In a nutshell, ammunition sized to SAAMI minimum dimension should be considered “wrong” for almost all rifles, but in its defense, it is “wrong, on the right side.”
To those who might know.......how much potential headspace and "bump" is there is the typical rifle chamber? I was looking at the SAMMI specs for 270 Win......chamber and cartridge......and it looked to me like it would be 0.006" between max cartridge and max chamber.
In principle, the difference in max and min is supposed to be - by design - tight enough that one firing will not cause case separation or shoulder rupture. It also should not be excessive headspace to the point that the firing pin interference with the primer is reduced to the point of failed ignition.
So yeah, the difference between max and min is intentionally still tight enough to prevent malfunction and catastrophic failure.
Or put another way, if the LEE dies (or any other standard set of dies) do what their material says they do......resize back to the maximum allowed by SAMMI specs.....how much is the bump? In theory, a "tight" chamber would be none. But what about a loose or "sloppy" chamber?
A chamber casting would have to be done to measure the actual dimension of the cut for anything other than headspace, throat length, or neck length. Games can be played with mandrel expanders for neck diameter as well. But in principle, dies should be able to size to minimum SAAMI spec, but also allow “sized for chamber” reloading practice.
Granted, “sized for chamber” only offers control over headspace length, without independent control over body diameter. Certainly shoulder diameter is never independent, or even really variable, and neck diameter is only controllable when using bushing dies.
Maybe an example here as a difference between two chambers: Swapping from one 6 creed barrel to another a few years ago, both measured out to the same headspace within 1 thou (as accurately as I can measure with the device I was using). However, it became obvious after swapping barrels that the old chamber was more generous in body diameter than the new barrel - some cases, even after resizing to proper headspace LENGTH, simply would not chamber. The sizing die was not tight enough diameter to sufficiently size the body and “base” of the cartridge to fit the new chamber. The result was that a portion of the cartridges - maybe 10-20% offered significant resistance in chambering - wedging an interference fit around the body before dead-length stopping on the shoulder - and 40-50% of them offered sticky bolt lift during primary extraction, as the cases “memory” was larger diameter and they didn’t springback to clear the chamber as they should. One pass through a small base die with the set of brass and everything was back in order - set to the same headspace length as the original sizer. (For those playing the home game, I DID screw my FL sizing die all the way to the shell holder, plus 1/4 to see if that die could sufficiently size the body - it did not, so a small base die was procured).
So sizing die dimensions mattered significantly in that case.
However, in practice, since we never truly have independent control over body/base diameter, we do commonly neglect that we may be squishing our bodies smaller than needed. In principle, if we force the conical shoulder of the case to dead-length stop against the conical shoulder of the chamber, and the bolt face and case head are “square” to the axes of the barrel and cartridge, respectively, then our cartridge will be centered and supported in the chamber in the way we desire for precision. However, I COULD, potentially, push the shoulder back in a 30-06 case to headspace in a 300 wsm chamber, leaving massive clearance around the body. In principle, the bolt face and shoulder would force alignment, but obviously the skinny ‘06 case doesn’t fill the WSM chamber, and will almost assuredly rupture if fired. But… it illustrates that we may be and can be more tolerant of extra clearance around the body than we really should be of the neck and shoulder.
There seems to be two issues or methods floating around. Standard dies setup as per instructions........are sizing to a standard. Setting up the dies to close a bolt throws that standard out the window......and sizes to the individual chamber. Just curious what the potential difference between the two might be?
It’s simply not apt to say sizing to fit your chamber is throwing anything at all “out of the window.”
Sizing to minimum SAAMI standard is a blind practice, meant to produce ammo which will fit any and all chambers. A “one size fits all” paradigm. It’s kinda like getting a charity event T shirt - where they saved money and only ordered XL’s so “one size fits all.” Which is to recognize, some guys in the husky end of the spectrum simply won’t be able to put it on, and smaller folks like young folks, half of men, and almost all women will be swimming in it - it’s not terribly apt to say “one size fits all” when you’re playing such a fast and loose game with the word “fit.” Just because someone is able to wear a shirt, such does not mean it “fits.” Similarly, just because a cartridge is small enough to chamber, such does not mean it actually “fits” the chamber.
Offering another extreme example for consideration: as noted above in my anecdote with my 6 creed, a case which is oversized in any dimension, even the slightest, will not chamber properly. So if I’m hunting my .30-06 this season and my sizing die backed out of the press ~1/4 turn without catching my attention, I might not be able to close the bolt on that round - despite the fact it is the appropriate .30-06 case, sized in the appropriate .30-06 sizing die. Alternatively, if I had an inattentive moment of Darwinism and loaded my rifle with some .308win rounds, my rifle would likely feed and fire with a smile on its face. Neither actually “fit” the chamber, but ~2 thousandths in the “plus” direction hurts more than almost 1/2” in the “minus”. Similar to the case of the pushed-back ‘06 case in a 300wsm chamber, the 308win will “fit” in the ‘06 chamber, but will not be sufficiently supported, so it doesn’t really “fit.”
Is it worth chasing......or are we picking fly crap out of the pepper?
Here’s where rubber really hits the road…
I’ve offered examples above for how and why chamber fit matters, micro scale in one direction which causes malfunction, and the absurdisms of macro scale undersizing in the other direction - both of which, unfortunately, will still chamber and fire, still function, even though the results could be catastrophic.
Sizing to minimum spec won’t hurt very bad in MOST cases - and if it does, it indicates you have something wrong with either your die or your rifle (creating excessive headspace because the die is too shallow or your rifle chamber is too deep). But outside of the context of making generic ammunition which must “fit” in any rifle, all you’re really doing is producing ammo which you know doesn’t actually “fit” your rifle.
