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> Twist Rates???
Twist Rates???
@ 06:16 am (GMT) |
Mark WhitakerNathan, I have been delving into the theory of twist rates and it seems to me that manufacturers make twist rates to cover most bullet weights as a compromise.The only calibre they seem to specialise in is the 223 - 1:8 for heavy and 1:12 for light. When loading for my 7mm08 with A-Max 162gr, the box suggests ideal twist is 1:8, but my Remington is 1:9.25. My question is, if I were to rebarrel 7mm08 for just A-Max 162gr, would you rebarrel in 1:8. If you were to make a 7mm Practical would you make it 1:8? Would you not have a higher RPM due to velocity with the possibility (theoretical only) of the bullet spinning too fast? IE: would you use the same twist rate regardless of velocity, or a slightly slower twist rate to compensate for the velocity? Mark |
Replies
@ 06:38 am (GMT) |
Mike DavisRe: Twist Rates???Nathan mentions this somewhere as a double edged sword for projectile makers..too fast of twist going too fast a velocity means too hard of projectile needed to keep it together rendering it pinhole material at any further distant shot.why not select projectile to SUIT the rifling you have got and see if performance is better??? |
@ 07:12 am (GMT) |
Thomas KitchenRe: Twist Rates???hi mark have you tried the amax's how well do they shoot in your twist rate. |
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@ 01:01 pm (GMT) |
Mark WhitakerRe: Twist Rates???The A-MAX works fine in my 7mm-08.As I said in the first post, l'm looking into the theory of twist rates. Another example would be the 338 Federal. In Nathan's second book, he says this would be a great long range calibre with the Rocky Mountain 225gr. 2800-3600 fps, ideal twist rate is 1:11. These velocities are ok for 338 lapua or Edge, but at 2400fps of the Federal then a faster twist rate would be better. As I said, just trying to understand the theory. As it's been stated in another thread, Nathan's Long Range Calibre book make great reading and re-reading. Mark |
@ 04:54 pm (GMT) |
Thomas PavelkaRe: Twist Rates???Hi MarkGlad you brought this question into the light because twist rate does have a bearing on bullets. Years back I picked up a Weatherby Mark V rifle in 7mm Weatherby Mag. It was a beautiful rifle with wonderfully grained and figured wood. It was one of the most pleasant rifles I have ever shot in as far as recoil went and I set out to work up a good load for the rifle. As memory serves me, I went through all of the 7mm bullets that Sierra had at the time and could not get a one of them to group worth a hoot. 3 to 6 inches at 100 yards is not wonderful. I then went to Hornady bullets with the thought that at the time Weatherby was loading their factory ammo with Hornady bullets and I thought sure that would fix the problem. As bad, if not worse than the Sierra results. Then I tried Speer bullets. Exact same with any Spitzer bullets. Now keep in mind, I was playing with different powders, different charges of powder, bullet seating depth, brass, and even primers. I burned up hundreds of rounds in an attempt to get that rifle to shoot well and could not get it to do so. There was rebedding, pressure points, free floating the barrel in between. I even glass bedded the rifle. Then one day I made a mistake when I bought a box of Speer bullets and came home with a box of 175 grain round nose bullets. What did I have to lose. So I loaded some up with H870 and went to the range. I fired a shot and it was 3 inches high dead windage at 100 yards. I fired another one. There was no other hole in the target. I must have pulled it. Fired another one. No new hole in the target. I jacked up the scope to 12X and I thought I was seeing things. It looked like the 1st hole was slightly larger. So, I fired two more shots and then walked down to the target. You couldn't sandpaper the smile off my face with 40 grit sandpaper. All 5 bullets were in the same hole! At last! And that rifle would do that every single time I laid it on the sandbags with that load. Wonderful. Now who the heck wants a 7mm Weatherby rifle that can only shoot 175 round nose bullets. If nothing else this proved the rifle was capable of superb shooting, but why only with a heavy round nose bullet? Papa Roy Weatherby was still alive at the time and so I gave him a call in California and told my tale. He said that there was a batch of the 7mm barrels that did not have the right twist rate and what was happening with the lighter spitzers is the cores of the bullets were actually molten by the time they left the barrel. The only thing that kept the lead in the jacket was the air cooling the tip of the bullet which acted like a cork to keep it in. He re-barreled the rifle and the problem was solved. I think in this day and age we shouldn't have to be analytical geniuses in order to get a band of bullet weights to shoot in a given rifle. Yet here is a table from Shilen Barrels on 308 cal bullets and twist rates: .308 - 7" * for heavy VLD bullets and/or subsonic ammo. - 8" for bullets heavier than 220 gr. - 10" for bullets up to 220 gr. - 12" for bullets up to 170 gr. - 14" * for bullets up to 168gr. - 15" * for bullets up to 150 gr. - 17" * for bullets up to 125 gr. My 308 with a 168 grain VLD Berger Bullet is right about on the fence at a 1:12 twist then, and this may be why I can't get it to group those bullets, or at least a part of it. At this moment I am loving my 270 as Shilen says 1:10 twist for ALL bullets. Now why can't they all be so simple. This stuff makes my head hurt. |
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@ 11:18 pm (GMT) |
Mark WhitakerRe: Twist Rates???I have recently come across a Winchester M70 Standard long action and am thinking of building a custom rifle in 280 REM, 26" magnum contour barrel and only using heavy projectiles no lighter than 162gr. I haven't rebarreled the 7mm08 and i will keep it so I have a light weight and a medium weight - long range 7mm rifle.As stated above a AMax works fine in 9.5" twist rifle, but there are now newer heavier designs stating 8" twist rates. As Tru Flight have 8" and 9" 7mm barrels would it not be prudent to build the rifle with an 8" twist barrel to utilise these heavy bullets now and any new designs in the future? Mark |
@ 06:57 am (GMT) |
Eloy VilavellaRe: Twist Rates???The faster twists are provisioned to load the longest vld bullets.Many times there is a relation with bullet weight but stability factors depend on lenght, contruction, speed, rifling etc.... Solid copper bullets require faster twists for the same grain bullet. They have to be longer to meet the grain because there is no lead. Also aome march bullets are longer like lets say the 107gr sierra 6.5mmSMK that is as long as a 123gr amax including the tip. So this is a nice bullet but with a greater differencial between the center of gravity and the center of pressure so if they are npt turned faster they show considerable yaw and mifht tumble. So I agree choose a twist that suits your bullet choices but also consider the benefits of a faster more versatile twist wo over doing it. |
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@ 12:07 am (GMT) |
Eloy VilavellaRe: Twist Rates???When I said VLD bullets I also had solid copper hunting bullets in mind as those also might require the extra twist due to their extra length.So again, weight says something about the bullet but not really the determining factor. The best thing is to start by following bullet manufacturers recommendations. |
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