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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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subscriber:
Thanks Bob,

Good point about bullet length advantage shifting as the speed of sound is exceeded.  As such, airgun slugs are more like handgun bullets than rifle bullets.

rsterne:
Absolutely correct.... However, the influence of a high SD on the BC still applies, as they are directly proportional.... The problem is, that in order to obtain a velocity in the mid 900s, there is a limit on the maximum SD you can use, based on the product of the pressure and barrel length.... For a 24" barrel, and assuming you can get the FPE up to 50% of the theoretical maximum (ie my "lofty goal"), there is a maximum SD (and hence bullet weight) for a given pressure to hit 950 fps....



The lower chart is simply an enlargement of the upper one.... It would be a rare PCP that would be able to achieve 950 fps with the above bullet weights.... For cylindrical bullets, the length is proportional to the SD, which means that to shoot a longer bullet at 950 fps you need more pressure, regardless of caliber.... All the bullets in the photo below (note the similar lengths) have a Sectional Density of 0.17....



That matches the blue line (3000 psi) in the above graphs.... For the larger calibers, these are very much shaped like a pistol bullet, only for the smaller calibers do they look more like a rifle bullet.... Sorry to get a bit off topic here....  :-[

Bob

subscriber:
Thanks Bob

All things considered, what can be achieved with just 3000 PSI peak pressure is pretty impressive.   It is not far below light black powder muzzle loader ballistics.

ballisticboy:

--- 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?  Assuming the center of form and center of mass are also further apart.


--- End quote ---

Although it will depend on the projectile shape to some extent, a longer projectile will tend to have a longer distance between the aerodynamic centre and the CG. On an aerodynamically stable projectile this will give more stability. Bullets are unstable, hence they tend to become more unstable as they get longer. With gyroscopic stability, there is also the problem that the transverse inertia gets bigger at a faster rate than the rotational inertia, further reducing gyroscopic stability. As a result, faster twist rates are needed.

subscriber:

--- Quote from: ballisticboy on September 15, 2021, 11:00:33 AM ---With gyroscopic stability, there is also the problem that the transverse inertia gets bigger at a faster rate than the rotational inertia, further reducing gyroscopic stability. As a result, faster twist rates are needed.

--- End quote ---

Yes, Miles. If you were stuck with one barrel, then long projectiles could easily drop below your imposed stability factor of 1.5. But, for this exercise you already sated that all projectiles would be fired from whatever twist barrels would be required to yield 1.5 stability factor, at a muzzle velocity of 950 FPS. 

The reason for my question was that your chart for predicted group size seemed to favor much shorter projectiles that I expected, even when barrels that has fast enough twist to provide a stability factor of 1.5 would be available.

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