The BR-4 is a benchrest primer which has a thicker cup and more uniform ignition. It can handle higher pressure than the 400. I use the 400 in 223 plinking loads for the AR. If I'm gonna run hotter loads I'll use #41 or 450s. In my higher pressure 223 bolt rounds I use the BR4 and 450s. I use the BR4 in my lapua 6.5 creedmoor brass. DO NOT use 400s in the small primer 308/6.5 creedmoor/6 creedmoor brass. It cannot handle the high pressures of those cartridges and you are playing danger/injury/possible death zone if you do. Bergara had a post on their website and social media about people damaging their guns because they were loading using the 400s. Be smart and you will live to be an old reloader some day my friend. Hope this helps you.
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