Light pellets at very very high velocity?

[Mr.P]

I have a .25 airforce that shoots 60 grain slugs at up to 130 fpe.  I know pellets lose stability when shot close to the speed of sound because of the behavior of air at these velocities.  But I recall seeing some graphs posted showing that projectiles can achieve a good deal of stability once you get well past the speed of sound, and it's just shooting right near the sound barrier that's the biggest problem.
 

so first, is this true?  Diablo at 900 ft./s is stable but it's unstable at around 1200, but would it be stable at 1600fps since it is a good number of FPS above the sound barrier?
 

I ask because I was wondering what would happen if I shot a very light lead free pellet with this gun at full power.  It shoots 50 grain slugs at over 1000 FPS.  How fast would a 16 grain lead free diablo shoot?  And what would it's trajectory be?

[Wayne52]

I know that the GTO's in .22 caliber surprised the heck out of me shooting them in the mid 900's just how excellent they shoot. 

[Scavenger]

If you've never seen it, Taofledermaus on YouTube has done a lot of high speed tests using Diablo (diabolo) shaped projectiles shot from shotguns at high speed. These are filmed with a high speed camera. Interestingly enough some of the lightest weight projectiles DID have good stability at well past supersonic speed, while utilizing that diabolo shape. Some of them also did not. . .I don't think the answer is cut and dried. Caliber has a lot to do with it as well.

[ballisticboy]

The problem with the pellet shape as it approaches the speed of sound appears to be caused by a rapid change in the aerodynamic stability from stable to unstable.The change causes rapid changes in the gyroscopic stability which increases and at some point will be infinitely largs not allowing the pellet to follow the trajectory or to correct any yaw. Coupled with this is a large loss in forward speed which, together with the very high spin rate from the high speed at the gun, increases dynamic instability further magnifying the problems.

All of the above will vary with different pellet shapes and how well they come out of each individual gun. Find a pellet which is well matched to your gun and you may well be able to fire at higher speeds. Barrels with lower twist rates may also be able to fire successfully at higher speeds. But remember the muzzle blast from a gun firing a pellet at over 1200 ft/sec is likely to be much higher than one firing the same pellet at 800 ft/sec and is much more likely to show up small problems when the pellet leaves the gun.

[Mr.P]

Here's a stupid question:  can I cast slugs out of pure tin in the same molds as I make lead slugs?  Or mostly tin?  Avoid the "drag chute" effect of a diablo but still get up to those higher velocities maybe. That would be interesting to try.
 

The comment about pellets losing speed quickly (and so a high speed pellet would be a short range solution only) had crossed my mind – and maybe that could be good for some?  Very flat high speed for short range with the confidence that you aren't going to damage things beyond 75m or whatever. 
 

i think a very light aerodynamic slug could be interesting to mess with. 

[ballisticboy]

Unfortunately if you make a slug from tin instead of lead you will reduce the gyroscopic stability factor by the ratio of their densities. If the lead slug was already over stable you will get away with it and may even improve it, but if the lead slug had the correct stability the tin version would need a faster twist rate or risk gyroscopic instability.

[Mr.P]

If it is shot at 1400 instead of 850 FPS, is it possible that the  that high speed down the barrel would compensate?  Going from 700 FPS to 1400 wild double the RPM, right?  Or am I not understanding?  I have Gregor's MP mold .25 round nose boat tail 48 grains. Stable out of LW barrel at low and high speed.  If I made on of these put of tin, maybe it would work?

Guess this is where testing or software is helpful. 

Can I cast pure tin using my lead Bullet molds anyone?

[darkcharisma]

subscribing to find out your actual testing.

i dont think pure tin will hurt molds.

[Mr.P]

How confident are you?  I don't see these molds in stock and can't afford to ruin one.

So, is it safe to cast with tin for sure, anyone?

Will this metal damage my barrel?

I should be able to cast round nose boat tail slugs with a weight of 31 grains (would be 48 grains if pure lead).   I should be able to shoot those pretty fast if the gun can hit 130 fpe with a heavy slug.

Thank you

[nervoustrigger]

There was a fairly lengthy thread on the topic here maybe 2 years ago.  I tried to locate it but couldn’t manage to pick the right combination of search terms.  If memory serves, the discussion ran right along the same vein…looking for something that lies in between a lead pellet and a lead slug.  That is, better drag characteristics than a shuttlecock shape but a lower sectional density than lead.

A broader internet search on casting with tin brings up a number of discussions on powderburner forums so it appears to be a well-traveled path.
 
