Before we even start to think about bolt down the reloading press to the table, lets understand something called headspace :)

A Very important thing to understnad and know why or dont why and what happens if.....

To correctly hold a cartridge in a chamber it needs two contact points, one to stop rearward motion (the breech) and one to stop forward motion. The point on the cartridge that stops forward motion depends on the type of cartridge and is called the datum line. A rimless pistol cartridge has the front lip of the cartridge stopping forward motion. This makes it easy to measure headspace on a rimless pistol cartridge as it simply equals the cartridge overall length. Headspace measurements in your guns chamber and factory made cartridge aren’t the same. The cartridge a factory made must be used in a lot of different guns. You, the handloader can adapt the cartridge much closer to your guns chamber.

A rimmed, or semi rimmed pistol cartridge and a rimmed rifle cartridge headspace on the rim.

For the rimless rifle cartridge, the headspace is the distance from the breech to the mid point of the shoulder.

The Magnum type cartridges headspace on the magnum belt of the case. In the early years of 1900 century the incorporated the belt to help the cartridge with the headspace when the case shoulder was minimal. A rimmed cartridge had troubles when used in magazine rifles so they moved the rim forward, hence the magnum belt of today. Cartridge and gun makers still headspace that type of case on the belt but the handloader will with minimum resizing get good results when using the shoulder and datum line for headspacing.

In the Handloading for rifles we talk about how you set the die for resizing the case. I mention there that you don want to screw down the resizing die against the shellholder and think that you will get a fully resized perfect cartridge for your gun. You may get lucky but most probably you will get a case with a too narrow headspace or to long depending on your gun. “Your gun” is not any gun that has the same brand, model number and cartridge designation stamped on it, its YOUR GUN. That rifle you have will not have the exact chamber measurements as other guns. One of the many reasons shooters start to reload is for adapting the cartridge to theirs particular gun. When Norma or any other cartridge company makes a lot of .308 Winchester cartridges, they can’t measure your gun or mine to get the right numbers. They work after SAAMI and CIP regulated numbers. For you that are a nice thing, you know that the cartridge will work in both your Sako, Remington, Winchester etc etc gun.
I said nice but is that the best thing for you? Nope, you can adapt that cartridge to your gun and that will perform better in the long run. BUT! If you take those custom cartridges you made and try to shoot them in another rifle? What then? Then the headspace will be wrong for that new gun, it may not be dangerous, just wrong and the gun/cartridge combination will perform as good as the first gun. You want to reload cartridges that can perform in similar ways in a lot of different guns then you have to do it like the big cartridge makers. Stay with the numbers provided by SAAMI and CIP. You can of course do that if you want but…why? Isn’t reloading about getting better cartridges to use in your gun?

Problems with headspace.

One of the things that give a lot of trouble when shooting reloads in guns is the wrong headspace. Above here, we can see that the 9 mm Luger cartridge or the .45 ACP have the headspace the same as the cartridge length. If you trim those cases too short, you alter the headspace and the firing pin has to push it forwards to a stop before it gets to whack the primer. If the case length is to long then the gun wont chamber fully (and some guns can fire that cartridge with bits and parts of primer/case and powder being redirected in too the face of the shooter)
What happens with a .38 Special, .44 Magnum etc? Those cartridges are of the rim type the same as the 45-70 rifle cartridges. Headspace there is the thickness of the rim. The total length doesn’t alter the headspace but the case has to fit the chamber, you can trim them to a maximum recommended length or measure your gun and use that length.
Now let’s talk about the rimless cartridges like 308 Winchester, 30-06 and all others of the same case type. The headspace there is to the datum line somewhere in the shoulder of the case. If the case is to long for the chamber you won’t be able to close the bolt after it.
If it’s too short it will chamber, and the firing pin will push the cartridge forward to a stop. When fired such a case will stretch to fill the chamber if not a misfire occur. Some extractors can hold the case and the firing pin does the work correctly. The case fires and the shoulders of the case stretch forward to fill the chamber. None of those cases are what we want, we want them to be as close as possible to our gun chamber, safety and performance will improve that way.