I just picked up a headspace gauge.As I was going over the instructions it occured to me that could it be possible to just bump the shoulder back a 0.001" and the gun will still function properly or must I "totally full length size" ? I have reloaded for this gun (Mod. 100 Win. 243.)numerous times and have always done a "total" resize. All of my other reloading experience is done for bolt guns.Also, could this "bumping the shoulder" apply to lever guns?Your thoughts and experences,please.

The question is what would you have to lose by FL resizing? I can gaurantee it won't effect your accuracy. For an auto, you want to set the shoulder back a minimum of 0.002-0.003 from the fired case dimension. Keep you primers at least 0.003 flush (0.006 ideal). Most industry FL dies error on the side of caution and all should meet SAAMI specs. Also, wipe out your chamber after each bore cleaning. As for levers, you should still FL resize even though you can use leverage to get the round chambered. BTW, Sierra devotes a separate chapter to gas gun reloading in their manual. And Glen Zediker's book "Handloading for Competition" was for me a fascinating read, espcially for gas gun shooting.

Thanks for the info.I have no clue how far back the shoulder is bumped,that is why I picked up that headspace gauge.I am trying to get a little more case life out of my brass.4-6 firings on this batch and thats pretty much what I have gotten out of other batches.I have been setting the FL die per the instructions and crank away without "fine tuning" the set-up.With the headspace gauge I can fine tune,or am I missing something(reading more, or less, into the equation) to set the shoulder properly?

You might get one of the stoney point datum ring gguage sets now marketed by Hornady.
I find them excellent for setting a sizing die exactly to the nearest .001.

The .243 and similar case design can stick two ways when chambered.
The .243 is so straight that it can stick on the body taper or it can interfere at the shoulder.
If you have some range pick up brass you can carefully size it until it quits sticking.

1. You might start by pulling the expander out of your FL sizer.
2. Set your FL with a .020 feeler guage on top of the shell holder.
3. If that does not work try .010 feeler guage.
4. When you get one that does work, back up .002 on the die and feeler guage until you get some interference.
5. Then back down .002 or so until it chambers with no drag.
6. Try it several times each way until you know you only have .002 or .003 clearance.

Yes, it is entirely possible to set the shoulder back only .001", there would still be a little contact between the case shoulder and the chamber shoulder and you should be able to feel a slight "crush fit" when chambering the case. That is called Partial Full Length Resizing. If you set the shoulder back more than .002" then you will lose the contact at the shoulder and that would now be Full Length Resizing.

I Partial Full Length Resize everytime and set the shoulder to the exact same dimensions everytime except on the first 3 loads of new cases. The first 3 or 4 times you fire a case the case has not expanded enough to fit your chamber. For example these are measurement taken with the Head & Shoulders Gauge on a 30-06

New case - 4.0400"
Once fired - 4.0485"
Twice fired - 4.0500"
3 times fired - 4.0510" (slight crush fit)
4 times fired - 4.0515" (crush fit)

So I then push the shoulder back to 4.0510" for a slight crush fit.