The effect of elevation to bullet stability

[A_Gamehog]

I have a favorite load for my 22-250 with a 12 twist. I like to shoot 200-400 yards with a Nosler 69 grain Competition .224 bullet.

At 100 the shots all touch, at 200-400 I get 1/2 MOA. This is at 4200 ft. elevation.

I shoot with friends at 1,000 elevation and last weekend these "accurate" loads were tumbling at 150-200 same gun

same loads. I realize that this twist is slow for this bullet in a 22-250 but it shoots great and consistent, my go-to-load.


My question is this, Why does elevation affect stability so much? and how?

 

[KevinThomas]

A_gamehog,

the question isn't why they tumbled, but why they managed to shoot at all the first go-round. These bullets need a 1x10" twist, minimum, even out of a 22-250 for decent stability. I'd say the first time around you were probably right on the raged edge of being stable, and any number of factors could have changed that to flatly "unstable."

I'd suggest you switch over to a much blunter bullet, like a 63 grain SMP, or better yet, drop back down to something lighter (shorter) like the 55s or (at most) 60 grain offerings.

 

[Nomad47]

Because it affects velocity. Apparently you are on the ragged edge at 4200'.

 

[Nomad47]

Kevin, I was typing as you were posting. At least we agree.

 

[Steve Blair]

Because it affects velocity. Apparently you are on the ragged edge at 4200'.

Bill,

It is the increased air density that causes more instability. Worst case is sea level. Think of shooting the bullets in a vacuum. Any reasonable spin would keep them straight. Both temperature and altitude affect stability for the same reason: air density.

 

[KevinThomas]

Lots of "worst cases" that go into the mix here, but he's definately touching on several. The reduced altitude, increased air denisty, decreased humidity, reduced velocity, all of these are factors that will make a borderline situation worse. It could also relate to a bit of a barrel issue, assuming it's a buttoned barrel. While it may be listed as a 1x12" it's not unusual for buttoned barrels to vary by +/- a half inch either way, even more in some cases. While bullet makers try to steer customers towards an appropriate twist, there's some variables that they can't really take into account. Anyway, if the bore is really something closer to a 1x11" or 1x11.5", he'll see marginally better results than what we'd expect from a 69 (in this case), than from a true 1x12". Of course, this is a variable, and it could just as easily go the other direction, leaving a 1x12.5" barrel that gave borderline results with bullets we would expect to shoot. Lot of variables here, but the sure fix is a lighter, shorter bullet. When it comes time to rebarrel, take a look at a 1x10" or a 1x9" if the 69s are on the agenda.

 

[A_Gamehog]

Thanks for the reasons and explanations guys. The barrel has about 2500-3000 rounds thru it. I got it 2 years ago for a Prairie dog, Coyote rifle but found out it likes the 68-69's the best by accident.

Kevin it is a savage factory button rifled barrel. 1-12 . But I see your point, it is a little under measuring the twist by cleaning rod. Yes it's almost time to re-barrel but I have another season of long distance PD shooting left in it. It really shoots these bullets in the wind well , and it's predictable. I have some other loads that shoot well at sea level, my 55-60 V-Max's don't have the BC of these but shoot good also.

Oh and it's a very hot load of varget to get the speed for this load. Flat primers but not excessive bolt problems or marks.

I was looking at a 22-250 AI in a 10 twist next time.