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Thought you couldn't modify a brace?
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You can do whatever you want 'BUT' don't do stupid. If the mod makes it easier to shoulder fire or it causes the brace to no longer function like an arm brace then that would be stupid. Heck, I've seen people making their own braces. There's a guy selling blade type braces that look like N1 stocks in the retro section. Not mine but below is a photo of one, looks pretty awesome!
Ultimately it's the ATF's opinion that will matter so I would be cautious. Be very sure that the BRACE actually works as a brace and doesn't have any sort of butt pad that could be considered shoulder fired. Sending in the completed pistol to the ATF to get the ''not shoulder fired'' evaluation would be a good idea if you alter or make a brace.