in this respect. But at 100 yards I had little tiny holes around the projectile POI hole. In other words, the bullet is coming apart at those velocities / RPM's, and yet they actually grouped pretty good considering. According to Speer, I was exceeding the recommended velocity for the TNT, by quite a bit, ya think?

But the part that has me scratching my head, is why don't I experience this with smaller caliber bullets like the .224" and .243". I also saw the same effect with light weight .308" projectile in 300 WM. Specifically 110 gr. SP at velocities around 3500 fps, once I get it down to it's velocity limit, it begins to settle down, and actually shoots pretty well really. I've shot those from a 30-06 at high velocity also, and it's the same story, but with a lower velocity limit for some reason.

Maybe it's rate of twist related? Well, obviously RPM's are a factor, but why is it so much more pronounced with the lighter weight larger diameter projectiles? External ballistics are not my strong point, I just work around the effects, rather than trying to diagnose the root cause. But I decided to bring the question here, where some of the brightest minds exist, and can possibly attempt to explain the cause.

The rifle is level. I use a string with a flat washer hung off the target, then I place a piece of tape at the top, and one at the bottom of the target to use as reference points in correlation with the vertical cross hair. I picked that up from a guy at the range many years ago. It's great, cause it integrates level with my view through the optic. I also like to use a single lateral reference point in combination with the vertical markers. This helps me shoot some tighter groups, it provides an elevation reference point. So when both points of reference are aligned with the cross hairs, I know I'm aiming at the same spot, shot after shot. Bright orange or red tape is my favorite, it provides good contrast against the black cross hairs, but any color will work.

GS
 
It seems odd that it will consistently be unstable as you described in the OP. An unstable bullet I would think to just go all over instead of up and to the right every time. Then come back? Hopefully someone will chime in with something.
 
Scope off axis

Rifle scope off axis is something to look at. The holes drilled in the action may be off a bit. The scope is adjusted internally, making it off axis. The scope adjustment must be centered and the scope mounts shimmed to get as close to zero as possible when installing a scope.
CENTERING THE RETICLE
The reticle was carefully set at the optical center of your riflescope at our factory. This
setting provides you with the ideal adjustment range from the center position. The
r i f l e s c o p e ’s adjustments are used to zero-in the riflescope.
It is wise to check the center of the optical axis before mounting. Do this by placing
the scope in a solid V-block (card b o a rd box with two slots). While looking thro u g h
the scope in a normal viewing position, carefully rotate the scope. If the target
moves in a circle larger than 1” from center (at 25 yards) in relation to intersection
of crosshairs, reset windage and elevation adjustments. Remove adjustment caps.
Set each adjustment to midpoint and recheck for centering. If target still rotates, use
adjustments to correct .
Click to expand...
http://bushnell.com/getmedia/c8da84df-a5a0-41f9-bfe0-e29e3577b5ed/Trophy_Riflescope.pdf?ext=.pdf Shim it. > http://bushnell.com/getmedia/43d8a20f-02b6-481d-b6b0-d4679f89b14a/shim_guide_2.pdf?ext=.pdf Mounts with windage screws should be used.
 
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Sighted in dead center at 100 yds, then at 150 it drifts up and to the right like 4", both in elevation and windage. At 200 yds it's about twice as much, barely on the paper. But at 250 it starts to return to center line, and at 300 yds. POI is almost completely center line.
Click to expand...
Wind. :D Up is ok and may be normal for some cartridges. If already moved to the right, its not going to come back to center unless something moves it. (wind)
 
Seems like it definitely could be an overstabilization issue with the lighter bullets due to twist rate. I have seen a similar phenomenon with one of my .50 cal muzzleloaders; if I shoot a round ball and use more than 75 grains of powder, the ball starts veering off (usually to the right but not always) to the point where it's almost off paper at 75 yds. I can load a heavier conical shaped projectile well above that level without losing stabilization. That particular rifle has a faster twist rate than another one I have; the second one I can load up a round ball to a higher level but it doesn't stabilize heavier conical bullets well at all.

In both instances (yours included) it appears that slowing down the lighter projectile to a certain point wins back stability and accuracy. Conversely, I have also noticed that pushing heavier faster in a slow twist barrel can often stabilize them sufficiently where they otherwise may be marginally stabilized. Bottom line is that I think you probably need a slower twist barrel in the 7mag if you want to shoot lighter bullets fast, unless you're willing to load them down a bit. The .22-250 typically has a slower twist (1 in 12?) because most of the popular varmint loads are 55gr loaded to high velocity. The 7 mag is more of a big game hunting cartridge and usually has a faster twist for heavier bullets more suited to that use.
 
That much drift is way to much to be a scope leveling issue unless its at a 15 degree angle.

The only thing I can add is the lighter 22 caliber bullets are usually designed to go 3500-4000 FPS out of 1-12 twist barrels because the manufacturer assumes that 22-250's and 22 swifts are the normal. Also because they have a smaller outside diameter, the jacket has less centrifugal force trying to pull it apart. Light 7mm and 30 cal bullets are definitely not designed for that type of velocity and have much more centrifugal force acting on them.
 
