Greetings,
100 grain projectiles can be marginally unstable in 10 inch twist rifles, specially boat tails. This may be the problem with some ammo. Almost all Remington .243 rifles have a 9 inch twist for this reason.
Reloading to save money seldom achieves that aim other than for target shooters. Handloading to improve performance and provide a more reliable supply of ammo is a different matter altogether.
GPM.