Gyroscopic Precession - more info please!


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TexasShooter59
July 28, 2013, 06:27 PM
Thanks to this post (http://www.thehighroad.org/showpost.php?p=9038946&postcount=6) I want to know more about this as it relates to ballistics and heavy bullets.

Just recently, I fired off my first 77gr. SMKs and the group at 50 yards was nothing to get excited about. Yet, this bullet is touted as being really good. (I was not able to shoot it at 100 yards due to time and range constraints.)

After running across RC's post mentioned above, I have to wonder if the bullets were in their gyroscopic precession stage, and at a further distance it would be better.

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MEHavey
July 28, 2013, 07:51 PM
How "nothing special" was the group size?

rcmodel
July 28, 2013, 08:04 PM
IMO: It is a waste of time & money to test rifle loads at 50 yards.

Lighter bullets will shoot bug hole groups regardless of how bad they are.

And heavy bullets may shoot 2 MOA at 50 or 100 yards yet sub-MOA at 200 plus.

If you ever have the opportunity, lay down behind a long distance rifleman and watch the bullets flight through a spotting scope.

You would simply not believe the gyrations a long heavy bullet goes through before it finally settles down and flies right.

rc

TexasShooter59
July 28, 2013, 08:22 PM
The group size is attached below.

I don't normally test ammo at 50 except for some carbine-specific tests. With this bullet, I was verifying pressure was okay by shooting incrementing charge weights at the 50, and I was not confident in the POI due to just changing scopes beforehand. Normally, I could have moved to the 100 hundred easily, but as stated above in post #1...

Walkalong
July 28, 2013, 08:22 PM
The sooner out of the barrel a bullet "goes to sleep" the more accurate it is, assuming is has the necessary construction etc to be accurate in the first place.

The U Tube video shows what happens to a bullet when the rotation slows down too much to continue to stabilize the bullet.

Bench rest barrels use as slow a twist as possible while still stabilizing the bullets. They go to sleep sooner this way.

TexasShooter59
July 28, 2013, 08:47 PM
So, does that mean a rifle with a slower twist than mine (1/7, 16" barrel) might stabilize this bullet at a shorter distance? Not that I'm trying to do that.

rcmodel
July 28, 2013, 09:57 PM
NO, more then likely a barrel with a slower twist then your 1/7 would not stabilize a 77 SMK at all, at any distance.

A 1/9 might, but it would be pretty iffy whether it would or not at 16" barrel velocity levels.

rc

mtrmn
July 28, 2013, 10:59 PM
IME, flat base bullets tend to be more accurate at ranges under 300 yds or so. Only at longer ranges do the boattails start to really shine. This also due to that gyroscopic thingie I would assume. YMMV

GLOOB
July 29, 2013, 02:08 AM
Gyroscopic precession means something entirely different. It doesn't occur unless an outside force pushes the spinning object in question off axis. I wish there was an explanation for why gun "experts" so often pervert and misuse fancy-sounding terminology.

dagger dog
July 29, 2013, 09:56 AM
google, nutation.

Buffalo Arms has Robert Rinker's UNDERSTANDING FIREARMS BALLISTICS on closeout, it's a book well worth owning.

higgite
July 29, 2013, 10:38 AM
Here's an informative article from the Univ. of Utah website. It touches on yaw, precession and nutation of a projectile, FWIW.
http://library.med.utah.edu/WebPath/TUTORIAL/GUNS/GUNBLST.html

ironworkerwill
July 29, 2013, 12:27 PM
@mtrmn: Flat based bullets deflect gasses in a prependicular direction to the axis of the bullet rotation. Boat tail bullets do not deflect but rather focus gasses in front of the bullet. The gasses are traveling several times faster than the projectile.

The JBMballistics.com calculator states that a 1:10 twist will be marginal for stability while the 1:9 will be stable for the 77grn smk. Factors of calculation are bullet length .994'', velocity 2620 fps, 59deg F, at 29.92 Hg air pressure.

From my understanding Walkalong nailed it. Bullet rotation has a sweet spot it will stabilize in flight the sooner the better. Latter as velocity drops off it will become gyroscopicly unstable. The more rotating mass it has the longer it will be stable. As a side note most BP shooters want as close to a full rotation as possible before the projectile exits the barrel.

TexasShooter59
July 29, 2013, 08:34 PM
Thanks for all the responses. I will shoot these SMKs at 100 and 200 (range's longest) to see what happens.

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