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melloairman
08-10-2010, 10:57 AM
Rifle is .357 caliber air rifle bullets 7 - 15 bhn lead , fps 800 -1000 with 145 plus grain bullets . Barrel 28 inches long . Normally made with a 1 in 20 twist rate . But a 9 mm barrel is avalible with a 1 in 14 twist rate . How much energy do you think would be lost due to the twist rate if bullets were resized to fit 9 mm barrel . Marvin

Artful
08-10-2010, 11:25 AM
Not enough information - what speed is spec'd with the 9mm barrel?
- if they both go the same speed you'll get the same energy

- a lot of my 9mm barrels are .357 caliber

- faster twist rate is for longer bullet length stability at any given speed - look up greenhill's formula http://www.recguns.com/Sources/VIIIB6.html

using someone else's calculator page to calculate

http://www.mountainmolds.com/helpGreenhill.htm
1:14 shows stablizing 1.37 inch long bullets
1:20 shows stablizing 0.96 inch long bullets

but they don't show velocity or which constant they used to calculate
with the subsonic speeds your showing, you should use 120 instead of the 150 constant used for faster velocity ballistic stablity calculations.

- I use 1:8 twist for long bullets at subsonic speeds in my 300 whisper.

- for another example the .243 caliber barrels that winchester supplied with their .243 WCF rilfes were 1:10 and stablized 100 grn bullets - but the 244 Remington was a .243 barrel with 1:12 twist and wouldn't work with the heavy bullets which guys wanted to use for deer hunting - Remington re-released the cartridge under the 6mm Remington name with a 1:9 twist to handle heavier bullets.

If you plan to use longer bullets go with the faster twist barrel.

Larry Gibson
08-10-2010, 11:39 AM
Given the same pressure for each shot there should be a small drop in velocity as it takes a smidgeon more energy to push a bullet through a faster twist. The energy used to do this then is not used to increase velocity. Whether this is measureable given an expected extreme spread of velocity of which the difference may fall within is questionable at such low end velocities. Only one way to know.

Larry Gibson

melloairman
08-10-2010, 11:46 AM
The speed would be the same . And yes more stablity is what I am looking for for 75 -100 yard sub sonic shoting . But I do not want to lower the fps due to twist rate . Marvin

Artful
08-10-2010, 12:45 PM
Once engraved and moving you would have a little more friction but I don't know much about airguns, I'd go with larry's response and guess it might be measurable but probably no more than 20-50 fps - you'd see it more I'd guess as the airtank dropped in PSI.

Have you asked the maker or googled for reviews of the gun?

Do you have a link to this beast?

44man
08-10-2010, 12:50 PM
Twist has such a minimal effect on velocity I would not worry about it. Stability is 100% more important.

geargnasher
08-10-2010, 03:39 PM
My concern isn't with the twist rate as much as it is with rifling depth, is that an air-rifle specific replacement?

Like has been said you need faster twist with a heavier (longer) boolit in general.

Gear

melloairman
08-10-2010, 04:29 PM
The barrels are B.P. the .357 has a .348 land and .357 grove. Most U.S. makers use the B.P . barrels with the deeper grove . Have had much better results with this type barrel compaired to the shallow grove . Just need to hear some opions about lose of fps due to barrel time and friction . Marvin

Artful
08-11-2010, 01:42 PM
Did you know there's another group of puffters talking cast boolit (well in 45 cal)

there over here

http://castboolits.gunloads.com/showthread.php?t=89212

you might want to look in and ask your question over with them.

geargnasher
08-11-2010, 02:40 PM
The barrels are B.P. the .357 has a .348 land and .357 grove. Most U.S. makers use the B.P . barrels with the deeper grove . Have had much better results with this type barrel compaired to the shallow grove . Just need to hear some opions about lose of fps due to barrel time and friction . Marvin

I'm not an airgun expert, and I have no doubt the deeper grooves make lots of things easier, but I would worry a WHOLE lot more about rifling depth (and thus surface area of the bore) causing a pretty dramatic velocity loss than I ever would friction from a slightly faster twist rate (more pressure/friction on the leading edge of the rifling engraves). If I understand you correctly, your concern is with what we call "spool time" in the performance diesel world, the inertia of the boolit has to be overcome and accelerated much faster for a given velocity with a faster twist. I think it's less of a concern in a .35-bore than in bigger guns.

If you're comfortable with the rifling depth, I'm comfortable with the faster twist barrel because my powder burning experience says you need it for the heavier boolits, but that's just my opinion, and it ain't worth much when the subject's pneumatic guns.

Gear

melloairman
08-11-2010, 09:59 PM
If I have done my math right a 1-20 twist at 1000 fps is 37,500 rpm , 1-16 is 45,000 rpm and a 1-14 is 51,000 rpm . I think I will ask the gun smith how much it would cost to have two barrels made and try them with like ammo . These numbers show you how slow the spin is for suitability .Marvin