Yes, they will often grow during resizing, so that's a good time to trim. However sometimes the difficulty is that the reiszed case now doesn't want to go over the case trimmer pilot! (had that happen before).
With the example you give, a straight-walled pistol cartridge, I've never trimmed one twice. Usually once with new brass to get them all the same length. Never had one 'grow' enough to need trimming again. Check, of course, before each loading.
I trim new brass before resizing for several reasons.
1. I use a progressive press, and it's much more efficient to complete the progressive cycle once it's started.
2. I've tested Winchester and Starline brass for growth after initial sizing and found it to be very uniform, thus trimming before sizing had no effect on the uniformity of the sized brass.
3. And the aforementioned problem with the sized brass not always fitting the trimming pilot.
I have also found that the brass can grow such that my press setting for bullet seating will need adjusting. After approximately five rounds of full loads, my 44mag. brass can grow such that it extended beyond the bullet seating channel. So I've found it easier to use brass for three rounds of full loads and then for target/plinking.
I use the Lee case trimming system with a separate pilot for each cartridge and a lock stud that fits on my electric screwdriver. Utterly convenient, and the pilots are correctly sized to accept brass after it has been sized. Hard to beat!
Straight casees don't seem to stretch much with expanding and forming. It's more squishing than pulling.
Shouldered cases do "hang up" on the expander coming out, some cartridges more than others. Turn of the century, small caliber, slope-shoulders seem to grow more than modern high angle shouldered rounds.
Using inside neck lube or hardware store powdered graphite (for locks) reduces the "pull" a lot.
I trim after forming because a lot of stretch can occur in the shouldered cases, taking you over the top. Straight walls aren't as fussy.
Interesting thread. Have thought on this subject myself. I am of the opinion that data in the reloading manuals assumes trimming will be done before sizing, as the trim to length, is generally about .010 less than cartridge max oal, if memory serves and I have my decimal in the right place.
I have witnessed some stretching of the case after sizing, but never in the neighborhood of .010.
So, if a fellow has a cartridge case where he wants maximum neck tension, like a short action 300 savage type, it makes some sense to me to trim after sizing, and go like...002 less than max case length, rather than the .01 "trim to" length. On my 300 savage, i try to get as much neck tension as possible, even though .008 is really not a whole lot of yardage.
On the long action cartiridges, it doesn't make that much difference to me, and I'll either trim before or after depending on what kind of mood I am in. Lately have been trimming after, just to maintain uniformity in sequence.
It's true that you don't have to trim the full 0.010", but if you don't, the obvious issue is that you can't use the traditional method of limiting the number of times a case is used, by the number of times it's trimmed. May or may not be an issue. I believe that most reloading manuals will suggest tossing a case before the 5th trimming is required, however, I've never had one last that long.
Also you might have to trim nearly every loading, an aggravation.
If you want max neck tension, might I suggest doing neck expansion with a Lyman "M" die? These work just great especially if you can smooth down the minor diameter of the expander 0.003 - 0.004" under bullet diameter, and don't run the major diameter of the expander into the brass very far. Then you also still have the option of crimping for more pull.
By the way, on some informal testing that I have done, doing the neck expansion via the "M" die does seem to help reduce the amount that a case grows during the reloading process, and also can help keep case necks concentric. It is even more help if you neck-size, especially with Lee's collet die.
Hope this helps. .300 Savage is a neat and under-appreciated cartridge.
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