wintertime.........ballistics! :)

[alinwa]

opinions please

Is bullet stability a function only of rpm's?

Scenario, the temperature drops and a given bullet wants to tumble. One 'fix' is to increase the velocity. I don't mean "add powder to maintain velocity," I mean increase velocity. I've done this. It seems to work. It seems that the increased rotational speed more than compensates for increased drag and buffeting or overturning forces....


Or not???? Am I dreaming?

How about with subsonic airflow?

I've recently started a subsonic 50cal project, I've ordered a 1:7.5 twist barrel because of the low velocity. This is half (or twice) normal twist rate of 1:15 for a BMG. I can't verify that it'll WORK, but I guess we'll find out when the barrel comes in.

TIA

al

 

[Vibe]

I've recently started a subsonic 50cal project, I've ordered a 1:7.5 twist barrel because of the low velocity. This is half (or twice) normal twist rate of 1:15 for a BMG. I can't verify that it'll WORK, but I guess we'll find out when the barrel comes in.

TIA

al

Al
The one thing I would worry about with that is "accidentally" dropping a factory loaded round into it. That might be bad. (In the Ghostbusters "Don't cross the streams" sort of way)

 

[Vibe]

This may not be applicable, but the colder air holds less water then warm air, hence , less drag.. (I’m assuming temps around the teens). I don’t think imo that temp would necessarily increase the tumble affect. But possibly the opposite. The bullet is almost red hot as it leaves the bbl creating a cone affect around the front of the bullet as sort of a buffer zone, causing pre-drag, then the dissipated heat behind the bullet causes post drag; the drag is what in fact stabalizes the bullet in conjunction with the buffering ahead of it, its sort of spinning in its own ‘placenta’. There is a perception that colder air is heavier which in fact its not in terms overalll pressure, it in fact makes the bullet travel further and straighter because of the molecular density of air. The twist I would go 8 or 8.5 because if I am so full of bs.

Smoke (and mirrors).

Cold dry air is in fact more dense than warm moist air. Water vapor is oxygen and hydrogen whos molecular weight is much less than "normal" dry air.

 

[Boyd Allen]

Del Bishop once told me that in the winter, in cold damp conditions, that a twist that would stabilize a BT bullet in warmer weather, might not give good accuracy, so I would guess that air density might be a factor, but that is only a guess. This was in reference to short range, with a PPC, where folks intentionally run closer to the edge on twist than they probably do for other types of shooting.

 

[alinwa]

Al
The one thing I would worry about with that is "accidentally" dropping a factory loaded round into it. That might be bad. (In the Ghostbusters "Don't cross the streams" sort of way)

It's a lopped off 338L case of my own design, shorter than a 308Win,

thanks for caring though

al

 

[Vibe]

It's a lopped off 338L case of my own design, shorter than a 308Win,

thanks for caring though

al

Sort of like a .50 Whisper?

Rotational spin degrades much slower than downrange velocity so stability increases as the bullet gets slower - bigger changes taking place in the overturning moment, and not so much change in the gyroscopic forces maintaining nose direction. Being "just barely" stable at the muzzle will only get more stable down range. But then "over-stable" does not nose over and track so well at real long range. So getting into the right range to begin with is still of some importance.

 

[alinwa]

But then "over-stable" does not nose over and track so well at real long range. So getting into the right range to begin with is still of some importance.

Uhhhhh, you might wanta' be careful with that idea. Sounds like the exspurts over a Rupprecht-Nennstiel and their like. They simply CANNOT get it through their collective noggins that bullets MUST nose over if they want to maintain any stability a'tall.

"You CANNOT "over-stabilize" a bullet!!!!! You can very slightly increase it's yaw angle in repose but it MUST orient itself to the airflow and a bullet a second or more down range has a severious belly wind going. It CAN'T "not nose over."


silliness

al

 

[alinwa]

Sort of like a .50 Whisper?

sorta' only without the problems

 

[Vibe]

Uhhhhh, you might wanta' be careful with that idea. Sounds like the exspurts over a Rupprecht-Nennstiel and their like. They simply CANNOT get it through their collective noggins that bullets MUST nose over if they want to maintain any stability a'tall.

"You CANNOT "over-stabilize" a bullet!!!!! You can very slightly increase it's yaw angle in repose but it MUST orient itself to the airflow and a bullet a second or more down range has a severious belly wind going. It CAN'T "not nose over."


silliness

al

So all of them vertical keyholes in the 1000 yard target were not from using too light(short) of a bullet for the twist? Dang. And it made so much sense at the time. (Still does). So yeah! - you dang sure can over-stabilize a bullet. And it's not real good for accuracy either. Over-stabilized will have a "severious" belly wind acting on it - it's just so set in it's direction that it don't care.

