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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Comparing the Precision of Tin and Lead Slugs

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rsterne:
The stability factor is 1.5 in all cases.... The calculations are assuming a given initial yaw, caused by imperfections in the slug, caused by the CG not being in the center of the form.... As the slug exits the muzzle, and the slug changes from rotating around the center of form (the center of the bore) to rotating about the CG, it yaws due to gyroscopic precession.... The longer the slug, and the faster the twist rate, the greater the initial yaw.... The more perfect the slugs, the less the initial yaw, of course.... Changing the relationship between the CG and the CP of the slug will change how that yaw damps out (the slug goes to sleep) or increases (tending to developing a spiral).... again all related to velocity and twist rate....

Bob

subscriber:
I thought that a longer slug provides a better lever for the correcting moment, to force the projectile to follow its projectile with less yaw?  Assuming the center of form and center of mass are also further apart.

Are rifle bullets "long and pointy" only for high BC?  Would shorter ones be more stable; and shoot smaller groups (at the expense of effective range)?

Obviously, an out of balance slug can upset things - all else being equal.

rsterne:
Miles can give you a better answer to your first question....

Regarding the second, yes, long bullets can have a better form factor, but a lot of their higher BC comes from their greater SD.... However, they do require a faster twist, and in a crosswind they will have greater "aerodynamic jump" (vertical deflection from that crosswind), and likely greater "throwoff" from bullet imperfections as well.... As the bullet turns to face into the "relative wind" (the vector sum of its forward velocity and the crosswind velocity) that yaw causes it to deflect upwards or downwards, depending on the direction of the crosswind and the direction of the twist.... and also depending on if the CG is ahead of or behind the CP.... This causes groups to slope in gusty winds, rather than being strictly horizontal.... However, if the BC of the shorter, lighter bullet is lower (as we would expect), it's crosswind drift will be greater....

As you can see, in real world conditions, a long bullet with a good BC has some advantages (flatter trajectory, less drift).... while a short bullet with a poorer BC, but shot in a slower twist barrel, has other advantages (less throwoff and jump).... This balancing act between long and short bullets is one of the reasons you seldom find bullets with extreme length/diameter ratios being use in benchrest competitions.... The upper limit is about 5:1 (with 4:1 being more common), and the lower limit is probably about 2:1 (to get a decent BC).... A lot of really great PB bullets are about 3-4 calibers long, but many of those are too high an SD for airgun use, because of our low pressures.... In addition, their advantages may be greater when Supersonic, so our "ideal" L/D ratio may well be 2-3 calibers long, (maybe even less?)....

The bottom line is, you still have to find the one that works best in your barrel, and/or vice versa.... Miles may have additional comments as well....

Bob

Back_Roads:
 So was the Lone Ranger onto something with his silver bullets ??? or am I confusing that with werewolf hunting  :-\ Either way I'm on a tin & lead budget.

subscriber:
OOPS:  Meant trajectory for bolded projectile.


--- Quote from: subscriber on September 14, 2021, 10:42:24 PM ---I thought that a longer slug provides a better lever for the correcting moment, to force the projectile to follow its projectile with less yaw? 

--- End quote ---

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