bearing surface length VS pressure

so ,
to those active in this discussion, is the .020 variation in my two lots enough to see on target variations ??

i would think that with the same load at plus or minus .03gr and bullets in the plus or minus 0.15gr variation...that if friction or force to fit the bbl, that some velocity difference might show up. the difference could be reverse engineered to show what addtional pressure was used in "fitting"/friction.


mike in co
 
I would think .020 in variation your going to see something! Just make sure you don't change anthing but the bullet. You will be testing at what distance.

Joe Salt
 
200 is the longest i have reasonable access to.....

gonna chrono it all try for a lite wind day...and see what happens.

mike in co
 
Duration

Dave Tooley said:
More to the point it's the duration of the required pressure to fully engrave the bullet in the rifling that affects the pressure curve.
Click to expand...

And, it is in that time that the bullet is subjected to its highest upsetting forces. (perpendicular to bore)

Ken
 
We are not going to solve the question on speculations its going to take a pressure gun to really answer that question correctly. In some cases you might see a spike of pressure if you working on the very fringe of safe pressures with a particular bullet. Changing the bearing lenght {longer} may give a spike in pressures cuseing hard extraction , blown primer or shiney spot on the case web.
Those would be the only indications that we have outside of sticking a bolt/
Each load has to be worked up carefully . any change can not only efefct accuracy but safe working pressures. Ive seen some shooters working on the very fringes of the pressure curves.
I guess the word is be very careful.
 
I have only one test where I compared the same bullet with different lengths over an Oehler 43. This test was done when I was exploring the affects of trimming meplats and tipping bullets. It was done in April, 2003. It was 6MM 105's and they came out of the same box. .030" difference in length of the bearing surface. This wasn't my rifle so I have to try and remember the comments made by the owner that day. Our goal was to only compare BC's. Nothing else. As I remember it he made no serious attempt at what I would call precision reloading for this test. Just dumped powder in, seated bullets and we were off to the races. Here's the data

9 shot group baseline .000" Avg muzzle Vel 3172 Avg BC .535
10 shot group +.030" in length Avg vel 3140 BC .526

Now one could look at those numbers and make some sweeping assuptions but here the rest of the story.

9 shot group the velocity spread was 112 FPS and the +.030" group the vel spread was 70 FPS

Other than the BC numbers the rest doesn't mean much.

Those of us who have used FB bullets in 1K comp have from time to time had issues with survivabilty. Some of those same FB bullets when a small BT was either machined on as by Henry Childs or added during manufacturing by Randy Robinette found the very same exploding bullets made it to the target. Some would say a reduction in heat from the BT reducing the bearing surface. I'm not so sure now. During my stubby barrel test I had several FB bullets mushroom on the base from the force. I did some things to reduce the friction and the mushrooming stopped. Since then I've done a few other things to manage the trauma as the bullet enters the rifling and have gotten promising results. As always your mileage may vary.

Dave
 
One part of this discussion that makes no sense to me is how do some of you measure the actual bearing surface?
Are you doing it with a optical-comparator?
Are you doing it on a X-Y table?

If your using a "base to ogive" from a comparator stand or with two comparators in a calipers, how do you derive at a increase or decrease in bearing lengths?
Isn't or couldn't the variance in lengths be actually ogive radius variations and not the bearing surfaces?

I have a friend who has access to a very expensive optical-comparator. He sorted bullets with a "base to ogive" stand, and by over-all length, then took the extremes of the sorting to the optical-comparator to find where on the bullets the variances were actually at. The optical-comparator revealed all the boat-tails and bearing surfaces to measure the same. All the variance was in the radius of the nose and the over-all lengths.

Over many years of segregating bullets with a "base to ogive" method (dial-indicator stand), I have never felt the variance in lengths to be bearing surface, and have always looked at them to come from ogive variance.
But no matter where the variances are on the bullet,.... I look at them as BC variance at the target.

From a box of 107-SMK's several years back, I had a spread of bullets of around 0.024" by base to ogive. I group tested the extremes at a 1000yds from a 1000yd Lt-gun and its pet load at the time. The dispersion was 27" of vertical between the two extremes of that box of bullets. At the same time, the segregated piles were printing 5-shot groups between 3.5" to 7".

I also will say "Seems like another discussion looking for a solution to a problem that doesn't exist or is ill defined"


to Dave - Thanks for sharing your "stub barrel" test !!!! - - very innovative idea !!!!


Happy Shooting
Donovan Moran
 
Last edited:
Keep in mind that the original question is somewhat simplistic in relation to the spin were taking.
 
Donovan measured bullets that differed .024 base to ogive and his measurements showed up on the target. That they showed on the target somewhat proves that he measured correctly. Not to hijack the thread, but I'm curious as to the mechanism of how bullets made from the same components and in the same dies could possibly differ 24 thou from base to ogive. In my experience, a bullet maker would have to work at it to create that kind of variance.

Pretty sure that if you're gonna measure bullets, measuring base to ogive vs. base to anywhere is apples to apples. No need to select a specific point.

Uh-Oh - logged in with my test ID

Wilbur
 
Donovan: If you have done this test, an there was a 3.5" to 7" vertical spread, how would you like just taking them out of the box and not now which one just cost you the world record. That is the reason I measure what you call Ogive to Ogive the part that actually touches the rifling. You guys must trust that every thing from the factorys is perfect, you probably think your primers can be taken right out of the box also. I don't have an Optical-comparator, but I no there is a reason when my rifle doesn't shoot the way I want it to.

Joe Salt
 
You guys must trust that every thing from the factorys is perfect
Click to expand...

Joe -

Been segregating bullets, brass, primers since 2001 (my first year of competitive shooting)... and always will !!!!
And would never expect 2 bullets with much difference in measurements to have POI similarity that is needed in competition.

At the time (2005) when I tested those 107-SMK's with 0.024" variance B-t-O, I also estimated them to have around a 8% variance to BC. The 27" of vertical ballistically showed the 8% BC variance to be a close estimation (the velocity ES was 14-fps between the sample at a 3033-fps average).

Happy Shooting
Donovan Moran
 
Donovan: So what your saying, hate writing it down, you and I are in agreement long and short bullets do make a difference. So children sort your bullets. I think thats all that everyone is looking for a good test.

Joe Salt
 
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