
The Redding “COMPETITION” shell-holders, do not always work properly - especially when used in conjunction with a press of another make: the overall height dimension varies wildly (industry tolerances, and indeed, Redding tolerances, for this attribute, vary substantially)! When the Redding shell-holders are inserted into the ram of a non-Redding press, it is not uncommon for the bottom surface to contact the top surface of the ram-groove, thus, incorrectly positioning the shell-holder to die relationship.
The shell-holder

ress-ram should contact should be only between the top face of the ram, and the bottom face of the shell-holder rim (my possibly incorrect terminologies - pics should clear that up). If the very bottom surface contacts the top surface of the shell-holder slot, there will be gap between the top face for the ram, and the bottom face of the shell-holder - very undesirable, but not uncommon!
In the pictured Redding COMP shell-holder set , the over-all height varies something over 0.007”! So, the difference between those tow shell-holders would cause an extra load (or, unload) on the press, and result in, at least, 0.007” difference, or, three times the stated difference in head-space dimension!
The height, from the ram contact face, to the top of the shell-holders varies within the tolerance range of the caliper - less than 0.001”. On this facet, all six measre 0.250 - .251".

Thus, presuming proper shell-holder:ram fit, and the ram at top of stroke, screwing the die into solid die:shell-holder contact, thus removing all ‘play/stretch’, any of the five shell-holders would interchange quite well.
That get us to the thickness of the case-head contact faces (caliper displaying 0.1200"), which, in this set, varies 0.010”, or, exactly the specified range: via [this] caliper, the “spacing” is not always the stated 0.002”, but, within the expected reliability of the caliper: 0.001” or, less.
I have observed this many times - sometimes, when I point this out, people have gotten mad at me, I suppose thinking I had just called them stupid . . . I’d be embarrassed to tell you how long I fought with my first set before figuring it out!
The solution: grind a bunch off of the BOTTOM of the shell-holders, clearance there is desirable, and need not be anywhere, “close” - that’s where you want the gap!
As a last resort, usually only needed for custom chambers, cut to minimum head-space, one may need to take a few thou of the top face of the shell-holder - yep, I’m made a few of those too - but only do this when you are certain of proper shell-holder:ram fit. RG
P.S. this same attribute can exist between all makes of shell-holders - the OAL/total height tolerances and the shell-holder groove tolerance must be, “all over the map”.