Tin is about 10x the cost of lead (on a volumetric basis, more on a gravimetric basis) so it’s unlikely to find much mass market appeal but it’s perhaps a fun thing to play with for the DIYer.  Tin melts at an even lower temperature than lead which means anyone with casting equipment can experiment with it.  However its thermal expansion is somewhat lower than lead so if you have a mold that produces a lead slug of a good fit, tin will likely be too tight.  For example, a .25 cal will be about 4 ten-thousandths larger when it cools…which doesn’t sound like much, and it isn’t for a pellet but it is for a slug.  That’s assuming you go ahead and melt the tin at the same temperature as lead.  If instead you take advantage of its lower melting point, the difference would be much greater.
 
Of course the reverse is true too.  If you have a mold that is producing undersized lead slugs, it might be interesting to try tin to snug up the fit and just see what happens.

[MagnumPI45]

I asked this similar question before but about slugs because I kind of thought the same thing.  Why not shoot really light slugs at very high velocities/well over the sound barrier through PCP's/really powerful springers if pellets are generally unstable right below, at, or over the sound barrier.  The diablo shape of a pellet aerodynamic wise is not suited for shooting over the speed of sound or in the beginning stages of breaking the sound barrier(transonic) so the next thought would be what about really light slugs shot well over the sound barrier (1400-1600fps).  The answer seemed to be since the slug would be so light to get those velocities from a PCP it would not hold its high velocity very long at all so it would be pointless, by 100 yards or even much less since it was so light it would already have slowed down to around the speed of sound or slower and the stability would be again thrown off of your standard slug shape plus you have the issues with what material you are casting and how it is stabilized, BC, twist rate, etc. 

With PCP guns you are limited to around 4500psi and that is just not enough power to push a heavier slug at velocities well over the sound barrier where it can maintain being above the sound barrier for long enough for it to benefit longer distance shooting, so you go from breaking the sound barrier to falling at it or below it way too quickly with a pellet/PCP gun.

If you use firearms with similar caliber sizes as airguns as a example, even lighter 22 Magnum rounds are still 30grains and they are traveling over 2000fps, a 40 grain 22 Mag is traveling at around 1800fps which would be enough to still be above the sound barrier past 100 yards, 17hmr is launching 17 grain bullets at over 2500fps so it is a heavy enough bullet traveling at a very high velocity to not fall into the sound barrier quickly and be accurate well past 100yards.  PCP guns just don't have the power to do that and even if they did you are now getting into them sounding like firearms/making a crack when they are shot, not being able to shoot them in areas where you can get away with lower velocity air rifles anymore, etc.

I have heard some people mention there could be more experimentation with the shape of pellets(shorter head/longer skirt) and slugs to try to stabilize them more in the transonic zone and speed of sound stages along with more experimenting with barrel twist rates so they can be shot at higher velocities more accurately but they are still more in the beginning of those stages.

I still would like to see someone experiment with shooting different very light pellets and slugs at really high velocities from PCP guns to see what the results/problems are.

[Raider03]

Maybe try a quick test with GTO .25 Cal, 16.54 Grains pellets. How do these perform at 1200fps and higher?

[Nvreloader]

You might want to do some reading/researching here,

http://castboolits.gunloads.com/showthread.php?420962-Casting-tin
See post #16 for other info.

The above site has lots of info on casting etc.

HTH's
Don

[Motorhead]

Another VERY important POINT I've not read yet ..... RPM !!!
You shoot any pellet light or heavy at those sort of speeds threw an AG barrel in the 15-18 to 1 ratio the said pellets RPM will be VERY OVERLY STABLIZED and it likely will spiral like crazy.

[rsterne]

My opinion on the above discussion....

1. It is possible to cast slugs from Tin instead of Lead, and has been done by at least one GTA member (unfortunately I don't remember his handle).... The density of pure tin is 7.31, and it has a BHN of about 9-10, compared with lead at 11.35, which has a BHN of about 5-6.... However, common lead alloys used for slugs have a BHN up to 12, (with 8-9 being the most common), so they will definitely work in an airgun, if properly sized for the bore.... Pure tin melts at a lower temperature than pure lead....

2. This places Tin at 64% of the weight of Lead, which means that to achieve stability, the twist rate in inches for Tin would be 0.80 times the twist rate for Lead.... ie if a 20" twist was perfect for a given Lead slug, you would need a 16" twist for the same slug in Tin, at the same velocity.... For the same shape slug, the BC of a Tin slug would be 0.64 times the BC of the same slug cast in Lead.... ie if you had a Lead slug with a BC of 0.200, the same slug cast in Tin would have a BC of 0.128 at the same velocity....

