I decided to measure the rim thickness of two batches of winchester brass I bought about five years apart. The headstamp is w-w super 303 british.

The first batch bought in 2001. Some rims varied in thickness on each case from .054 to .061. More rims varied from .057 to .060 and .057 to .063. Most rims varied from .058 to .062.

The other batch purchase in 2005 or there abouts most were .060 +/- .001.
The worst was .058 to .061 and the best was .060 to .0605.

It seems to me a rubber "o" ring would be better to use then a monofilament because the "o" ring can fill in broader variations in a individual case rim thinkness then a monofilament could.