RPM ACCURACY THRESHOLD

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  • Last Post 16 January 2016
CB posted this 25 November 2007

I'm reading references to an “rpm threshold” beyond which accuracy diminishes. Can't find anything on the search places. Can somebody tell me what it's all about or direct me to a source? Has top do with twist and mv etc.

Thanks;

joe b. 

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John Alexander posted this 16 January 2016

No more posts are being accepted on this thread.

John

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Hamish posted this 16 January 2016

"I want it to be known that this forum is not going to become the new RPM or 6.5 Swede battle ground as long as I'm a moderator."

Please and thank you.

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45 2.1 posted this 15 January 2016

pat i. wrote: A topic bought back from the dead. I want it to be known that this forum is not going to become the new RPM or 6.5 Swede battle ground as long as I'm a moderator. People are allowed to believe whatever they want and as long as the conversation remains civil I say discuss your heads off but if things start going south the thread will be closed. A certain bughole shooter from another forum is already talking about signing up here to get the battle underway but I suggest he thinks twice about it because his stay will be short lived if I have a say in it.

Bravo......... those are good intentions, BUT..... it's already going South here:

http://www.castbulletassoc.org/forum/view_topic.php?id=12453&forum_id=63&jump_to=91259#p91259>http://www.castbulletassoc.org/forum/viewtopic.php?id=12453&forumid=63&jumpto=91259#p91259

Time to do something!

Nice to see you again Pat and Mike.

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OU812 posted this 10 January 2016

runfiverun wrote: nope it's centerline just like hornady shows in the front of the reloading manual. that's how you get around the rmp thingy. It's Centerline, RPM, less fouling and a 30 pound BR rifle.

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357Maximum posted this 10 January 2016

pat i. wrote: A topic bought back from the dead.

I want it to be known that this forum is not going to become the new RPM or 6.5 Swede battle ground as long as I'm a moderator. People are allowed to believe whatever they want and as long as the conversation remains civil I say discuss your heads off but if things start going south the thread will be closed. A certain bughole shooter from another forum is already talking about signing up here to get the battle underway but I suggest he thinks twice about it because his stay will be short lived if I have a say in it.

Quite the thread necropsy.   I do not post much here, I mostly just read, but I applaud what you wrote there Pat.  Good form.:dude:  The cancer has already ruined one forum for all and I for one am glad you are ready to cut off the infected digits before it spreads here. THANK YOU, Mike

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pat i. posted this 10 January 2016

A topic bought back from the dead.

I want it to be known that this forum is not going to become the new RPM or 6.5 Swede battle ground as long as I'm a moderator. People are allowed to believe whatever they want and as long as the conversation remains civil I say discuss your heads off but if things start going south the thread will be closed. A certain bughole shooter from another forum is already talking about signing up here to get the battle underway but I suggest he thinks twice about it because his stay will be short lived if I have a say in it.

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runfiverun posted this 10 January 2016

nope it's centerline just like hornady shows in the front of the reloading manual. that's how you get around the rmp thingy.

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linoww posted this 22 December 2012

I had thought for a given twist a larger bullet has more surface area to heat up and otherwise get out of shape.I figured by the time a small bullet gets down the bore it as had much less total surface area rubbing the bore.That's it for my theory stuff.

George

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pat i. posted this 22 December 2012

George I'm a simple guy with a simple mind so the best way for me to explain what I think is going on is to use a washing machine example. The spindle in the middle of the tub is the centerline of the bullet, a single piece of clothes on one side of the tub would be an inclusion. When the tub first starts the spin cycle and is going relatively slow there's not much commotion going on but once the thing really gets up to speed it's banging and hopping around like crazy. You'd have to have an even simpler mind than I have to think for one minute that our cast bullets aren't full of defects and inclusions. The larger the bullet diameter is the farther away from the centerline a problem could exist. Maybe I'm all wet but from my own personal experience I don't think I am.

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linoww posted this 22 December 2012

is it just surface area related ?

