"...plus coal for each starting at..." Use the max OAL given in your manual until you have the most accurate load. Playing with the OAL is a tweaking thing.
Once they're out of the tumbler, do one operation on all the cases then change dies. You don't or shouldn't need to trim every time. Not a bad idea to check every time though. Set your calipre up as a guage. Lock it at the max case length.
"...don't understand the reason behind trimming..." Most rifle brass stretches the most on its first firing. Consistency is important when reloading any cartridge. Critical in a semi-auto. Too long will cause feeding issues.
If the cases are different lengths, you'll have seating issues if you're crimping and your loaded ammo will be different lengths. That's not good for accuracy. When a case is trimmed it needs to be chamfered and deburred too.
Once they're out of the tumbler, do one operation on all the cases then change dies. You don't or shouldn't need to trim every time. Not a bad idea to check every time though. Set your calipre up as a guage. Lock it at the max case length.
"...don't understand the reason behind trimming..." Most rifle brass stretches the most on its first firing. Consistency is important when reloading any cartridge. Critical in a semi-auto. Too long will cause feeding issues.
If the cases are different lengths, you'll have seating issues if you're crimping and your loaded ammo will be different lengths. That's not good for accuracy. When a case is trimmed it needs to be chamfered and deburred too.