The BATF has changed its mind and decided that shouldering the Sig SB15 Brace is illegal. They claim in shouldering the brace you're remanufacturing the firearm and creating an illegal SBR. However, is the brace itself illegal?
Throdsen's letter to the BATF: https://princelaw.files.wordpress.com/2015/01/atf_letter_pistol_builders_pack_rev_03.pdf
The BATF's response to Thordsen's letter: https://princelaw.files.wordpress.com/2015/01/atf_response_letter_for_ar_pistol_pack.pdf
Prince Law's interpretation: http://blog.princelaw.com/2015/01/01/ringing-in-the-new-year-atf-style/