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Hi All

BEARING SURFACE MEASUREMENTS and RESULTS

Been threatening add this review / findings for while so bear with me.

Working on the principle that if you cant measure it you cant sort it I happily purchased on of Ovenpaa's Bearing Surface Comparator
http://shootingshed.co.uk/oscom/product ... ucts_id=48

Had 5 x 100 Berger 308 155.5 BT that I then proceeded to measure and sort.

METHOD
I first measured the bullets and split the into 4 groups according to bearing surface - called A, B, C and D (I know very original).
I then weighed each bullet and segmented them W1 = 154.90 : W2 = 155.0 : W3 = 155.10

RESULTS
Underneath are the results in pictures

The post continues......

BEARING SURFACE MEASUREMENTS and RESULTS

SHOOTING SHED BSC OPERATION and USE
To sort by bearing surface took me about 3 hours on evening on the dining room table.
It was very quick and accurate. Compared to the vernier method it was streaks ahead!
This tool rocks! :goodjob:
Once completed I did a random sampling of 10 bullets from each group and remeasured them.
I had a change of about 2 out of 50 - call that user error! sign01

SUMMARY - see pic below
By Bearing Surface and % of volume
A 45.2% you find W1, 2 and 3
B 34.7% you find W1, 2 and 3 - more even 2 and 3
C 14.0% you find W2, 3 and 4
D 3% you find W3 and 4 only

Some extremes were excluded 9 or 10 bullets
The lightest bullet weighed 154.84 and the heaviest 155.44.

CONCLUSIONS
Should you believe that the Bearing Surface of a bullet is a variable you wish to mitigate then this is a very worthwhile exercise with the RIGHT TOOL!
There is a significant distribution of bearing surfaces within the same lot of bullets.
A great tool that I value highly :cheers: !

ENS

Well done on an excellent post, I haven't done bearing surface checks but have batch weighed 200gn Sierra Matchkings and found them to be varied +- .2 grains

Alan

Do you have the dimensions for the bearing surface differences?

j0e_bl0ggs wrote:Do you have the dimensions for the bearing surface differences?

Sorry obvious lapse on my part!

Bearing Surface Categories
"A" was defined as base value of 0.0010 and smaller (so it could be 2 categories those under 0.0000 about 0.0015 spread in total)

"B" was 0.0011 to 0.0020

"C" was 0.0021 to 0.0030

"D" was 0.0031 plus

SCRAP or "E" was about 5 to 6 bullets that were wierd / damaged

In effect the BS extreme variation was just under 6 thou (ie 0.0060 including outliers)
The bulk 98% were within a variation of of 4 thou

CONCLUSIONS
I saw a distinct correlation with the Weight distributions with Bearing Surface categories
Longer BS had no lighter bullets.

My hypothesis is that when the lead plug that is inserted into the empty jacket it is ever so slightly longer and thus when the dies forms the ogive it is that fraction longer... my rough guess - heck it could be extra bnding liquid for all I know.

TESTING
I still need to properly test a set of 10 x "A" versus 10 x "D" over a chrono to validate the average variation in average speed.

When I tried I had some chrono display problems that compromised accurate readings
and I dont feel like WAG's!

ENS

Thanks!