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Post by dougedwards on Apr 26, 2010 17:54:40 GMT -5
I was wondering if a bullet that is recommended for 1/8 rate of twist barrels could be helped along in a 1/10 barrel if it were shot faster. In other words, is speed a factor at all when rate of twist if calculated for any particular bullet?
Doug
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Post by cfvickers on Apr 26, 2010 20:49:33 GMT -5
I am no guru but I believe that higher velocity would be more of a detriment than a help. could turn a slight wobble into a downright tumble. I would try the bullet, sometimes they will shoot fine regardless, just depends on the gun and the bullet. It is usually ok to go with a bullet made for a 1 in 10 in a 1 in 8 barrel, but not the same vice versa. if you work up you will likely find a happy medium somewhere, but your range will be shortened almost certainly, how long can the bullet stay stable at 1 in 10? The higher velocity will either knock it way off kilter or it will make it hold longer.
In short, there is no way to say that I have heard of, just gotta try it.
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Post by rossman40 on Apr 26, 2010 21:11:34 GMT -5
For years people used the Greenhill formula which didn't quite work with newer designs specially when you get to VLDs and ULDs. Velocity plays a big roll when comparing say a 223 and a 220 Swift. Most .220s had 1:14 and .223s had 1:12. Sometimes if your right on the borderline another 200fps can make the difference. My AR has a 1:9 and has no problem with 69gr SMKs but if I try 75gr Hornadys or 77gr SMKs, they just do not work. If the Hornadys are loaded hot they come close. A guy I know can shoot the 77gr decent with his 1:9 AR but it is stroked with a bolt carrier weight and a adjustable gas port and they are being pushed to the max+.
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Post by dougedwards on Apr 29, 2010 20:24:59 GMT -5
I have never been much on physics but I would think that shooting any particular bullet faster through any rifle barrel would also mean that bullet was spinning faster. If a bullet requires a certain spin rate to stabilize, couldn't that same bullet achieve the same spin rate in a barrel with a slightly slower twist by shooting it faster than normal? I think that Rossman may have answered the question but I am slightly hard headed. Is my thinking wrong here?
Doug
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Post by ET on Apr 30, 2010 7:43:59 GMT -5
Dougedwards
No expert here but this experience may shed a little light to your question.
Few years back I did some reloads for my hunting partner’s .222 in a Rem. 660 model.
I load 50gr and 55gr bullets for him to try. They both shot well in the summer time but when Fall with temps near freezing hit the 55-gr bullet started to key-hole the target. Well having a seasonal working load was not acceptable to him. With ambient temp being the factor I could only see a small velocity change occurring that was enough to affect the stabilization of the 55gr bullet.
Ed
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Post by dougedwards on Apr 30, 2010 20:18:44 GMT -5
Berger 6.5mm 130 gr VLD's are rated for 1/8 twist barrels and my Swinglock is 1/10. Shot them at 3200 fps and the first shot from a clean barrel hit one inch high and 3/4" to the left. The second two shots landed 1/2" right of the first shot touching each other at 110 yards. It was 80 degrees F so I didnt' push the envelope by raising the powder charge. I am thinking of moving down in powder charge next time to reduce the speed to see the results of that.
I bought the Bergers without noticing that they were 1/8 and based on what you guys are saying I wouldn't know for sure until I tried them. They are match bullets but could very well be my long range choice if they continue to shoot this well.
Doug
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Post by billc on May 1, 2010 17:40:36 GMT -5
Doug,
You eventually have to obtain a spin rate - revolutions per second (RPS) - to stabalize a bullet. RPS is a function of velocity and the rate of twist. Both terms are variable and you can get the RPS either way. That doesn't mean that accuracy will be the same as changes in velocity would affect the barrel harmonics.
But, to roughly illustrate your question, using the Load From A Disk program, optimum twist function with the following conditions: a Sierria 123 gr HPBT MK, 6.5 mm bullet, length 1.308 inch, BC 0.51.
At 2400 fps the optimum twist is 1 in 7 with 4114 RPS.
At 3000 fps the optimum twist is 1 in 8 with 4500 RPS
Bill
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Post by ozark on May 1, 2010 19:38:12 GMT -5
years ago I heard and read about gain twist. It seemed like a good idea to me to start bullet spin slowly and then speed it up as it traveled down the bore. Haven't heard anything about that lately. Did it die out because of lack of merit?
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Post by billc on May 1, 2010 20:56:52 GMT -5
A few manufacturers make gain twist barrels. The only one I recall off the top of my head is Anschutz on some of the top dollar 22 target rifles.
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Post by dougedwards on May 1, 2010 20:57:24 GMT -5
Doug, You eventually have to obtain a spin rate - revolutions per second (RPS) - to stabalize a bullet. RPS is a function of velocity and the rate of twist. Both terms are variable and you can get the RPS either way. That doesn't mean that accuracy will be the same as changes in velocity would affect the barrel harmonics. But, to roughly illustrate your question, using the Load From A Disk program, optimum twist function with the following conditions: a Sierria 123 gr HPBT MK, 6.5 mm bullet, length 1.308 inch, BC 0.51. At 2400 fps the optimum twist is 1 in 7 with 4114 RPS. At 3000 fps the optimum twist is 1 in 8 with 4500 RPS Bill billc.....that confirms my suspicion. |
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