rate of twist.
i heard reciently that velocity can also effect what rate of twist you need to stabilize a given weight bullet? like in a 223. you need a 1/7 twist for a 75 gr bullet going what 2900fps? so if you had that same 75 gr bullet going faster would a slower twist still work? how much faster would it need to be to work?
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quote:Originally posted by joshmb1982
i heard reciently that velocity can also effect what rate of twist you need to stabilize a given weight bullet? like in a 223. you need a 1/7 twist for a 75 gr bullet going what 2900fps? so if you had that same 75 gr bullet going faster would a slower twist still work? how much faster would it need to be to work?
Can't answer the question, but..
The .50 cal muzzleloading firearms and .50bmg have about the same weight bullet.
.50 cal minie 1/22 twist rate @900-1100fps
.50 cal spitzer 1/12 twist rate @2750fps
Both highly accurate
I've always heard the length of bullet determines twist rate. Joe0 -
The stability of a bullet is related to the fourth root of velocity. It would take a large increase in velocity to change the minimum twist very much.
Actually a 9 twist .223 will sometimes stabilize a 75 grain bullet, so a good long smooth 8 twist .223 would be fine. A lot of service rifle shooters use 8 twist for up to 80 grain bullets.0 -
twist rate depends more on bullet length than weight. The longer the bearing surface of a particular bullet, the faster twist needed. The longer the bearing surface, the faster it must be twisted to stabilize it, given a set diameter.
Do a search on the "greenhill formula".0 -
joshmb1982,
I've got a little break here so I thought I would add a bit to your discussion about twist rates.
First and foremost, twist rate is determined by the length of the bullet primarily as stated. The velocity of a bullet is not only secondary but can be a non-existent factor as far as standard bullets and standard velocities are concerned. The exceptions to this are bullets launched at extremely low velocities as compared to the standard high velocities and when the bullet length is on the edge of one twist or another based on velocities and when the environmental factors kick in. Ask shooters who routinely travel from hot climates to cold climates or those of us who shoot at high altitudes and then try the same things at sea level.
The generic reference to bullets by weight can be misleading simply because of bullet construction. A classic spitzer bullet with lead all the way out of the nose of the jacket as compared to a Match Hollow Point where the lead is only 3/4 of the jacket volume. The spitzer is usually significantly shorter than the Match VLD bullets and therefore will be accurate in a slightly slower twist.
In your example using the question about the 75 gr. bullets, I went back through some development notes and found that I was shooting the 75 gr. Hornady BTHP Match (0.990") bullets at 2880FPS using a 1:9" twist barrel as an accuracy load. When I switched to the 75 gr. Hornady AMAX (1.070") bullets I needed to use a 1:8" twist for proper stability and accuracy. By calculation, the numbers run 1:9.6" for the BTHP and 1:8.8" for the AMAX. Since I had both barrels and twists on hand it was no problem to set them up for testing. Bear in mind that the manufacturers making standard barrels will usually run in whole inches with only a couple of exceptions. Barrel makers who make cut rifled barrels can adjust the twist rate in smaller increments for fine tuning your loads for specific uses. Competition shooters will often require twists in decimal because of their fine tuning and natural assumption that since someone won with the decimal twist that is next big secret to winning... Little do they realize that most great shooters will win no matter how many places after the decimal the twist is.
As a counterpoint to this description, I have also done a lot of development with the wildcat .22-243 Ackley Improved using the very same bullets. During some of the preliminary tests I used a barrel with a 1:10" twist just to see what the effects would be. The longer (1.070") 75 gr. AMAX performed the best in the barrel with the slower twist. The biggest difference in the test barrels (.223 vs .22-243AI) was the velocity created by the cartridges, 2880FPS vs 3600FPS. That rifle/cartridge/bullet combination is one of the most accurate I own to this day. Barrel life will definitely be shorter with the larger capacity cartridge...
This follows the suggested rule for accuracy as established by the benchrest folks: use the slowest twist rate for the length of bullet based on the distanced being fired. After all, it does you no good if your bullet runs out of steam before it reaches your target.[:D]
Here is a link to a bullet database for checking lengths:
http://www.robsoft.nu/databaser_en/kulor/bullet.asp?MainID=4
The server is down right now but try it later.
Be aware that gain twist barrels are making a resurgence here in the U.S. They are not using the huge range as once produced but smaller, fine tuned differences as small as 1/2" of twist in some cases.
Best.0
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