Re: 280 JDJ or 280 Ackley Improved
Yes, if you do not you will have excessive headspace and in while in a tight custom chamber this would probably not lead to a firearm failure, it will at best severly stretch your case heads dramatically shortening case life.
At worst, you will have a total case head seperation and then things get exciting.
TO be honest, for the pressure limits the Encore system should be subjected to, I would just use the correct 280 Rem brass. If oyu push the loads hard enough to see any advantage in the thicker case head of the 270, you will be getting frame flexing as well.
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