I think it's a diameter, length, rifling, and velocity issue.
I have come to believe that each caliber and to a lesser degree, each rifle has an optimum range of bullet weight.
That's why I've never been a big fan of trying to use one gun for multiple purposes.
 
Suspect it has more to do with it being a 7mm RM vs a .22-250 or .270. It's an apples and oranges comparison. Plus you'll be dealing with the rifling twist made for a heavier bullet.
 
precession could be causing this phenomenon. probably due to the design of the bullet. try a different bullet in the 7mm mag and see if the problem goes away.

murf
 
I have a savage model 12 BVSS in 223 which came with a factory 1 in 9 twist barrel. That gun absolutely would not shoot any bullet under 60 grains worth a damb. With50 grain bullets, every 20th bullet or so would go though the target sideways, and with one particular 50 grain HP about every 30th shot the bullet would disintegrate into a puff of smoke 50 yards in front of the muzzle with nothing hitting the target. I consulted the bullet manufacturer who told me the twist was too fast for that bullet.

I sent the gun back to savage who replaced the barrel with another 1/9 twist. Same key holing problem and disintegrating bullet problem when I got it back, so I replaced the barrel myself with a 1 in 12 Krieger. Problem solved.
 
Ya, I agree it's definitely something to do with twist, over velocity limit for that twist with light bullets, in which one can only get so much out of a given caliber with a given bullet weight. That's how I've always worked around it anyway. I would imagine if I were to use a slower twist barrel with say the 7mm RM, then I could scream those 100 grainer's at 3700 fps without much trouble, maybe?

Thanks though.

GS
 
I can remember someone posting advice about overspin not too long ago, said top shooters only spin the projectile fast enough to stabilize it and no more and this probably makes sense. But as a practical matter most of my drifting rounds have been due to muzzle upset of one sort or another from a less-than-ideal powder/projectile combination which permits blow-by when the bullet leaves the muzzle. Darndest thing: if I use a fast powder with good breech pressure the powder runs out of steam and the bullet ceases to obturate properly, allowing blow-by. But if I run a slow powder the pressure builds so slowly that gases simply go around the bullet instead of slamming it into shape, again allowing muzzle upset. After solving this riddle I still have the problem of overspin on the fouling shot(s). Does any of this overspin involve the fouling shots?

From this morning: fouling shot(s) still drifting right.
 

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Many times you have stated that you load your ammo close to the book max or even a bit above it. There is usually more than one accuracy node when working up a load. I often will find three of them. I will develop a load that uses the MOST accurate node and 99% of the time I find that it is in the lower to middle of the recommended charge range when I average several sources to get a load to use. I was trying to remember if any of my accuracy loads were at max or above without consulting my data but none jumps out at me. I also think that over revving a particular barrel / bullet weight combo at any caliber will cause odd things to happen and do not want this to happen as it throws in another variable to deal with. Also I believe that lower pressure stresses the firearm less (throat erosion/bolt thrust) as well as saves on propellant. This is a win-win as far as I see it.
Your method works well for you and so far I am really seeing great results with my loads as well. Is there a "best" way to do it, who really knows. You may get the same results you see now with lower velocity loads as well but I have not noticed this with mine so far. YMMV
 
22-250 is a ridiculously flat-shooting cartridge from just about any gun. I don't understand why you're trying to compare it to a 7mm, or anything else for that matter. You are trying to compare two different calibers with two vastly different bullets shot out of two different barrels. Velocity is not the only stabilizer in such a complicated formula.
 
Barrel heat and change of point of impact

Sighted in dead center at 100 yds, then at 150 it drifts up and to the right like 4", both in elevation and windage. At 200 yds it's about twice as much, barely on the paper. But at 250 it starts to return to center line, and at 300 yds. POI is almost completely center line.
Click to expand...
A factory barrel, when it goes from cold to, to hot to touch, can walk the point of impact away from zero. After the barrel cools, the point of impact will return to zero. Not all barrels will do this, only some.
 
Spitballer, now that you mention it, the fouling round only has a minimal effect. I think you might be onto something there.

And I've been running slow burners, I always do. And considering reducing from full tilt charges eventually gets thing stabilized. I'm gonna try a different / faster burning powder and see what happens.

GS
 
You are so right Frog0207, I do run almost everything at high table or + max, I like to squeeze everything in terms of flat trajectory out of my weapons. But also I my defense, I've never just jumped into a charge, I always you some degree of a development process, which is how I find the velocity cut off for this effect. I was just curious if anyone could maybe explain why this might be happening, no biggie though, I just work around it.

And this my friends is exactly why we reload, so we can develop and test various loads, thus locating the optimal combination of components, man I love reloading.

GS
 
gamestalker said:
And this my friends is exactly why we reload, so we can develop and test various loads, thus locating the optimal combination of components, man I love reloading.
Click to expand...
Absolutely! And this is a great story to explain that to those who don't handload / reload.

Be sure to let us know about what turns out to be successful for you.
 
We are talking about a 6 MOA shift in POI over 50 yards That is not a small amount and is not be explained by spin drift, or a scope out of level, or a load out of its accuracy window, or barrel heating or a wind gust or any other such small detail. 6 MOA is a massive amount of trajectory change. The only thing I can think of to explain it is that the bullet is coming apart, but I would not expect it to be consistent or to return to POA like you described.
 
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