He's probably right. I'd double check the case measurements after resizing, but there really isn't much difference between the rim measurements of the two. The WSM rim diameter spec is 0.535", while the 7mm Rem Mag is 0.532". The rims are the same thickness. The WSM was designed to allow the use of the standard magnum bolt face. The only real difference is that the 7mm RM has a belt that is the same diameter as the rim, while the 7mm WSM has no belt, but is a rebated rim design where the rim is smaller than the body of the case at the web. So it's possible that a shellholder designed for the standard belted mags like the 7mm RM might be too thick to accommodate the WSM, but not the other way around.

I just looked up the charts for shellholders and found the following, where the first number is the shellholder for the 7mm Rem Mag, and the second is for the WSM:

RCBS: #4/#43
Redding: #6 for both
Hornady: #5/#35
Lee: #5 for both
Lyman: #13/#34

Here's the link to the chart: http://www.midwayusa.com/midwayusa/s...lder_Chart.pdf

So some brands use the same shellholder, others don't. The best thing you can do is try it (but save your receipt).

Mike