Blind mag? If so, what kind of stock? Factory, aftermarket? Brand/style?
What cartridge is the mag box for? I'm assuming .308 since that's what you said the gun was originally. Should have a cartridge number stamped on the side of it. .308, 22-250, etc.
What exactly is the feeding issue? Not picking up rounds? Nose of bullet hitting above or below chamber?
Details make all the difference.
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