FN's SPR rifles are based on Winchester Model 70 actions, and they are available with detachable magazines. They're available in .308 Win and .300WSM. Whether or not FN will part with the bottom metal as separate parts isn't anything I can quote on, but its an avenue to investigate. The magazines themselves are listed on FN Herstals website, one would imagine a gunsmith could order the magazine well/trigger guard assembly for you.