[WESHOOT2]

First, I own two 4" GP's, and have loaded 148- and 158g, and I have used Bullseye and AA5.

Next, YOU need to define (at least for yourself) "best accuracy". If you are TOTALLY committed to the very very best possible accuracy then EVERYTHING matters; if you wish for "very good accuracy" then less rigorous measures may be employed.

For BEST: new cases (recommend Starline) trimmed to uniform length (doesn't matter what length, just uniform). This can be accomplished inexpensively with the LEE trim tools.

Use Federal or CCI primers.

Suggest Bullseye for best accuracy; also try W231. Time-proven.

Seat bullets to uniform OAL using REDDING Competition Seating die.

Crimp.......crimp should be EXTREMELY UNIFORM. Highly recommend crimping with the REDDING Profile Crimp die. I use a fairly heavy crimp; suggest starting with an extremely LIGHT crimp and SLOWLY increasing crimp until best results are found.

Now for loads..........

You can perhaps exceed some manual's recommended MAX loads, but you MUST understand the risks. (ALWAYS ALWAYS ALWAYS wear safety glasses!)
GP's have a large safety margin, and are among the very strongest 357's available, so rest easier.
BUT.....
You can blow up any gun if things go badly LOL.

I have loaded as low as 148g/1.8g Bullseye and still had the bullet exit every time (biggest danger -- bullet lodged in bore!).
I have never felt the need to push this bullet, but have loaded the 158's to fairly high velocity, based on specific bullet. Not all bullets are equal, even when configured the same and of equal weight. The material is the difference, and sometimes the shape changes the friction surface (the portion that physically touches the barrel).

Do you have a chronograph? (Recommend PACT ONE.)

Can you list your specific components and loading tools?

What manuals do you have?

Isn't this fun?