Just put this setup on my AR pistol and really like it. It's replacing one of the original Spikes pistol buffer tubes. Great deals through two companies. $100 shipped through Universal Mania (second order from these guys in Fayetteville, NC. they have been stellar both times) on the brace and $50 shipped on the tube from Alex Pro Firearms.

I was leaning towards the KAK super sig tube but the specs on length were iffy (possible manufacturing change), so I emailed KAK to get it straight from the horses mouth. I had heard that the tube was shorter than the SBM4 brace. I wanted to get a full length tube for the extra anti-rotational friction it will cause. Got a reply from the KAK owner that simply said "did you google it". WTF really?

Did some more searching and gave a call to APF who make a similar tapered stop tube but they have two offerings of differing length to match different braces. Great folks at APF ( turns out they are a spin off of Alexandria Profab, a manufacturing company in Alexandria Mn.) The gent I spoke with dropped what he was doing and grabbed one of the tubes, did some measurements for me on the fly and got the tube in the mail that day. Tube part number I got was #558 Sig TAC Buffer tube. Probably best to give them a call to make sure you match up the right tube length for the brace you plan to use.

Here are some measurements that I did off the tube I received:
OD 1.18" thick enough to make a tight fit without stretching out the brace.
Length aft of the tapered step 7 3/16" comes out to the end of the brace. Actually the brace hangs over the tube .088" top / .049" bottom.
Length end of the threading to the end of the step/front of the brace 1.35".
Overall length 10"
LOP back of plate to butt 9 1/4"
SBM4 overall length 7 1/4"
SBM4 ID I should have measured prior to install, but it appears to be approx 1.16" producing a .002" interference. So far this has been tight enough to keep it stable without rotation. Time will tell if the brace stretches, I haven't been able to shoot with it yet.

I prefer this lash up over others I've seen, may not be the best for all but it will work well for me. It did cost a few shekels more and weighs more than a blade setup but I can pull it up fast without pounding my collar bone.

D