Also, when purchasing headspace gauges, purchase the set from one manufacturer, there are variations between brands, that can stack giving false readings if mixed together. Here is the blurb from a Manson gauge entry on Brownell's site:
" . . . MIXING AND MATCHING BRANDS OF HEADSPACE GAUGES - Don't do it!! That's the short answer, here's why. It boils down to tolerance stacking. Each Headspace Gauge manufacturer works within a range specified by SAAMI. Manufacturer ''''A'''' may work at the high end of the range while Manufacturer ''''B's'''' gauges are in the middle of the tolerance range. Mixing the two could give an inaccurate [result]."
Here is a page with info about headspace: http://surplusrifle.com/shooting/headspace/index.asp
Just so you know, I have been a loader and cartridge wildcatter for 50 years, and build my own swap barrel rifles. Changing barrels and bolts to swap between cartridge head sizes requires adjusting headspace, which is easy, but must be done with precision gauges.
So, let's say I buy a rifle with safe maximum headspace, where the bolt will not close on a FIELD gauge, that is otherwise safe to shoot, or a lot of new brass is undersized for a particular chamber. This is not a problem for the handloader. What is needed is to adjust the headspace by forming the cartridge case to match the chamber, and this applies to belted, rimmed and rimless cases. It produces more accuracy and cases that stretch less on firing that last longer.
Sometimes the headspace difference is minor, where backing off the full-length sizing die a couple of threads when sizing new brass will prevent chambering. That’s good, because all that is needed is to turn the die down an eighth of a turn and resize, then checking if the bolt closes, and repeating until the bolt just closes, establishes headspace for that chamber. Then I set the die lock ring. Thereafter that die is adjusted for that particular chamber. If however, the chamber is too large or the brass too short, then the need is to neck the case up a caliber or two and reset the shoulder.
The photo above shows how I accomplish this. The case on the left is a .223 Remington as it came from the factory.
The middle case has been necked up to .243 in a 6mm PPC F.L. die by running it over the expander ball, stopping short of actually sizing the neck. Almost any 6mm/.243 die that is short enough will work, my alternative is a .243 WSSM die. I don’t use Forester dies for this operation as the expander ball is just below the neck sizing chamber inside the die making it functionally longer. The other alternative is to purchase a dedicated neck expander die. Most manufacturers sell tapered neck expanders for cartridge conversions including RCBS and Redding.
The case on the right has been partially sized back down in a .223 Remington F.L. die. The double diameter neck can be seen that will become the headspace stop or double shoulder. To fit this case to the chamber, the die would be turned in no more than an eighth of a turn at a time, the case then resized, and the process repeated until the bolt just closed. On a bolt action I prefer just a bit of resistance as the bolt closes. The die lock ring would be locked down. Firing this case would establish minimum headspace and it could be either reloaded using the F.L. die or a neck only die adjusted out to avoid pushing the neck/shoulder back.
I use this same technique on rimmed cases, where the case is converted to headspace on the shoulder instead of the rim thickness. I used this method in forming cases for the Contender pistol and others, using bottle necked cartridges. It also works on rimmed bottle necked rifle cartridges such as the .30-30 and .348 Winchester, and wildcats based on them. I set the 7mm Remington Magnum and belted wildcats to headspace on the shoulder in the same way. The only disadvantage is that brass has to remain separated and marked for the rifle it was set for, and the die should be kept for use only for that ammo. This is not a technique to allow use of a rifle with excessive headspace outside of safe limits, rather it is a means of adjusting cartridge headspace to minimize case stretch and inaccuracy. If the rifle bolt closes on a FIELD gauge, do not use the rifle or this technique. If headspace is too excessive, the case head will not expand enough to seal the chamber, allowing gas to escape into the action, a very dangerous condition that can cause injury or death. This is a discussion of my experience only. Use caution and common sense, as I can not assume liability for individual judgement and interpretation.