BHs recommendations will likely work but you need to realize that cartridge brass is just like any other metal. When its exposed repeatedly to high temps, it eventually tempers the brass and it looses alot of its elastic properties meaning that it starts to act more like a spring then soft brass.
So, after a case gets many firings on it, the brass will start to act more like a spring. Meaning, when the case is fired, it expands and instead of returning to a dimension similiar to its original form, it stays expanded.
Then when you FL size the brass, the die forces the case smaller in dimension but when the case is pulled out of the die, instead of taking on the new dimensions, it springs back to a similiar dimension as it came out of your chamber when fired.
The more the case is fired, the more it retains the fired dimensions.
Now, we all know this happens in the neck and shoulder area. Thats why the case necks split, because they get work hardened and heat tempered and eventually they are just to brittle to expand anymore and they split. This will not happen with the case walls just ahead of the case head because they are very thick compared to the neck but they do get tempered.
You can anneal the neck and shoulder, you can not anneal the case wall just ahead of the case head.......
There really is no real cure for this. You can increase the amount that the case is sized down. This will make the case dimensions smaller after spring back but it is a temporary fix as the problem will get worse and worse the more the cases are fired.
To be honest, consider how much it costs to shoot these cases per shot, even at $2.50 per case, if you get 10 firings, thats a quater for each firing. Not cheap but I think your getting your moneys worth. If a custom Small base die will get you another 3-4 firings, all the better.
Just want to make sure you know that this is not a problem that you can completely cure, only postpone a few more firings.
I would also be curious what rifle your shooting the Lapua in? The reason I ask is because rifles with small thread shanks chambered in the Lapua with some stainless barrels will have increased chamber swelling during firing. In most cases, this causes no real problems but it can allow the case to expand more when fired even if its only by 1/2 thou or so. When the case gets old, it will expand more and stay larger. Because of the great amount of case taper in the Lapua, you likely will never know the difference because the mechanical extraction cam on the rifle will pop the case out of the chamber easily before the bolt gets to the top of its lift stroke so you will never know if the case is slightly sticking in the case or not.
Just curious what rifle your using.
