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H and IMR are not the same deterrent chemistries, so the temperature you are developing the load under can matter. Also, your primer can affect some powders more than others. It may turn out with different bullet weights there is less difference, too, or one may favor lighter bullets more than the other. Don't know what your friends were shooting?

Flattened primers alone are not a greatly reliable pressure sign. Most military surplus load I've had will flatten them out pretty well. When they start to show, you usually still have some leeway. If they mushroom or crater, then you are getting into potential primer piercing and probably need to back off (though a wide tolerances firing pin tunnel, or the chamferred firing pin tunnels in some newer Remingtons can cause cratering that may not mean pressure is high).

In any event, I wouldn't sweat the powder differences too much. Just remember they are not interchangeable and you need loads worked up separately with them.

Your mentor is speaking of his experience with his seating depth practice. It will work, but be aware it will raise peak pressure 5,000 to 10,000 psi from normal SAAMI seating depth limits, so you want to drop your load at least 10% and work back up if you start seating out there. You didn't say if he turned the whole die or just the seater stem, where threads are usually finer? A quarter turn on a large threads is about 17 thousandths off the lands, which is what I expect you meant. That moves the bullet back 17 thousandths off contact with the lands of the rifling at the throat. I've seen people swear by seating bullets 10, 20, 30, and 50 thousandths off the lands, and also by seating a bullet's bearing surface a whole caliber into the case neck. But as Gunpa said, it's not a universal truth for any gun.

In the Precision Shooting Reloading Guide, the author of the benchrest loading chapter tells how he always believed 0.020" off the lands was best. One day he accidentally adjusted his micrometer type seating die the wrong way and wound up 0.050" off the lands instead of 0.020". He didn't discover the error until he had 50 rounds loaded. At that point he faced pulling the bullets and reseating them or just shooting the 50 for practice. He decided on the latter. To his amazement, they shot smaller groups than anything he'd ever put through the gun before. That would be about 3/4 turn on your outer die threads.

I've taken to backing down 10% on my anticipated load level and running the seating depth from actual bullet contact with the lands all the way down to as deeply as the bearing surface of the bullet can go into the neck without putting the ogive below level with the mouth. I usually work in 0.020 increments because my seaters have the micrometer adjustment that makes that easy. You can work in 1/4 turns. Usually you find a group minumum or two in that range. Not super tight, but once you have the number of turns involved, you can go back and refine it with 1/8 turn steps to find the center. Once you have it, then tune the powder.

Dan Newberry's round robin method for locating the right powder charge can be adapted to work with seating depths. That's what I use. Read his whole OCW site, since you are a newbie. It provides a very useful systematic method of finding sweet spot loads.
 
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