sighting in target

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bsouthernau
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Joined: Sun Mar 11, 2012 4:31 pm

Re: sighting in target

#16 Postby bsouthernau » Sat Jun 15, 2019 5:13 pm

Tim L wrote:
As I understand it , that is dependant on spin rate being suitable from the start. Spin will slow more slowly than the bullets forward motion will, so the bullet remains stable. I don't know if it becomes 'more' stable as the ratio of spin to forward motion increases but I guess it's possible.


Spot on with the first part Tim. If you have acess to Litz's book have a look at pp 137-138 or you can see the full catastrophe at https://www.fulton-armory.com/faqs/Genl ... /index.htm (hope I got that URL right) among other places.

Cheers

Barry

Tim L
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Location: Townsville

Re: sighting in target

#17 Postby Tim L » Sat Jun 15, 2019 5:39 pm

It was actually Bryan that explained the second part to me. He gave a lecture at the SWNs and explained why Berger had reduced the twist rate on the 215s from 1:9 to 1:10. In a conversation later on I asked about the dangers of running in the "marginal stability" zone of the Berger calculator because I was running 185s down a 1:12 at the time and they seemed to be working well.

I had had a few rums, but the above is what I took away from it.

Edited having read some of the link,
"Thus, if a bullet is gyroscopically stable at the muzzle, it will be gyroscopically stable for the rest of its flight. " is a pretty definate statement, so I obviously took away some erronious understanding.
I do recall that shooting projecties in the marginal stability range can lead to istability. It must be due to some other mechanism though. Varying exit velicities and conditions look likely. I'll have to read the rest now
Last edited by Tim L on Sat Jun 15, 2019 7:28 pm, edited 1 time in total.

bsouthernau
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Joined: Sun Mar 11, 2012 4:31 pm

Re: sighting in target

#18 Postby bsouthernau » Sat Jun 15, 2019 7:14 pm

Tim L wrote:..... Berger had reduced the twist rate on the 215s from 1:9 to 1:10.


Early boxes of these said "Minimum twist 1:10", last lot I bought said "Optimum twist 1:9". I find 2720fps from a 1:9.5 is doing it for me and that's what I'm taking to the UK this month. Never tried them in a 1:10


I was running 185s down a 1:12 at the time and they seemed to be working well.


What works is what works. In theory there is no difference between theory and practice........

Tim L
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Location: Townsville

Re: sighting in target

#19 Postby Tim L » Sat Jun 15, 2019 7:43 pm

There was a bit of history in the 215s Litz had been shooting them the previous year from a 1:9. The reduction to a 1:10 was, iirc due to the extra spin of a 1:9 accentuating errors from jacket wall thickness when there was no need to spin them that fast. That said, he seemed to do pretty well with them.
I shoot 215s from a 1:10 at 2650 with no problems and Chris Allman does pretty well with them from a 1:10, not sure what speed though.
I guess the point may not be quite settled. I'll stick with 200s until it is :)

Good luck in the UK.

lonerider43
Posts: 427
Joined: Thu Sep 01, 2016 6:55 pm

Re: sighting in target

#20 Postby lonerider43 » Wed Jun 19, 2019 10:26 pm

ok,500yrds.sunny,light switching winds.
s1-2,s2-5..4,2,3,5,4,6,4,3,4,4 = 49/60
s1-5,s2-4..6,4,4,4,4,4,3,4,4,5,4 =46/66
there was a small tricky breaze with minor mirage running.
he's a new shooter and wasnt keeping up too good with the changes but did start on the target.
his skinny 24" barrel and hunting stock wasnt really helping things,but he was having fun.[and thats what counts the most]
he is getting an fclass rig built but its a few months off.so for now.... this rifle is what hes got and what he gets to shoot.
i wouldnt call it bad,just good practice...
Australian's Against "Gun-A-Phobia"

bruce moulds
Posts: 2900
Joined: Sun Jun 19, 2005 4:07 pm

Re: sighting in target

#21 Postby bruce moulds » Thu Jun 20, 2019 5:23 pm

Tim L wrote:It was actually Bryan that explained the second part to me. He gave a lecture at the SWNs and explained why Berger had reduced the twist rate on the 215s from 1:9 to 1:10. In a conversation later on I asked about the dangers of running in the "marginal stability" zone of the Berger calculator because I was running 185s down a 1:12 at the time and they seemed to be working well.

I had had a few rums, but the above is what I took away from it.

Edited having read some of the link,
"Thus, if a bullet is gyroscopically stable at the muzzle, it will be gyroscopically stable for the rest of its flight. " is a pretty definate statement, so I obviously took away some erronious understanding.
I do recall that shooting projecties in the marginal stability range can lead to istability. It must be due to some other mechanism though. Varying exit velicities and conditions look likely. I'll have to read the rest now


a definite statement indeed.
and a statement of urban fact.
spin decay is real and has been proven by testing at I think the Aberdeen proving ground, USA.
there is a surprising % of decay that they found, using the 308 as a test medium.
I cannot remember the amount so will not suggest one.
note that we are recommended to use an s.g of 1.5, and now some recommend an s.g higher, possibly for this reason.
other reasons for higher rather than lower s.g. being optimum include issues with dynamic stability, going through the transonic zone, which includes mach1.2, and windshear.
windshear is of greater interest to black powder shooters, as they get the double whammy of that plus transonic at the same time from the muzzle out to 600 yds, so there can be some stacking of problems there.
on the other hand, smokeless can be going transonic when spin decay has happened out past 1000 yds, so here is a real danger point for smokeless rounds.
bullets in flight have an angle of repose when they settle down, and this is the result of vectors of force on the bullet, one of which is wind direction.
when wind direction and strength change, that force on the bullet changes, causing it to attempt to alter its angle of repose.
if it can do this successfully it will remain stable, and if not it will have a little wobble, then repose, or tumble.
shooting with a good helpful marker has proven this to be the case, although I have never actually got one to tumble.
shortening the bullet solved the problem.
0.050" change in bullet length can make an amazing difference to reliability of stability.
whatever the angle of repose, the bullet, and particularly our long bullets will steer a little that way, hence spindrift and lower shots when wind is from the left.
a lot of this stuff is related to what happens to motor bikes that become airborne in a crash while their wheels keep spinning.
flying bikes can perform surprising and deadly antics.
we pay more for good bullets in part to be able to spin them faster with reliability of accuracy.
our longer bullets require faster spin, and thus have a higher yaw of repose, and thus greater spindrift.
so when forces change on these bullets, like windshear, thing can go wrong more.
oh to be able to shoot 100 gn 308 bullets with slow twists at long range.
bruce.
"SUCH IS LIFE" Edward Kelly 11 nov 1880
http://youtu.be/YRaRCCZjdTM

Bigtravoz
Posts: 293
Joined: Sat Apr 22, 2017 7:43 pm

Re: sighting in target

#22 Postby Bigtravoz » Fri Jun 21, 2019 7:26 pm

Think mytarget is the mob I download mine from. They’re free and have many different ones. I use the moa square ones. You can calculate it all off them.


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