[Unclenick]

The terminology is confusing and inconsistent. If you read Denton Bramwell or some other authors, the “base” is the back end of the casehead; the part that includes the rim and the headstamp and the mouth of the primer pocket. Clearly no die can size these parts if they are securely below the level of the shell holder.

So, I called RCBS and chatted with them for a bit. In their terminology, the base is the part of the casehead that lies forward of the extractor groove in a rimless case. It is the point where the .470” diameter datum is taken on the .308 case, and apparently includes the junction between the casehead and web where the pressure ring forms, though I had some trouble getting clear information on that point. In any event, this is the back end of the taper of the sidewalls, and perhaps that is where the name comes from? The base of the conical section that taper defines?

According to RCBS, their small base die should be set up about an eighth of a turn further than contact with the shell holder, and at that position will narrow what they call the “base” very slightly (exactly how slightly wasn’t given). This is to ensure easy loading and extraction. The die will also set the shoulder back about 0.002” more than a standard full-length seating die does, thus shortening the headspace by that much. Between that narrowed casehead portion and the shortened headspace, they have pretty well guaranteed a fit. They recommend the small base dies for both autoloaders and lever action guns and pump guns. Basically, anything that either has to strip a magazine fast or has to take a cartridge from an angled carrier.

Incidentally, if you have feed problems with an RCBS small base die, you can return it to them along with five un-resized, fired cases. They will measure the cases and adjust the die to accommodate your chamber, so long as that chamber’s dimensions are still consistent with safe practice. If their case measurements indicate your chamber to be outside safe dimensional limits, they will inform you that it needs to be corrected. That’s a good service to know is available. I suspect Redding or other small-base die makers would provide a similar service, but you would have to ask?

The dimensional numbers from the RCBS tech come to within what my experience with dies has been. Due to spring-back, many cases don’t return fully to their original size in a standard sizing die. In .30-06, with the relatively stiff Lake City military brass, fired in a chamber that was almost 0.007” over minimum, it would return to about .004” over minimum on the first run through a standard sizing die, and a second stroke into the sizing die allowed to rest for five seconds would bring it out about 0.002” over minimum. A small base die should get you that extra .002” on the first try, and take it to SAMMI “0.0” on a second resizing with rest. At least, in theory.

Nick