When cutting down 500 S&W cases to 500 JRH cases.

Should I size the virgin case first before cutting off excess case?

Depends on the diameter of your pilot, it needs to turn freely inside the case ?neck?. I usually size mine first.


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My Lyman trimmer instructions say that pilots are based on properly sized case.

I trim mine before sizing,otherwise they are a little to tight in my RCBS case trimmer. Sizing dies are all just a bit different in size. The difference between my 454 hornady die and my 454 RCBC die is considerably. The RCBC die being tighter, and about 60 fps faster over the hornady die loaded with same bullets and powder. I guess what I'm saying is there are many variables, and the only why to know for sure is try it both ways.

I cut all of my 500 S&W cases to JRH length before I sized them.

As long as you are consistent in your process it doesn't really matter if your cases are .002" shorter or longer than the spec. That isn't going to change the internal case volume to any appreciable degree as to impact the powder burn.

What will be impacted is the crimp. If your cases are .002 short but they are ALL .002 short, the crimp consistency will be there. If the case lengths vary between say +.001 to -.002 then the crimps will vary and you will likely see performance/accuracy differences between the long and short cases.

As an example, look what Hornady does with their Leverolution ammo. The straight wall cases on those rounds are notably shorter than spec, but they perform well because all rounds are crimped the same.