I have given these a close look, shouldered a pistol built using one. It definitely functions as a buttstock. In fact the idea of an "arm brace" is rather silly because it works better as a buttstock (though a bit short) than an arm brace imo. I'm VERY surprised the ATF gave the go ahead. They claim it's not designed to be fired from the shoulder? I know the ATF made up some conflicting rules and regs, but geez. You don't look at it and say "oh my arm goes through here" but rather, "wow, this buttstock is a bit short". It's a buttstock, not an arm brace but I like the way they appear to have sneaked it past the man for the time being anyhow.
My real question is if I installed one on an AK pistol would it be an SBR even though the basis for it's invention is the installation on the buffer tube? Does the SIG 556 have a buffer tube? I don't believe it does. Their letter would suggest I could indeed install it on an AK pistol and be legit. To me it seems like a good way to get your wires crossed up with the law, regardless of the weapon it's used on.
No it's not, and Sig/Century didn't sneak anything past anyone. Have you read this thread or even viewed the video showing the history of it being promoted by a veteran?
If it bothers you that much, it's a free country and you're more than welcome to petition the ATF to change their mind. I doubt many of the amputees etc that are thrilled with it will appreciate your efforts though.
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