Hey,
I've read 2 different books about powder charges and bullet velocities. RCBS and Hornady. I have also visited the Hodgdon website, and looked at their information.
The thing I don't understand is why they are all different in their data! I wasn't able to find a single thing the same between the three, based on the same powder/bullet/caliber/weapon. Plus, some of the variations in powder charge are pretty large! Not sure what to believe.
Anybody know why that might be?
Anybody know why that might be?

They test with different barrels. Some test with complete, factory guns. Others test with a universal reciever and screw different barrels in. Every barrel is different. That's why we must work up loads from minimum. The guns we are loading for might hit safe max before the one the company used, but they do tend to be conservative for safety's sake.

AS mentioned the companies test with differing components. The barrels, test equipment, bullets, primers, cases, and powder, are all different from lot to lot, and from what you will have as an end product.

They test the components and develop average safe loads using the components they have on hand. Their controlled lab temps are stable to some degree and will result in across the board pretty reliable test results.

On our end, we select the powders, primers, and bullets, based upon our intended uses. Then after comparing several sources, for this collection of components, we start out at a start load and slowly work up from there. If for exapmple using a 150gr .308 bullet one company uses 48grs of H4831 as a start load and another uses 46gr, and another uses 49grs, you could simply look at the bullets for differing profiles, or average the three differing start loads and go from there. This is of course using start data only. A lot of times the make up in the bullets jacket or the bearing surface length will make the differences in data.

In other cases you simply choose your intended top end powder velocity and start with the data listed for that powder from that manufacturer's start data. They cannot control every aspect of every firearm or final overall length. This is why you always start low and work up. Even when you have a great shooting load, and you only change out the same weight bullets you always back off and work back up. It's much better than finding out when you drop the hammer that you should have.

Different rifles behave differently.

Sometimes quite a lot.

As an example, we had 5 Mannlicher Luxus rifles, all identiacl, all new, in 270 Winchester.

We fired Norma 150 grain SP ammo in them, and the velocity difference between the fastest and slowest was 150 fps!!?

None exhibited any signs of pressure. Still, that was an incredible difference in velocity.

My guess would be different powder lots/batches.

Everything used to develope the data was different. Each lab had its own equipment, and techs running it. The bullets likely were various brands and styles, and if not were from different lots. The same with the primers and brass. Powder was probably from different lots and the chambers, bores, throats, and their conditions were also different.
It is suprising they come as close as they do IMO.

Gottamax ----- You have already gotten good information here, particularly that referring to different barrels giving speeds that may vary. What you have to remember is that the data gathered by the powder companies refers to that particular barrel-bullet-primer-action-lot of powder, brass used on that day, nothing more. Change days with the same components and you change results to a degree. They also play it safe when publishing those facts. Generaly their high end loads today are short of those published in past manuals, probably the result of years of experiences with any and all types of firearms being used for to their loads. They have to publish to the weaker of the lot. You can take their advice and that of other published material and make an intellegent decision as you own reloading, as this bunch of fanatics have done for years with much success. Good luck and good shooting.