Recently I obtained new reloading data from A.Arms and Vith.( the little booklets that come for free when you buy a can of their powder) I noticed that maximum charges in the latest editions for certain caliber/bullet/powber combinatins vary from earlier editions; in some the max. charge is notably higher, for others it is lower. I have seen changes of 10%!
How can these be explained? Has a given near max. to max. load that had proved te be safe in a given rifle four years ago to be considered not safe anymore?

Welcome to reloading.
This is the muddy water issue.
This is why Bubba's pet load may blow up your gun.
This is why you never start with a maximum load, always sart load development at least 5% below any listed maximum.
Every gun is an entity unto it's self.
Reloading manuals are only guide lines, and should not replace common sense, and safe reloading practices.

Yep,what he said! Nate

In addition to every rifle being an individual, the pressure instrumentation today has improved. It can detect pressure variances that weren't visible with other means.

Note that the pressure recorded in the manuals is not for just one firing - the same load varies from shot to shot, so they use an average of several shot to determine pressure. The closer the recorded average pressure in the charts is to SAAMI max, the lower the average variance was for that load. The higher the variance, therefore, the less powder they can put in the case and still avoid going over the max.

Using today's pressure testing equipment, they see the variances more clearly, and to avoid going over the SAAMI max pressure for that cartridge, they back the powder down more than they did years ago when they didn't see the variances as clearly.

Jaywalker

Another reason is powder formulas & coatings can change from production lot or year to year. If you are running loads near max. it is advisable to reduce your load & work back up whenever you buy new powder especially if you haven't bought powder in several years.

Reindeer

AA doesn't produce any of their own powders to my knowledge.

Where one powder may have been made in an eastern European country a few years ago may very well be produced in China today. When companies have a very popular powder and it selling well they aren't real excited about changing names because it’s now manufactures powder is a hair faster or slower. So you see the load books change from time to time.

Some of this activity is also due to the impact that lawyers have had on our sport. Remember if there’s a dollar to be made a Lawyer will try to show you the way as long as there is more than his fare share in it.