Temp430;
Here's what's going on: When any rifle is fired, the barrel vibrates, primarily in the verticle plane. If the muzzle is in mid-cycle when the bullet leaves the barrel, the muzzle "kicks" the base of the bullet. In other words when the tip of the bullet leaves the barrel the muzzle is pointing in a slightly different direction than when the base of the bullet departs. For best accuracy it's nice to have the bullet leave the barrel when it's at a dead stop & changing direction in the vibration cycle. That's what us handloaders are chasing as a target nirvana. Very subtle things can influence the departure timing.
If you're using factory ammo, and it sounds as though you are, you just buy & try something else. With a very popular caliber like the .243 Winchester, there's hundreds of choices, not all of which may be available to you in local stores. But it sounds as though you already have found something that gives perfectly adequate accuracy. Therefore, if you insist on using a Barnes bullet, you're probably going to have to take up handloading. If you don't want to do that, use the SST, there's nothing wrong with it.
900F