Dave, an XMR chart is used only in cases where you want to use an Xbar chart but can't afford to take multiple samples and average them.  It allows you to take advantage of the central limit theorem at the expense of averaging your data with data that may be old.  

I don't see any relevance to using it for defectives.  If you're interested  in the ratio of defectives to sample size than that is what you need to plot.  The calculation of control limits that change with the sample size is the only difficult part.  That can be automated using a computer program (maybe a spreadsheet) to perform the calculations for the operator and then charting the result, or buy a canned SPC program to do it all.

Another approach is to structure the data collection plan so constant sample sizes can be used.1  That way the control limits can be stable.


1.  Chrysler Corp. etal, Statistical Process Control Reference Manual, 1995 

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