Joe is the most correct so far (JCUNCLEJOE). Check those new reloading manuals. They are using different brass, primers, and their own bullets, usually in a shooting fixture. NOT a gun, but in a shooting device in their factory or testing facility. What is the rate of barrel twist? What is the length of that barrel. They are using computer software and chronographs to generate their velocity and load data, that works in their fixture.
Find some of the old loading manuals. They are using powders of the day (likely different than as manufactured now) but they are usually shot in an actual gun. May have chronographs that worked per technology in those days, and most pressures were extrapolated (no computers) using calculations with pencil, pad, and equations that I wont go into.
Elmer Keith made some accurate bullets, came up with some accurate loads, and guestimated pressures, but had no equipment to do any measuring. It was all in his head.
So, if you are like me, you will eventually end up with about a dozen manuals of various age, that you will use when working up loads. You will get some info from other shooters, compare the info in all of your manuals, take out pad and paper, and start extrapolating down about 20% below the apparent max load, and start working your way up in small increments.
If you have one gun, this may cost you a little to start, but you should be able to find a tack driving load. If you have lots of guns like me and some of the other shooters on this board, you end up with boxes and boxes of various loads to try when you get a round tuit. Sometimes, if you are married, those round tuits are hard to find.......unless you get a new Freedom Arms or something that just can't wait to blast ragged holes.
Go check Amazon, Ebay, and a few other places. Pick up a few more manuals, including Ken Waters if you can find it. You will learn a lot and learn to see why they are so different.
Good luck with your loads. May the moa God's smile on you.
Steve