Ducksoup,
Perhaps to "ease" your mind a little, or perhaps to just discuss the topic...
To simply assume that a firearm is OK with loads loaded above maximum listed loads is a bad idea. I've personally seen good quality rifles (one Ruger 77 in 7mm mag comes to mind) that wouldn't handle anything over about a medium load with the powder/bullets we were loading. In contrast, my own rifle (same cartridge) will handle max loads without a problem.
Data from various loading manuals usually varies as well, depending on the results they get from their test barrels.
So, loading manuals are a good guideline for loading data, but they can't tell anyone what the maximum load is for any particular rifle. They're a great reference to go by, but actual results will depend on the firearm. Even rifles of the same make and model can vary.
Fortunately, there are signs to watch for that will guide a knowledgeable handloader to know when (s)he's reaching the max load for his/her firearm. These are signs that EVERY handloader should watch for, no matter what the book says "may be" max load.
And, as long as a handloader is careful and knows what (s)he's doing, it is possible to safely load ammo that's above "book" max in some rifles. Again, it's not something that should be attempted blindly, or based on an assumption of "strength" of a firearm.
SAAMI (Sporting Arms & Ammunition Manufacturer's Institute) spec's are generally formed around the weakest firearms chambered for any particular firearm. Good examples of this are the old 45/70, .45 Colt, and even the newer .32 H&R mag. Load data for these are generally found with low pressures in mind, because a lot of older firearms won't handle high pressures (in the case of the .32 mag, it's because of the weak break-top revolvers made by H&R that the cartridge was designed for). Even so, in modern firearms they can each be "pushed" quite a bit by those who know what they're doing. Some loading manuals recognise this, and include data for newer/stronger firearms, but others don't. Even when they do, the loads should be approached with an aire of caution, just as any max load should (book or actual).
But, all these things are done with a known powder type, and experimentation can be gradual, deliberate, and the results checked to some degree.
In the case of tumbling loaded cartridges, it's the unknoiwn that might cause problems. If powders get altered by tumbling, then we're no longer dealing with a known substance, and results may become unpredictable.
And THAT's scary to me.
Daryl