Batching by bearing surface

[Jase PTRC]

Recently i posted up some photos of variations in base to ogive length in some Berger 155.5 full bore's. It has me wondering about bullet sorting.
When batching bullets by length either base to ogive or by bearing surface how many thou variation is acceptable to batch by? also what tool does everyone use to measure bearing surface?
I have a Hornady comparator but obviously it only measures from one end so im measuring base to ogive.

[AlanF]

Jase,

The main reason for sorting bullets is to get uniformity of drag in the air. I suppose bearing surface length could affect pressure in the barrel and hence muzzle velocity, not sure how much. Bullet characteristics affecting drag in the air are length, diameter and shape. I've found over the years that the types of variations can differ between bullet brands and styles. What I do with any new type of bullet is measure a small (say 30) sample for many things (weight, diameter, base to ogive, meplat size, bearing surface length, and OAL). Base to ogive has been my favourite in the past, but lately with Bergers, its been measuring OAL and inspecting meplat size, because I believe the variation in these is the major cause of drag variation. On your question of how best to measure bearing surface length, you could get another nose piece like your Hornady and put it on the other caliper blade. As far as how many thou to batch by, I usually do 500 at a time with base to ogive, and sort into piles with up to 0.002 variation.

Hope this helps.

Alan

[Jase PTRC]

Thanks Alan, i do check weights but only about 10% of the lot (so 50 pieces) out of 500. if thats consistent then i have been measuring base to ogive but was only doing a sample of the lot so i wasnt really batching more so just checking that sample and seeing if there were any huge variations. I want to step up my game a so will start measuring all of the batch from now on and batching accordingly.

[RAVEN]

Jase
think of Standard Deviation the smaller the difference between batches the less variation you should have at the target in a perfect world
I wouldn't bother doing it for the short ranges.

RB

[Old Trev-39]

I check every dimention to .002 for all my 1000yd projectiles, including length and weights to 1/10 grain. Weighing is done after the metplat. I then batch into groups of 5 projectiles as I shoot light gun I only need 5 for each group.
Order of measuring.
(1) Base to ogive including visual inspection for any deformaties.
(2) Metplat
(3) bearing surface
(4) Length
(5) weight
(6) Package into 5.
Cheers,
Trevor.

[Normmatzen]

For what it's worth, Brian Litz, the ballistician at Berger Bullets, stated that practically speaking base to ogive is 1. easier to measure than bearing surface and, 2. any error in measurement relative to bearing surface is trivial.