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pat i. posted this 22 December 2012

Don't know why but can only assume that it's because the inclusions and defects are closer to the centerline of the bullet. I found this with my 6.5 PPC which I could run at 1700 fps and 156,000 rpm with the 8 twist barrel. My only conclusion and one I've written in the past is that the smaller the caliber the higher the twist limit is. Remember I said that accuracy wouldn't be AS good after an optimum twist was chosen not that it wouldn't be completely usable for it's intended purpose. Personally I think for 30 caliber about 130,000 rpm or so is at the upper end for best accuracy and I think the CBA match reports and records bear me out. Maybe for 6.5s it's 160,000 and for .22s it's 190,000. I don't know because except for the one example all my cast bullet shooting has centered around the .30 calibers.

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linoww posted this 21 December 2012

22 cast bullets seem to respond well(or better) to higher velocity.Why?

george

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RicinYakima posted this 21 December 2012

Oh No Pat! You touched the sore spot, again. The “keyboard” and “internet” shooters are going to be unhappy. If it doesn't happen under a CBA match program, in front of other shooters, it doesn't exist. As far as accuracy goes, if it doesn't happen at CBA, ASSRA or some other nationally recognized organization, it is BS. They know it and we know it.

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pat i. posted this 21 December 2012

I have 30 caliber barrels in 10,11,13,14, and 17 twist chambered in either 30 BR or 30 PPC and no one will ever convince me that barrels don't have a twist limit/RPM threshold. People who actually tried different twists and don't claim to shoot 1/2 inch groups with everything they put on the bench don't say that they go from one inch groups to 5 inches once the twist limit is reached but accuracy will deteriorate over a certain RPM no matter what they do fit/reloading trick wise. What should be a glaring red flag by people who claim the limit is BS and that guys who can't best it just don't know how to reload or shoot is their reluctance to prove it in public. It's easy for folks to bad rap the CBA match program and claim there's better shooters out there but there's no ignoring 2 shots out of a 5 shot group or shooting at 25 yds and claiming it's 100 when there's other shooters sitting right next to you that you're not related to.

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runfiverun posted this 21 December 2012

it was allready done with a 308... that testing was done in conjunction with a pressure graph test,and a lineal dispersion test. all that was proved was that certain boolit molds didn't take the higher velocity's better. and the “threshhold” was just a jumping off place to needing a better boolit/case fit,better alloy,and specialized loading techniques. the “threshold” is the lower limit when all of the above are applied. better results and higher velocity's are achievable with a slower twist rate without employing them.

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EDG posted this 21 December 2012

It seems there would be a good answer for this tempest. Simply take a sample of identical barrels with varying twists and chamber for the 38-55.

The fast twist can be 1-10 and the slow can be 1-20. There are also 1-12, 1-16 and 1-18 twist barrels.

Just pick a half dozen reasonable loads and try them in all the twists and see what the results are.

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madsenshooter posted this 05 November 2012

What seems to have been determined on cb was that the term “RPM Threshold” was just that. Not a ceiling which one could not go above, but a matter of semantics, someone's terminology for the point where everything went to h### with a given alloy/load. In some instances, the RPM of the bullet may have been the factor that got the load to that point, then again, maybe not. Pressure seemed to be my biggest nemesis when I was working with 6mm 5R rifled Obermeyer barrel. Bullet fit was also a factor. I have to get back to that experiment, got a new mold to try out that'll fit the bore better.

http://castboolits.gunloads.com/showthread.php?t=121224&highlight=6x45

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Chargar posted this 22 October 2012

I went to a Pentacostal church one time and heard folks speak in tongues, but Dr. Mcoy can write in tongues.

I like simple!

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Ed Harris posted this 22 October 2012

Ric, I agree with you. The accredited and authoritative references were listed for the benefit of readers who are not satisfied with anything simple.

73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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RicinYakima posted this 22 October 2012

Ed,

Herr Nennstiel's paper is the best for we layman, but sometimes it is better for us to just say “It's magic” and let it go at that. The why's can get very messy with my old brain.

Ric

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