Where are you getting this information??? The small base dies I have used {RCBS} as well as any others that I know of reduce the base diameter by .001" and they are not made to reduce the headspace length by anything. Like any full length resize die you can set them up to reduce headspace length, but you probably wont get .003" shorter than a "standard" die out of them. Unless of course you shorten the shell holder or cut off the bottom of the die.
There are those that say brass cases do not harden with age and that it can only harden by being worked. The guys that made this claim also claimed to be "metallurgist". There was a long thread on here a few years back about this. But I have seen way too many old cases that did nothing but sit around and they certainly had very hard necks. Air harden, work harden or time harden....the end result was the same, either the necks split just from sitting with a bullet in them or I could hear and feel a noticeable "chirp" when sizing over the neck expander. Proper annealing put the cases back in service. The other thing to really watch out for with older brass cases is corrosive primers. I think the magic year was 1944. Everything from then on was non-corrosive.
Trust me I know how far my dies push the shoulder back and small base dies reduce the case to minimum SAAMI dimensions. And again this is why small base dies are noted for case head separations because they bump the shoulder back more than a standard die.
And below my RCBS and Redding small base dies "WILL" push the case shoulder back more than a standard die. And below are not all my 223/5.56 dies, I worked in a sporting goods store and got a discount and had a curiosity about how much dies vary in size.
NOTE, I have a standard Lee 223 full length die that will size the case smaller in diameter and push the shoulder back more than my small base dies.
Bottom line, chambers and dies can and do vary in size, meaning in diameter and shoulder location.

Below a Colt field gauge at 1.4376

Below the same headspace gauge in my adjusted Hornady gauge.

And a fired Lake City case from my AR15, and this case will be resized with a RCBS small base die and a Redding +.004 shell holder for .003 shoulder bump. Meaning if a standard RCBS shell holder is used with press cam over the shoulder would be bumped back .007 or more.

Below Dillion dies are small base dies and they even warn you to adjust the die for proper shoulder bump.
Dillon Carbide Rifle Dies (Individual & Three-Die Sets)
https://www.dillonprecision.com/dillon-carbide-rifle-dies-individual-three-die-sets_8_4_24498.html
"Sizing/Depriming Die
The sizing/depriming die is full-length and sized to minimum tolerances to size cases down to function in both semi and fully automatic firearms. We strongly suggest that a headspace case gauge be used to correctly adjust the size die."
Last edited: