Different companies have different labs where data was collected with different brass (brand and lot), different primers (brand and lot), different chambers, different barrels, different pressure measuring equipment, different atmospheric conditions, possibly different temperatures, different lots of powder from different years, different ballistic engineers advice/opinions, and different lawyers requirements for liability. With all these differences, how can the data from two different manuals ever be exactly the same? The fact they are even close is a testament to human engineering and the precision of firearms and component manufacturers.
Guess what, the rifle you're loading for is different too as well as all the other variables mentioned. This is why a load should be worked up for a specific rifle and a single load isn't universal for all firearms that use a given cartridge. That's just absolutely basic common sense.
Sunray isn't wrong in this instance and what he's saying isn't "nonsense". He is oversimplifying things somewhat since, yes, data for solid copper, cast, jacketed, etc. are all different (there's that word again). Generally speaking the starting loads will produce safe pressures with any common type of bullet of the same weight. You can use data for a jacketed lead core for a solid copper at the starting values. It might not be optimum but it'll work. I prefer to use data specific to solid copper since the lower starting value is often where I find the most accurate load but the higher starting loads of cup-and-core bullets wont produce dangerous pressures.
At the end of the day, a 55gr lead core bullet with a copper jacket is similar enough to any others of the same weight and construction that data can be interchanged perfectly safely. The pressure exhibited at starting loads in any manual I've ever heard of is significantly below the maximum rated pressure for the given cartridge which is, again, lower than the proof testing cartridges any decent firearm was tested with at the factory.
