Quote from: AK73 on July 29, 2014, 08:28:49 PMQuote from: Motorhead on July 29, 2014, 08:12:16 PMSmaller the frontal area greater the penetration at equal speed and weight.Unless a VERY GOOD SHOT, shooting small pellets really fast will more times than not go right threw your game delivering little to no kinetic energy. Unless you hit vitals or a brain shot you can pretty much expect quite a few runners unfortunately. But don't you have to hit the vitals to get a clean kill regardless of caliber kinetic energy, etc? Sure you have a bit bigger margin of error, but you still have to hit the vitals...I don't think he was implying that necessarily. I think Motorhead was meaning that the larger calibers have a larger available margin of error.
Quote from: Motorhead on July 29, 2014, 08:12:16 PMSmaller the frontal area greater the penetration at equal speed and weight.Unless a VERY GOOD SHOT, shooting small pellets really fast will more times than not go right threw your game delivering little to no kinetic energy. Unless you hit vitals or a brain shot you can pretty much expect quite a few runners unfortunately. But don't you have to hit the vitals to get a clean kill regardless of caliber kinetic energy, etc? Sure you have a bit bigger margin of error, but you still have to hit the vitals...
Smaller the frontal area greater the penetration at equal speed and weight.Unless a VERY GOOD SHOT, shooting small pellets really fast will more times than not go right threw your game delivering little to no kinetic energy. Unless you hit vitals or a brain shot you can pretty much expect quite a few runners unfortunately.
I have no idea what "static displacement" is, you're way over my head.... To me, "SD" is Sectional Density, which is the relationship between bullet weight and diameter (area).... Keep experimenting and sharing your results, I'm sure we can all learn from you....Bob
For those of you that are interested, I ran a simple 21 gr. .177 pellet (cylinder with a round nose) through the bullet design and twist calculators.... A 21 gr ends up 0.35" long, and with a 16" twist the Stability Factor (SF) is as follows:500 fps = 1.5 (ideal)700 fps = 1.4900 fps = 1.251050 fps = 1.10 (marginal)Mach 1 = 0.96 (unstable)If the pellet is heavier (or longer) it would have less stability and require a faster twist....Bob
Youch! Do you wanna hold it? Or rely in it? Didn't Lloyd block&bolt down a PCP supply and barrel, and try something like this, in his lab? If it wasn't him...I don't blame him, he's a sharp cookie, so I could be wrong.
Quote from: rsterne on July 30, 2014, 07:18:09 PMFor those of you that are interested, I ran a simple 21 gr. .177 pellet (cylinder with a round nose) through the bullet design and twist calculators.... A 21 gr ends up 0.35" long, and with a 16" twist the Stability Factor (SF) is as follows:500 fps = 1.5 (ideal)700 fps = 1.4900 fps = 1.251050 fps = 1.10 (marginal)Mach 1 = 0.96 (unstable)If the pellet is heavier (or longer) it would have less stability and require a faster twist....BobSo it looks like my gut was a bit off again and for the theoretical 900 fps .177 28 grainer it would be marginal, not unstable and the twist rate it would want is about 1 in 12-14 ..?
But why would you ever limit yourself to the same mass in all three calibers?Why not shoot heavier pellets as the increasing caliber allows?
Basically the longer the projectile and the smaller the caliber the more difficult it is to stabilize...So now you are talking a Minie ball that does not have the self stabilization of the shuttle cock /diablo shape... the 1/16 twist of most air guns is about right for the subsonic diablo shape...It has been a long time since I did a cursory (not super in depth) study of external ballistics but my guess is a long .177 Minie like pellet would need about a one in ten twist rate,here is a link if you would like to do the calculations.http://www.ebr-inc.net/articles_Greenhill_Formula.htmlThe smaller the projectile the harder it is to cast with consistency, swaging them is more reliable IMO. My brain and memory does not work as well it used to so if I am off base I expect to be told so...
Ran a few quick calculations on what would be required to get a 21 gr. pellet up to 950 fps (42 FPE) in a 24" barrel.... It would require an AVERAGE pressure during the shot of at least 2000 psi, and porting approaching full bore-size.... plus of course enough valve dwell.... This is not the kind of thing you will do with a MSP with a relatively small valve volume.... it will require a reservoir/plenum of at least 42cc running 2200 psi to maintain the required pressure throughout the shot cycle.... and even then it would be a challenge....Bob