I've been loading belted mags for many years. The Larry Wills collet is good if you want to use belted mag brass that was fired in a different chamber and will not fit your current chamber. Or if the Chambers Headspace is longer than optimum allowing the brass above the belt to bulge after repeated firings.
A Headspace comparator is what you want so you know exactly how far you're bumping the shoulders during Full Length sizing (I aim for 0.002" to 0.003" shoulder bump).
(click pic to enlarge)
Here is a pic of a sectioned 300winmag piece of brass after 10 firings showing no brass thinning above the belt. This shows how important it is to know the exact shoulder bump on belted mags. If this brass was FL sized 10 times it would show significant brass thinning above the belt. This is Remington 300winmag brass in case anyone wants to know.
(click to enlarge)
^^^^
Never mind the short neck of this brass... I cut it to make a chamber neck length gauge.
If anyone wants to know how to make your own chamber neck length gauge here is a link.....
http://bulletin.accurateshooter.com/...-length-gauge/
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