This has probably been asked before but here goes.
I am starting to reload for my Win 100 243. From past experience I know the gun does NOT like 100GR factory loads,it is much more consistent with Remington 80gr PSP. I am using the for whitetails.
Any help is appreciated.

If you like the 80 grain PSP Remingtons you can usually find them at Natchez, Graf's, or Midway in bulk.
My son has a .243 that really likes 85 grain Sierra HPBT (this is a deer bullet not a varmint HP)His rifle also likes 90 grain Speer Deep Curls (a Bonded Bullet)we've only killed one deer with each of them and both were one shot kills, dropped on the spot.
Nosler makes a 85 grain Partition if you want a tougher bullet.
I would recommend some of the lighter Barnes tsx and TTsx bullets but being all copper they are a longer bullet and may shoot more like 100 grain bullets in your rifle.
The only way to know what your rifle likes is to try them.
For my uses IMR4350 works great in the .243 for bullets of 80- 85- 90 and 100 grains.
Good luck with your rifle!

This was on the small caliber sight and discussed at some length.

What is your most accurate 243 Win Load?

You may find some info there to help..

First, make sure you have the factory recall modification; these old 100s can kill you. There used be a web site on it; not sure now; anyway, before you shoot it, get the bolt mod done.