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Air Gun Home Forum Index » Rifles » My Version of the Millenium Pumper Goto page Previous  1, 2
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PostPosted: Sun Dec 16, 2012 12:29 am Reply with quote
rsterne
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Somebody asked what the FPE was if I maxed out the settings at 2000 psi.... With 42.4 gr. EunJin Points, it hit 858 fps, which works out to 69.3 FPE.... Mind you, it took half the air in the valve to do that, and MAN was it LOUD !!!

Bob

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PostPosted: Sun Dec 16, 2012 12:47 am Reply with quote
toadmyster223
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That is beautiful and amazing. I would pay you so much money for one of those if I had any.
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PostPosted: Sun Dec 16, 2012 11:56 am Reply with quote
gicos
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You achieved what I'm after without even having to stand on the pump! Very encouraging. Besides being gorgeous, that's a .22 Short multi-shot pumper you've got there, Bob!

I ran the numbers to see what's happening inside the barrel. This one was a bit challenging, as the valve closed about 11/16 of barrel travel. So I had to start with barrel volume broken down into eights. The barrel at 23.8" long with a .254" groove diameter is 19.77 cc. Broken down into barrel cc eights, this is:

2.47cc 4.94cc 7.41cc 9.88cc 12.35cc 14.82cc 17.3cc 19.77cc

Since the reservoir is 28.5cc, the pressure percentages in eights from breech to 3/4 barrel are:

Breech -> 91.4% -> 82.7% -> 74% -> 65.4% -> 56.7% -> 48%

which shakes out in psi to:

2,000 -> 1,828 -> 1,654 -> 1,480 -> 1,308 -> 1,134 -> 960

Since you had half a reservoir left, the valve closed about half way between 5/8 and 3/4 valve, or about 11/16 barrel, 68.75% of the barrel travel. Pressure at 11/16 barrel is 1,047 psi. Since the valve is now closed, the equation starts over.

68.75% of the barrel is now filled with 1,047 psi air, leaving 31.25% of the barrel unpressurized. 68.75% of the total barrel volume is 13.59cc and 31.25% of total barrel volume is 6.18cc. 6.18 is 45.47% of 13.59, so pressure at the muzzle will be 54.53% of what it was when the valve closed. So muzzle pressure is 571 psi, a pressure difference of 476 psi.

So, the pressure curve at 1/4 barrel, 1/2 barrel, 11/16 barrel (valve closure), and the muzzle now looks like:

2,000 -> 1,654 -> 1,308 -> 1,047 -> 571

Since we now know the pressure at 11/16 barrel, we just need another 1/16 to find the pressure at 3/4 barrel. 1/16 of the total barrel volume is 1.23cc. There's 6.18cc left to travel, so this is 19.9% the volume remaining, meaning the pressure at 3/4 barrel will be 80.1% of what it was when the valve closed. Meaning 848 psi.

So, finally, here's the curve in fourths:

2,000 -> 1,654 -> 1,308 -> 848 -> 571

Or for a more detailed look in eights with half barrel in bold:

2,000 -> 1,827 -> 1,654 -> 1,481 -> 1,308 -> 1,134 -> 848 -> 710 -> 571

Some significant work is definitely being done in the last half of barrel, which would explain why it's so loud. Whew, I think I sprained my brain on that one.

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Jason
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PostPosted: Sun Dec 16, 2012 12:13 pm Reply with quote
rsterne
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My calculator shows that this is a "dump shot" not that all the air is used, but that the valve is still open when the pellet exits.... The muzzle report would confirm that....

Don't forget, a 30 gr. pellet will NOT provide the same FPE in this gun, more like about 60ish FPE.... It does 57.5 FPE with a 25.4 gr.... In addition, the .25 cal has 33% more base area in the pellet for the air to push on, adding greatly to the acceleration.... and hence the FPE....

Bob

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PostPosted: Sun Dec 16, 2012 9:50 pm Reply with quote
gicos
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ARRGGHH! Must not have had enough coffee yet when I did those. My apologies. The pressure would drop by half if the barrel volume ever hit 28.5cc, which it doesn't. So, the percentages listed are percentage drops of 1,000 psi, not 2,000. Or, the pressure drops should be half what I listed above. How's this look:

Breech -> 91.35% -> 82.7% -> 76% -> 65.35%

2,000 -> 1,827 -> 1,654 -> 1,520 -> 1,307 -> a nice breeze for the trees

I did have enough coffee to catch the surface area increase of the 25 cal over the 22. I'm showing the surface area as 14.4% greater, though. 33 thousandths more must be what you meant. 14.4% greater pressure would look like:

2,288 -> 2,090 -> 1,892 -> 1,739 -> 1,495

And that's pressure cookin' at the muzzle. I definitely need a higher starting pressure and less volume.

Heavier chunks of lead are perfectly good in my book, as long they aren't so slow that accuracy suffers. I'm all about using the bullet (or pellet) that best enhances any given setup. I chose 30 grains to work with since the bullets are readily available and cheap, but I'm by no means chained to this if I can get more out of something heavier.

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Jason
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My Version of the Millenium Pumper 
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