OK. Your link didn't work, but your photo did. If that photo is correct then I'll admit I was wrong on that, though it still looks like it's a bad idea to use a carbine or buffer rifle tube if you even possess the stock at all, because it says if you do you could be nabbed for possession of an SBR. So it may not be 100% illegal like I said, but it could easily be illegal if you simply own a stock for it.
I wish you had shown me that BATFE letter sooner instead of going off on a tangent about stuff I wasn't talking about. But I still thank you for the correction. From now on I'll simply say it could be considered constructive possession to use a carbine buffer tube on a pistol.
0331: "Accuracy by volume."