HS Precision has had something similar for a long time. You use their long action bottom metal and they have a 6.5x284 magazine that allows for a 3.1" COAL. It will work with a SAUM cartridge. I used this exact setup years ago on a 6.5 SAUM. It'll only hold two rounds (vs. 3 rounds of 6.5x284). This option is nice if you're stuck with a Remington 700 style BDL stock. Sometimes this bottom metal just drops in, sometimes it's minor sanding inside of the stock (out of view, no ugly mods) to get the bottom metal to seat properly.
HS Precision detachable magazine for HS Precision long action bottom metals. Magazine is Stainless Steel with a Teflon coating. The bottom of each magazine box is surrounded with a polymer boot.
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