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Even'in Gardien,
Yep, the answer is simple, and boils down to the fact that the different companies are using different test rifles/barrels, more then likely different lots or powder, primers, cases as well as the different bullets.
Personally I look, if possible, at a number of books when begining to develop loads and then start toward the bottom of the listing for my first tests.
Normally I will progress with one grain increases - example 40 grains to 41 grains - until I am approaching the upper listed loads at which time I begin to go in 1/2grain increases.
If the books said something like a starting load of 35gr. and a max of 41, my load series might look something like this -- 3 rounds 35gr., 3 at 36, 3 at 37, 3 at 38, 3 at 38.5, 3 at 39, 3 at 39.5, 3 at 40, 3 at 40.5 and 3 at 41grains.
How low I begin, somewhat depends on how much experience I have with that rifle in other tests.
If you begin to experience any of the signs of high pressure - read lots and heed the warnings!! - it is time to stop, go home and pull the bullets on the hot loads.
I like to run my tests over the cronograph at the same time I am shooting groups, as watching the velocities as well as pressures and group sizes at the same time can provide a lot of information.
Back awhile, I used to always shoot 5 shot groups, but shooting 3 shot groups is much cheaper when using bullets such as the Nosler partitions, and it provides a good working idea of what the potential of the load is.
A few years back, I worked up loads for a 300win mag, and then needed to buy more powder, which of course was then of a different lot #.
Just the change in powder lots, decreased my velocities by about 50fps.
Be very aware that using cases, even of the same brand, can create high pressures providing they are not of the same manufacturing lot #s.
Because of past experience, I try to avoid using odd lot cases for anything other then "banger" or lower pressure practice ammo.
Buy your brass as many of one lot as you can afford, and once you have found a good powder for your needs, buy a number of pounds of that powder making sure it is all of the same lot #.
But, beaware that for example if the "book" says the max load is 45grains and you find your rifle's sweet spot at 43 grains, when you change lots of powder you may need to adjust up or even down to again find what your rifle likes.
It is fantastic that we have such great products, but everything is made within tolerences, a fact we must take into consideration during load development.
Read, be safe and enjoy!
Keep em coming!
Crusty Deary Ol Coot
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