also slightly steeper. Each of these characteristics is recognized as being beneficial toward increasing barrel life. However, the 243 case neck is significantly shorter, which evidently makes all the difference!
Ballisticians have long recognized two significant things when comparing these cases. First, the 6mm is always well behaved – no surprises; conversely, the 243 is notorious for generating unexpected results, including unexplained pressure spikes. Second, inexplicably, the 6mm offers significantly greater barrel life!
The only reasonable explanation for the latter fact (which may also explain the former quandary) is that the long neck of the 6mm somehow protects the barrel throat. I believe this is precisely the situation. I suspect that a long case neck saps heat out of the perimeter of the propellant plug (as that plug begins to push the bullet into the bore). If the case neck is long enough, the cool brass can extract enough heat from the plug to quench the burning, at least along the front end. In this instance, a significant length of unburning propellant follows the bullet into the leade (beginning of rifling). This would allow the steel at the bore interior to cool slightly, after being heated dramatically by deformation and friction during bullet passage, before it is assailed by the full brunt of the subsequently passing incandescent propellant gases.