The rest of the story . . . .
I designed the Belted Magnum Collet Resizing Die. So, let me know if you have questions. I have been (and still am) continuously available to handloaders by phone and email since this unique die hit the market, back in 2001. At this time, there are well over 6,000 shooters using it. Unlike conventional resizing dies, this patented collet design squeezes the case inward only, and it can't plow brass rearward.
Belted magnum case bulge almost always occurs at (or soon after) the 3rd firing. This is caused from cumulative case stretching. The first firing is especially bad, because a NEW belted case has over than 3 times more shoulder clearance than a non-belted case. So, the first firing is a REAL case stretch. Most of the excessive case bulge happens during the reloading process. After a few years, I re-wrote the instructions because I found that too many shooters were using this die to over-resize their cases.
You only need to reduce case diameter .001" smaller than one of your FIRED cases. That makes the die much easier to use. (Most shooters figured that out.) The drop gauge on top of this collet die just shows how your cases fit in the average SAAMI spec chamber - which may or may not be the same size as your chamber.
Measuring case diameter above the belt should be done with the WIDE part of your calipers. This clearly shows the difference between a new case, a once fired case, and a case that won't chamber. Once a handloader sees this, it doesn't usually take long to understand what is needed to make reliable fitting handloads.
