My Montana 243 was one of the toughest rifles I have ever shot to get to shoot well consistently. Most of it was the nut behind the bolt. Being so light weight, it let me see every mistake in my system. I use to load H414, 55 ballistic tips, & Fed 210's at plus 4000 FPS. It was a country mile away from the lands. I used Nosler's OAL. It was a joy to carry, but other rifles are much easier for me to hit reliably with.
I would suggest using a powder that is much more temp insensitive than H414. I used it, but I always had to tune it. No matter where I hit the coyote [beeep], he was always anchored. Every single time. I am a much better reloader these days. I use to have fliers. I had it rerbarreld with a Douglas 1-12. For what ever reason, the Douglas was hundreds of feet slower than the factory. And it wouldn't group the 55's!