I wouldn't buy a Skeets again. The .001 jump isn't critical at all, it's just something that bothers me when I use a micrometer.

The Skeets instability is not temp-related. It's construction related. However, they are accurate enough for shotgun bores and chokes.

You can get inexpensive ID standards for calibrating gauges on Ebay but they might be weird dimensions - that doesn't matter. So what if it's .607 as opposed to .600.

You can also get them at MSC or McMaster-Carr.