I m interested in why it matters if you do full length resizing. Please explain. How will it bite you?
That's a fair question.
Let me begin this summary with a bit of an introduction. I'm not going to get into a big discussion of SAAMI minimums and maximums for chambers and cartridges as you can read all about that elsewhere ad-nausia. Suffice to say that cartridge maximums are set by SAAMI to always be less than chamber minimums in order to ensure that all new brass will always fit in all properly headspaced new chambers. However, these standards are for factory loaded ammo, not for reloads. Reloading die manufacturers understand full well that the difference between SAAMI minimum chamber and maximum cartridge is not conducive to best accuracy. In fact, this difference is just one of the many reasons why we reload. It surely isn't really about cost savings...
That's just what we tell our wives.....
ideally, we want to get rid of all that slop that SAMMI builds into its chamber and cartridge specifications so that our reloads are more closely aligned concentric to the bore when chambered. So most full length sizing dies get rid of most of the slop and size brass cases to be as close as possible to what is required to fit all chambers with as little slop as possible. Cheap dies may not be so close. And of course, neck dies don't size the body of the case at all to eliminate the slop.
With that as background, here is a sample of the kinds of things that have been encountered and resolved on this forum:
It turns out that very few full length sizing dies really size back to OEM brass specs. Using a high quality micrometer, measure some new never-fired brass in multiple places. Then fire them and resize them. The new brass is almost always still smaller than the resized brass in numerous places.
Although not as effective, but faster, you might also be able to see this by removing the neck expander button and depriming stem from the die and dropping in some new unfired brass. If it drops in without any effort its obvious proof that unfired brass is smaller than resized brass will be. For most dies, the new brass will probably even rattle around in there.
Note - lest anyone think this is a bad thing, rest assured that it is not. Ideally, brass should NOT be resized back to OEM size. It should be sized just enough to chamber freely with a little allowance for dust for hunting, and no allowance for targets.
In addition to that, the brass web will grow/expand over time and full length sizer dies seldom size the web itself, just the case immediately above the web. This is why many die manufacturers make special "small base dies" for sensitive chambers (eg Browning BAR).
The above all assumes a high quality full length sizing die set to full depth on the shell holder. Any less than that is potentially worse.
Best accuracy with full length dies is usually obtained by "bumping" the shoulder a small amount. Doing anything else won't be reaching the full accuracy potential of the rifle.
Very few case holders are the same size. In fact, Redding sells custom case holders with different lengths to allow the reloader to customize the sizing of the case.
Cases spring back about a thousandth of an inch after firing or sizing. This dimension and effect also grows over time.
And last but not least, even the very best reloading press has some spring in it. No matter how stiff or strong the press is, it will bend a little as it resists the load - and so do the dies. It's impossible to do otherwise because there is no such thing as infinite stiffness. As cases age with use, they get stiffer too and this causes even more flex in the dies and press.
In summary, the likelihood of problems is somewhat dependent on the manufacturer of the rifles, the chamber dimensions, the manufacturer of the dies and the dimensions they chose, the number of times brass has been resized, and last, but not least - the dimensional variance between the different rifle chambers and bolt headspace involved. Time is not your friend in this game as it takes time for the brass to grow, while at the same time it makes us more and more comfortable until.... time runs out.
I trust that is sufficient to explain why problems are inevitable. Your last question was "How will it bite you?"
The problems that eventually occur are:
- stuck brass in the rifle chamber
- bolts that won't close
- bolts that won't open
- broken bolts
- galled bolt lugs
- deformed and damaged bolt cams
- separated heads
- blowback in the face
If you look, you can find examples of each of these on this and other forums. Nobody wants even one of these problems with any rifle at the range let alone miles from help out hunting.
Its so easy to avoid. Using brass in just one rifle makes sure the brass won't grow beyond the dimensions of the chamber and headspace and also reduces the amount of full length sizing required so that maximum accuracy is achieved. And yes, I know that excessive loads cause similar problems. But that's another subject.
I mark all my ammo boxes with the caliber and the rifle that ammo is used in.
Cheers!