 

[alinwa]

So all of them vertical keyholes in the 1000 yard target were not from using too light(short) of a bullet for the twist? Dang. And it made so much sense at the time. (Still does). So yeah! - you dang sure can over-stabilize a bullet. And it's not real good for accuracy either. Over-stabilized will have a "severious" belly wind acting on it - it's just so set in it's direction that it don't care.

so what you're saying is....... the bullet is flying with it's nose held 20degrees or so out of it's airstream......

??

al

 

[Vibe]

so what you're saying is....... the bullet is flying with it's nose held 20degrees or so out of it's airstream......

??

al

The holes in the target were what was saying that. I'm just reporting the results.

 

[alinwa]

The holes in the target were what was saying that. I'm just reporting the results.

hmmmm, I'ma' hafta' ponder this. Seems to me those bullets hadda' be near about ready to tumble.

From "over-stability."

hmmmm

al

 

[Vibe]

hmmmm, I'ma' hafta' ponder this. Seems to me those bullets hadda' be near about ready to tumble.

From "over-stability."

hmmmm

al

Maybe. But all 10 holes in the "group" (I use that term very loosely) were keyholed - and all consistently pointing in very close to the same direction, indicating a nose up attitude when they passed thru the target.

 

[alinwa]

I useta' hadda' Sako 6PPC USA that would keyhole 70gr NBT's at right around 50F @ el1000'MSL, 3250fps

This would vary or walk up and down the scale maybe +/- 10F for reasons I didn't explore.

al

 

[CYanchycki]

My experience

with a Rem 700 6BR with a 10 twist. Built using a 10 twist because of an article that I read. Fella said would stabilize a 95 gr Berger or SMK. Cannot remember which one for sure.

Anyhow, winter time shooting -27 Deg Celcius. Loaded with H4895 and the 95 gr's would not stabilize. The chronographed velocity was the same at -27 as it was at +20 for the exact same powder weight. Temp climbed to -15 and they shot like they had in the past.

Why?

My only thought was the borderline twist and the much COLDER temps. The velocity was the same but what was the difference? The only thing I could see is that the colder air is denser because molecules get smaller in cold and expand in warm. There was that much more resistance that the marginal twist barrel would not let them shoot in the cold.

Now relating this to the 6PPC and say bullets on a .825 or .790 jacket. The twists we use for our short range rifles are adequate. Some of us have talked up hear in the great white north about barrel twists and jacket lengths and bullet stability. In theory it makes sense that when the temps are cooler and say more consistent as they are in Manitoba using a 13.5 twist and say a bullet on a .825 jacket how much better would a .790 jacketed bullet shoot???? I have started shooting the 790 jacketed bullets and have been happy.

I feel that up in Manitoba that a 13 twist may be needed for the bullets on .825 jackets.

I know I will get a lot of well they have shot fine for years in such and such configuration, the key today is the shooters are getting MUCH BETTER at diagnosing accuracy related problems.

Just my opinion. What do I know coming from a farm in the middle of nowhere???? Loved the hunting and fishing though.................. ;-)

Calvin

 

[Andy Cross]

"You CANNOT "over-stabilize" a bullet!!!!! You can very slightly increase it's yaw angle in repose but it MUST orient itself to the airflow and a bullet a second or more down range has a severious belly wind going. It CAN'T "not nose over."
silliness
al

Well this is both correct and incorrect. If the centre of gravity was positioned perfectly in a bullet then over stabilization couldn't happen. If the CG is not in the correct position in reference to the overall length, shape and weight then you can indeed over stabilize it. The rate of spin does drop off at a slower rate than the velocity and in many cases the two curves being optimum spin vs velocitywill often cross paths on the graph at more than one range.
As one of my lecturers once told me. We didn't create the laws of physics, we can't alter them but we do have to live by them.
Andy.

 

[Andy Cross]

Cold dry air is in fact more dense than warm moist air. Water vapor is oxygen and hydrogen whos molecular weight is much less than "normal" dry air.

Adding anything with some mass like hydrogen and oxygen aka water vapour to the air would increase the density of the air acting on the bullet but its effect would be so small you would need some pretty fancy equipment to detect it.
Andy.

 

[Boyd Allen]

And then there are those 62 gr. Watsons that I have about 1300 of...made on .650 jackets. Perhaps we need winter and summer bullets or barrels. Thanks for the info.

 

[Vibe]

Adding anything with some mass like hydrogen and oxygen aka water vapour to the air would increase the density of the air acting on the bullet but its effect would be so small you would need some pretty fancy equipment to detect it.
Andy.

Adding lighter elements to a mix does not increase density. Quite the opposite - just like alloying tin with lead does not increase the density of the lead you started with. And the laws of partial pressures control how much actual volume gets changed. Also a 20% relative humidity at 40°F represents much less moisture (absolute humidity) than 20% relative humidity at 60°F.