Charge Weight Accuracy

[gbal]

While waiting for the rugby match yesterday,I did some ball park calculations on this 'kernel' issue.

 

Powder kernels vary between powder brands ,but taking an average of those who have actually weighed/counted kernels,lets take the weight of a kernel to be .02g (range has been .025,.02,.167.in various counts).

 

So one kernel is .02g;and therefore 5 kernels weigh .1g (the usual basic precision start place),and 50k weigh 1g

 

Lets take a 30 cal with 155g bullet loaded with 43.4g (viht140 max load).

 

This load has therefor 43.4x50 kernels,which is 2170 kernels

It gives a MV of 2650fps.

 

So each kernel 'contributes' 2650/2170 fps,which is 1.22 fs (average,of course some burn hotter etc than others)

 

OK so IF we measure to a kernel with sophisticated scales etc,we should have ammo with that consistency of MV-and to the extent that powder is the only factor in MV,we should have that consistency ie a SD of zero...but lets allow 1kernel,or 1.2 fps (double if you like-it isn't critical,as you shall see.......

So MV SD 'should' be 1.2 (or 2.4 allowing some error between scales and brass)

 

But reported MV SDs are a bit more than this ,from fastidious reloaders with top equiment and good chronos over decent samples.

Not many get much below 10fps,but lets take an SD of 6fps MV.

 

That note is 5x our 'powder only' value-which was a rather generous 1.2 only (and 'really' zero)

 

Soooo,at best a kernel precision in powder accounts for only 1/5 of the MV SD

 

It rather follows that there must be other factors (or one factor) that contribute(s) far more;five times as much...

 

Likely factors would include variance in case capacity and neck tension,bullet weight,possibly primer ignition effects-and of course,the complex possibility of some ineractions between these factors.

 

Hmmmm we should all be grateful that some devotees have done the careful powder wok needed to isolate that variable.

Case capacity should be fairly readily measureable,bullet seating neck tension has no direct measure readily available,but the K&M in line seater allows batching,pretty closely,so good enough,esp after annealing.

 

OK, another way to consider all the factors involved in an outcome -ultimately here it is precision of the ammo- is analysis of variance-but we lack the basic data (eg for measureable neck tension).However somewhat similar analyses give interesting potential comparative improvement scores (target hit %s) for various components. Assuming a reasonable baseline (not eg 17Rem for 1000y shots),refined reloading comes in about 10% ,and wind reading,and ranging are at least three times that-each.

 

A laser (field shooting) and basic anemometer might prove sound investments...discuss. But all this is for another wet Sat/Sun,when there is no potentially gripping rugby scheduled ('gripping' is a 90% component in scrummaging,but should be zero % in off the ground high kick ball catching.... :-)

 

gbal

[Re-Pete]

You all need to get out more...............

 

RePete

[VarmLR]

I'm just wondering if a simple way to measure neck tension differences, or at least frictional differences between cases, is simply to rig up a test rig that uses dummy rounds with seated bullets and have the bullet clamped firmly, then apply a pulling force to the case head and measure (in Newtons) the force needed to overcome friction and get the bullet moving. That has a direct link to neck tension since the elastic compressive force acting on the bullet case is proportional to the force needed to pull the bullet.

 

It's something that someone with a little know-how could rig up at home with the right kit. Just as an experiment rather than for any real world application.

 

The alternative would be to go to the pub

[1066]

I'm just wondering if a simple way to measure neck tension differences,

 

When I'm working up a new load and have the charge weight about right, I often load a few up at that charge but seat them as long as I can. I head to the range with a Lee hand press and slowly seat them deeper depending on results.

 

What I have noticed is, if I leave it a week or so before trying to seat them deeper the bullets definitely "stick" a bit before they start moving - I don't feel this if I seat them deeper soon after loading.

 

Nothing straight forward in this game is it. Is it because the cases are too clean? should I dry lube the bullet base/case neck before seating? Does the same thing happen with Moly coated bullets?

 

Does it really matter? Probably not for any practical purposes - just an observation.

[VarmLR]

 

When I'm working up a new load and have the charge weight about right, I often load a few up at that charge but seat them as long as I can. I head to the range with a Lee hand press and slowly seat them deeper depending on results.

 

What I have noticed is, if I leave it a week or so before trying to seat them deeper the bullets definitely "stick" a bit before they start moving - I don't feel this if I seat them deeper soon after loading.

 

Nothing straight forward in this game is it. Is it because the cases are too clean? should I dry lube the bullet base/case neck before seating? Does the same thing happen with Moly coated bullets?

 

Does it really matter? Probably not for any practical purposes - just an observation.

 

I wonder if that is galvanic action (dissimilar metals) causing some minor corrosion "stiction" between jacket and case? You have a point as any neck tension test would have to be done soon after seating. I can't think of the purpose of doing it with anything but a series of dummy rounds but all it'll show is variation before anything's shot but not necessarily the reasons why so arguably pointless as an exercise? Again, one can get too anal about these things when being perfectionist in case prep and using the best quality dies etc (and adjusting run-out etc. if required) is all realistically I guess the target shooter can do. Propellant batch variations and perhaps even primer variations will always be beyond the control of the shooter.

 

Pub it is then!