I recently purchased 2 boxes of 300 Win Mag Lapua brass in preparation for a custom rifle
being built. I normally follow benchrest case preparation techniques. Before neck turning the 300WM brass I decided to measure the headspace of the virgin brass. Using a Redding Instant Indictor the Lapua headspace was .015” to .017” shorter than the SAAMI minimum. In my past experience with virgin .308 Win Lapua brass the headspace is normally .002”-.003” shorter than the SAAMI minimum.
Because the shoulder is so far back on the 300WM brass I can not FL resize the brass to the same shoulder/neck junction dimension. Therefore I can not get a consistent neck turning shoulder bump cut. I imagine this brass is going to grow like crazy after fire forming and the shoulder and neck are going to move around considerably. This is another reason neck turning seems futile at this step. I can always neck turn after fire forming, however I don’t particularly like this option since it requires 2 fire forms before the brass is “ready to go”.
Belted cases are notorious for thinning around the web due improper headspace-belt chamber alignment. It seems like the Lapua brass will grow at least 15-17 mils assuming the chamber is cut to SAAMI minimum. Is 15-17 mils of growth something to worry about? Has anyone else seen such large headspace setbacks in new 300 WM brass?
One way to solve this problem would be to cut a custom chamber with a reduced headspace to match the Lapua brass. Since this handloaded brass will be married to this rifle I am not worried about factory ammo/brass chambering issues. Does anyone have comments (good or bad) about this option?