3. ALL projectiles have a huge (4-5 times) increase in drag in the Transonic region (Mach 0.8-1.2).... That means that when travelling Supersonic, they slow down a LOT faster than if travelling Subsonic.... Pellets, for example, can lose several hundred fps in just the first 10 yards if launched Supersonic.... Slugs are, of course, better in that regard, but you would need Centerfire velocities to keep a typical airgun slug Supersonic at even 100 yards.... A .22LR high velocity slug (40 gr. travelling at 1250 fps) drops below Mach 1 at about 75 yards, for example....

4. As we increase the muzzle velocity past about 1000 fps, the mass of air it takes to propel the slug increases dramatically, so less of the total FPE available ends up in the slug, and more is wasted in accelerating the air itself.... In other words, the faster you want your MV to be, the less efficient your airgun becomes.... Using Helium can help in achieving Supersonic velocities of course, because it has only 1/7th the density of air....

5. As we increase the MV past 900-1000 fps, the wind drift INCREASES, because it is related to the drag.... In fact, for a typical airgun slug, you would have to launch it at over 2500 fps to get back to the same drift as you have at about 900 fps....



As you can see, staying in the 900s is the best place to be to minimize wind drift.... When you add up all the above factors, trying to launch any projectile with air and keep it Supersonic all the way to target is a muggs game.... and IMO a total waste of time.... and that is without even mentioning accuracy.... 

Bob 

[ballisticboy]

On the question of gyroscopic stability, the absolute RPM is not what determines stability. The gyroscopic stability is determined by the ratio between the RPM and the forward velocity of the projectile which is determined solely by the twist rate when the slug leaves the barrel where stability is normally at its minimum. This is because the destabilising aerodynamic moments increase with speed just as the gyroscopic stabilising moments do. Thus increasing speed to increase the spin rate will not necessarily make an unstable slug stable, it will all depend on the changes in the aerodynamic moments.

[Mr.P]

On the question of gyroscopic stability, the absolute RPM is not what determines stability. The gyroscopic stability is determined by the ratio between the RPM and the forward velocity of the projectile which is determined solely by the twist rate when the slug leaves the barrel where stability is normally at its minimum. This is because the destabilising aerodynamic moments increase with speed just as the gyroscopic stabilising moments do. Thus increasing speed to increase the spin rate will not necessarily make an unstable slug stable, it will all depend on the changes in the aerodynamic moments.

 This is something that I was thinking about and was trying to formulate how to ask.  I was understanding that a higher twist rate would be needed for the lighter projectile but I was thinking that with the tire velocity it would achieve that by going to the barrel faster. But if as you say it's all about the ratio of spin to velocity then that affect cancels and we would need to have a different type of barrel it seems.

I  casting Gregor's huben k1 MP mold slugs.  He tested those in LW choked barrels (older k1) And I watched a video and she reports their accuracy across a very wide range of velocities.

So it looks like this is a foolish endeavor, but maybe by some chance these particular slugs that perform so well with this particular barrel at arranged Speed below the speed of sound I also do OK at higher speeds. And I read on a post somewhere that the shape of the projectile of a bullet like this is similar to standard 22 rifles which shoot at about these velocities as well (1200-1400 FPS maybe?)and deal with transonic issues ok, or am I wrong?

Anyway other than the hassle of cleaning out the melting pot, seems worth a try.  When I get some other items on my list taken care of i will try this out and and will gladly share the results here.  No doubt terrible groupings confirming what is already well known by those with the education.  If so, don't judge me I'm in this for the hands-on and who knows - maybe a hyper (?) velocity lead free slug with reasonable energy and accuracy out to 50 yards would serve a purpose.

[Back_Roads]

 I'm the guy who has a pot of tin, I have cast pellets, and slugs, initial test have shown promise, worth setting up a cheap pot and pressure pour ladle and trying it, so far I have done up to .50 cal. but have not had a chance to test the .50 yet.
 Sizing them is a must due to dropping larger and being slightly harder.

[Mr.P]

Man that last post of mine is a mess. I posted it in a hurry and speech to text is not very good I guess

[subscriber]

MrP,

I am surprised that the experts have not suggested that you cast a lead slug with a hollow nose cavity that is 1/2 the slug diameter and extends 2/3 to 3/4 down the length of the slug.

This will reduce the slug weight while keeping a "flywheel" of lead, to maintain sufficient rotational momentum for stability.  In terms of design philosophy, think FX Hybrid slugs.

If you must cast slugs from tin, here are some threads that might help:
https://www.gatewaytoairguns.org/GTA/index.php?topic=165931.0
https://www.gatewaytoairguns.org/GTA/index.php?topic=173399.0
https://www.gatewaytoairguns.org/GTA/index.php?topic=175771.msg155986447#msg155986447