From Lapua:
Hi Russell,
Given that we developed this case specifically for competition shooting, which normally takes place in considerably warmer weather, I would definitely recommend using a magnum primer. This is especially true if you're wanting to use a ball powder, as they tend to be harder to ignite. This may sound like odd advice coming from a sales department, but I'd seriously consider using some other brass, specifically something that utilizes a large primer. The large primers generally don't give as fine accuracy as the milder, small rifle primers do (which is why we chose to use them), but in the extreme cold, that's not that much of a tradeoff to ensure consistent ignition in very bad conditions. But yes, at the very least, use magnum primers for this application. Warm summer days on the competition range, the smaller standard primers are no problem at all. Just have to match the tools with the conditions.
I hope this answers your question, but if there's anything more that we can do to be of service, please don't hesitate to let us know. As always, we'll be glad to help.
Sincerely,
Kevin Thomas
Sales Manager