I started loading for my 308 Norma Magnum again and was surprised last week to find NO magnum primers available at any of the shops. I have thousands of primers but had only one single box of large rifle magnum primers: CCI 250. Wanting to assure a supply of whatever primer I used so for my initial loads, I just used regular Federal 210 primers. Groups were pretty horrible. Largely vertical and 1.5 to 2 inches using 70 grains of H-4350 as fuel under 165 grain SSTs. The vertical tendency seemed to spell 'ignition' so this week I again searched out some primers and finally found 1000 Remington 9 1/2M (mag) primers to use in today's loads. With only the change in primer, the groups were now 1/2 to 1" in a pretty nasty cross wind... and this with just a fore-end rest.
Though the use of magnum primers when lighting large piles of slow burning powder is pretty much common sense, today's experience was a reminder of the times in my shooting career when simply changing primers made a dramatic improvement in accuracy. Don't be afraid to change up primers when a load isn't doing the things you think it should.~Muir