Rem 243s are 9 1/4" Twists. I would guess that you just did not find the load for the 100g, or the harmonics of your particular barrel did not like those heavier bullets.
If your barrel likes the 85g Sierra, then the barrel would shine with the 85g Speer btsp(IMR 4831) which is a deer bullet, not a varmint bullet with a better BC. Most of my family Remingtons get 95g Hornady SST, 100g BTSP or Nosler 95g Ballistic tips with IMR or H 4350, R#22, or another 200 fps with R#26. Barrels will like what they like, and there is no getting around that fact.
Raven Rock, a vendor on this site is selling a 100g bonded core Ornx bullet that should be an awesome performer with R#26 at 3150, performing way beyond its pay grade.
The last deer I shot in Kansas was a 260 lb buck at 320 yards, 243 Win with 100g Hornady btsp, with a very accurate hot load of IMR 4350. He took two steps and fell over. He heard me using a rattle bag and grunt call, at 700 yards. He came running, slowed to trotting, then stopped in his tracks with me hiding behind a round bale of hay. I had my gun up resting on the hay bale, with the Leica range finder shooting distances all the way. When he stopped, I shot the range, held over, and took the shot. This was a frontal quartering shot, with the bullet entering just inside his front left shoulder, taking out the use of the shoulder, then lungs and heart. This was not a big buck for that area, but time was getting short, and the weather was about to turn hot, so I took him for the meat....a very heavy 8-pointer.
A friend here has taken a LOT of deer with a Barnes 80g TTSX with a very hot load of IMR 4064 with Federal 210 primers in the 3400 fps area, he shoots a Remington and Browning A bolt factory barrels. We skin our deer and the damage from the 80g Barnes TTSX is impressive.