Oh, okay – I think I got it now. For significantly reduced loads in your 7mm Magnum you’re going to want to use a different powder. Every reloading manual I have states in bold letters to not reduce powder charges below the minimums listed. And at 60 grains of H4831 behind a 140 grain Sierra Game King, you’re already .4 grains below the minimum listed in my latest Sierra manual.
H4831 is a relatively “slow burning” rifle powder. For reduced charges it’s better to use a “faster burning” powder, such as SR4759. You can explore your options for reduced loads in loading manuals, which you should have a couple of. I recommend Speer and/or Lyman manuals.
There are quite a few reasons why you shouldn’t go below minimum powder charges listed in loading manuals. One reason applies to slow burning powders in large capacity cases and it’s so controversial I won’t even state it. But I will state this; on pages 62-63 of the Speer Reloading Manual #13, Speer warns against excessive load reductions of slow-burning rifle powders in large capacity cases. H4831 is a relatively slow-burning rifle powder and the 7mm Remington Magnum is a relatively large capacity case.
To answer your second question about IMR4350 - yeah, it should work well in a 7mm Magnum, though I personally have not tried it. But the thing is, according to the charts, IMR4350 is not all that much faster burning than H4831. A little faster maybe, but not enough so that you'll be able to significantly reduce powder charges.
I'm having a bit of a tough time explaining what I mean to say. See, duplicating 7mm-08 or 7mm Mauser power with a 7mm Remington Magnum is not a problem with either of the powders you asked about. Where you're likely to run into problems with those powders is when you try to duplicate 7mm BR or 7mm IHMSA powers with a 7mm Remington Magnum. With those cartridges, you're talking about kicking that 140 grain Sierra out at only around 2000 fps. To do that with a 7mm Remington Magnum case, you should be using a lot faster burning powder.
