[Bart B.]

No die I know of sizes the belt down in diameter. That collet die sizes down the case body right in front of the belt.

That collet die's a good thing for reloading belted cases. Before Willis made his collet version, people took a standard full length sizing die, cut its bottom off about 1/10th inch above the die's belt clearance area and the same amount below the body-shoulder juncture. Then squaring up its bottom and making a slight radius on its inside edge. After full length sizing belted bottleneck cases setting their shoulder back about .001", that "body die was set in the press so it sized the case body all the way back to the belt which standard dies do not. That ridge is more pronounced with higher pressures and thinner brass. It usually looks like a fine line around the case a few thousandths in front of the belt.

That body die sized down the ridge often left on belted cases a few thousandths in front of the belt when fired in chambers with excessive belt headspace to have the belt's front edge a few to several thousandths back from the chamber's headspace shoulder. Otherwise, that tiny ridge often interferred with the case being driven forward by the firing pin and hit the edge of the chamber headspace ridge such that extra vibrations caused the barrel to whip differently from shot to shot. Sizing that ridge down helped accuracy enough that those "body" dies were popular. Most folks winning long range matches with belted magnums used their own home-made "body" dies. Its use after (sometimes before) full length sizing was about the only way to get accuracy as good as what new belted cases produced.

I've used one I made back in the early 1970's from a .338 Win Mag sizing die and it's worked resizing cases for all my 30-338 magnum barrels worn out in competition. If one makes their own, be sure the body taper of the die you're going to modify has the same body taper per inch as your belted cases have. Otherwise, you may size part of the case too small between the belt and shoulder.

Best thing to do for greatest case life with belted bottleneck cases is to full length size them such that their shoulder's set back no more than .001" so the case headspaces on its shoulder instead of its belt. The collet die helps somewhat, but not as much as minimal full length sizing. Best advantage of the Willis collet die is it pushes the case body into the chamber radially instead of by wave action normal full length sizing dies do. With minimal fired case shoulder setback and use of the collet die, 20+ reloads per belted case is the norm with normal, max peak pressures. Hotter loads shorten case life and typically hurt accuracy.