Friday, February 4, 2011

The changing bullet diameter, sized and unsized. It can help, or it can ruin your groups.

Curiosity about how bullets change size over a period of a few weeks got me started on a test. With different bullets poured and measured all at the same time, some of the results are interesting. 

A good single representative view makes it easier to see what is going on. Other bullets followed a similar pattern, they vary a bit in the amount of change from week to week.

Here are 2 bullets from the same cavity of a 358-429 mould. The green line was lubed in a Lyman 450 with a .360 die within hours. The size remained the same because it wasn't big enough, but it was lubed. The blue line is the other bullet, unsized and unlubed. Maybe not the perfect scenario, but that's the way it worked out.

It's a three week period with each bullet measured on the seam.

Below is a description how this bit me: (click on the graph for a larger view)

Pretty fascinating.  The growth and shrinkage are different for bullets cast from the same cavity within seconds of each other.  The lubed bullet stayed consistent.  My assumption is the coating of lube protects the lead from the effects of air over the first few weeks.  However it matters less why this happens, just that it does.

The most accurate loads I have became erratic, producing a shotgun pattern instead of a tight group.  The cause was a change in my process, and related to the bullet growth in the chart.

In the past, the process that worked is:
  1. Cast a few hundred bullets
  2. Each week prior to shooting, lube/size & load 200 for the week

Pretty simple, each week I lubed and loaded the rounds for that week.

Then, in an effort streamline and improve things at bit, here's what didn't work:
  1. Cast, lube & size all the bullets
  2. Each week prior to shooting, load 200 for the week
Using the knowledge from the chart, the past process sized fairly large bullets, after they had aged (on a Lyman 450 lubrisizer).  It turns out sizing large bullets in this case produces bigger bullets than the sizing-aging approach. 

The new process produced bad results because it limited bullet size.  They never got big enough to produce the best groups at the range.

When groups of 1 inch or smaller are common, a 2.5 inch shotgun pattern is a huge disappointment. 

Finding and solving the problem is a reward all it's own.

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