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Author Topic: Comparing the Precision of Tin and Lead Slugs  (Read 604 times))

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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #20 on: September 15, 2021, 12:15:43 AM »
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.
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #21 on: September 15, 2021, 01:22:30 AM »
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
« Last Edit: September 15, 2021, 01:25:54 AM by rsterne »
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #22 on: September 15, 2021, 01:34:29 AM »
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.
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #23 on: September 15, 2021, 11:00:33 AM »
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.


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.
« Last Edit: September 15, 2021, 11:04:50 AM by ballisticboy »
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #24 on: September 15, 2021, 11:43:32 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.

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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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #25 on: September 15, 2021, 12:25:31 PM »
This might explain why my regulated Bandit sometimes has great groups and sometimes doesn't with my Dynamic TM-1 .177 9.5 grn tin pellets. In one 9 shot magazine I might get three different size groups of three shots each. On the other hand my Daisy 901 fires the H&N 6.64 grn pellets with extreme accuracy at 9-10 pumps. A few days ago I ordered some 19.91 grn .25 H&N green I want to test in my Eagle Claw. When this storm clears I want to see if I get better groups with the Bandito using the copper plated H&N Barracuda Power 10.65 grn pellets.

If heavier tin pellts/slugs become available I would gladly replace my barrels with an appropriate twist rate. I just don't know anything about getting a "custom" barrel.
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #26 on: September 15, 2021, 12:47:49 PM »
This thread is about slugs, not pellets, however there is another by Miles in a Sticky below you may find interesting....

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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #27 on: September 15, 2021, 01:13:10 PM »
This thread is about slugs, not pellets, however there is another by Miles in a Sticky below you may find interesting....

Bob

Thank you, I'm reading it now.
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #28 on: September 15, 2021, 08:34:39 PM »


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.

It is probably down to the relative increases in aerodynamic moments compared to inertial moments and their effects on the yaw wave lengths, which tend to be longer on longer projectiles. As a result, longer projectiles will be slower to react to yaw angles than short projectiles and thus have larger dispersion and group sizes.
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #29 on: September 15, 2021, 09:43:55 PM »
Thanks, Miles

While your calculated dispersions seemed to favor shorter slugs; the dispersions for the longer ones seemed to top out near 1 MOA.  Hardly something to gripe about.

I think that slug yaw at the muzzle can be greatly reduced by means of an effective air stripper.  This should make a difference in the precision on target; else they would not be so common for airguns.  Or am I missing something?  Perhaps, that air strippers help more with pellets than slugs.
« Last Edit: September 15, 2021, 09:46:14 PM by subscriber »
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #30 on: September 15, 2021, 11:15:49 PM »
While the muzzle blast can certainly cause yaw, and that may indeed be reduced by a good air stripper.... the type of yaw that Miles is calculating is a reaction to bullet imperfections.... When the bullet is released from the bore, it changes from rotating about the center of form (axis) of the bullet to rotating about the CG.... If the CG is not exactly on the center of the bullet, when the bullet changes its axis of rotation, it yaws in reaction to that.... It is that initial yaw Miles is using, as I understand it....

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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #31 on: September 15, 2021, 11:21:09 PM »
Thanks Bob

This is where projectile quality comes in.  Poor ammo will shoot poorly.  Also, design can have a major effect on the risk that the center of mass might be slightly offset, such as dual core for military bullets.

All of the above makes me wonder how benchrest shooters ever achieve quarter MOA groups.  Sure, if it is 3 or 5 shots, then luck will occasionally land tight groups.  Not for more shots; and not if they can repeat it.
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #32 on: September 16, 2021, 01:21:35 AM »
Miles would be the guy to ask about that.... I would like to know as well what is needed to produce consistent fractional MOA groups..... I presume extremely well made bullets is the key, but are there other factors as well?....

Bob
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #33 on: September 16, 2021, 09:15:03 AM »
While there are dozens of factors affecting precision, in the end for the projectile it all comes down to two main groups, how the projectile is launched from the barrel and how the projectile reacts to how it is launched. At short ranges the first group of factors is more important than the second, but as ranges increase, so to do the importance of the second group.

If a projectile leaves the barrel perfectly with no yaw or yaw rates, it will fly straight and consistently to the target and give good precision. This is the method used for most airguns or small arms, in that they try to minimize any problems through precise manufacture and fit to precise barrels. This all comes under the first group.

In the second group of factors, it is accepted that a projectile will never leave a barrel with zero yaw or yaw rate. Precision at the target is achieved through reducing the reaction of the projectile to the yaw or yaw rates so that the errors at the target are minimized.

Ideally, you will have both groups of factors working for you, but optimizing one usually comes at the expense of the other. For airguns, it is the minimization of the yaw and yaw rates that is used rather than the projectile design, particularly with pellets which are very poor at dealing with any type of yaw.

All the above assumes of course that the barrel is pointing in the right direction when the projectile leaves it and that it is not flapping about like a flag.
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #34 on: September 16, 2021, 09:26:13 AM »
Thanks Miles
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #35 on: September 17, 2021, 01:33:59 AM »
Miles would be the guy to ask about that.... I would like to know as well what is needed to produce consistent fractional MOA groups..... I presume extremely well made bullets is the key, but are there other factors as well?....

Bob

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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #36 on: September 17, 2021, 01:06:50 PM »
Yes, it's pretty obvious that you have to start with a good barrel, and one not "waving around like a flag" as Miles said.... My question assumed that, but I remain curious about the relationships between shape and drag, and shape and stability.... There is no question that slugs can "go to sleep" as they travel downrange, and actually produce smaller MOA groups at longer distances.... I know Nick Neilsen has seen that often with his slugs shooting better at 100 yards than at 50....

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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #37 on: September 26, 2021, 06:03:03 PM »
I know this is about tin vs lead, but what about copper vs lead? Copper is closer in density to lead than tin. I assume the cost is the main reason it isn't used, but might still be viable for hunting.
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #38 on: September 26, 2021, 11:47:49 PM »
Copper obviously works as a projectile material for firearm bullets.  The snag with copper airgun slugs is the high friction that would result from a slug with a cylindrical shank.  The snag would start with the force required to imprint the lands in the projectile.  Making that engagement shallow to reduce the starting force would probably allow a significant amount of air to blow by the projectile.

Swaging a copper slug results in a cylindrical shank.  To take a lesson from powder burner monolithic copper bullets, copper slugs are often lathe turned, with half the bearing area removed in the form of a bunch of shallow "grease grooves". 

You could probably turn copper slugs with just a narrow front and rear driving band, that might work from airguns.  Still, with airgun barrel land and groove diameters varying so much, the problem would still be that the slugs would rattle in some barrels, and be tight in others.  Both lead and tin are soft enough to make them more forgiving with respect to diameter variations (and passing through FX's tight chokes).

Finally, at airgun velocity and energy levels, copper slugs are not going to expand very well.  Not unless they have pre-slitted nose petals.  Another machining operation, that would chase up the cost even further and doom copper slugs for airguns completely. 

For instance; a .326" long copper slug with bearing band OD of .218"; having a nominal radial barrel land to slug shank clearance of 0.004".  The slug shown below weighs 15 grains.  It could be made with thicker walls, but would be even harder to expand:
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Re: Comparing the Precision of Tin and Lead Slugs
« Reply #39 on: September 26, 2021, 11:56:40 PM »
A more realistic hollow cavity bumps the above copper slug to 17 grains:

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