Background: I finally built my first AR pistol, and at the moment it has an SB-Mini brace. I put the SB-Mini on my build because that configuration will probably survive the proposed BATFE rules update (I read the proposal). My only issue with this build was that the brace was loose on the pistol buffer tube, and it would easily rotate to a bad orientation or even fall off. I know it is generally illegal (or very unwise) to modify a pistol brace in any way, so I instead wrapped blue painter's tape around the buffer tube until the SB-Mini fit securely onto the buffer tube. It worked great! Then I removed the tape, because I felt I was in a gray zone with current law.

Questions: I am not looking for official legal advice here, I am simply asking for opinions. I would like to hear your opinions on two matters: (1) do you perceive the blue tape as a modification to the firearm/buffer-tube or to the brace?, and (2) is writing a letter to the BATFE for clarification a good or bad idea? With respect to (2), I really don't want to be on their radar if I can help it. But then again, I don't want to go to jail and lose my job, all my money, and guns because I put some blue tape on a buffer tube.

I am a new member to these forums as of today, and this is my first ever post (so please go easy on me!).

If I get time tomorrow I will post some pics, but for now, get a Phase 5 pistol tube.  Get the one designed for the SB mini.  It has a ledge at the end of the tube which prevents rotation.

Agreed.

I don't see how the tape could but who knows. You could also use 100% silicon and it should hold it in place. Just make sure it is where you want it before it dries.

This is the back end of a Phase 5 tube.   You can see the ledge on the tube, along with the shelf on the inside of the brace.   There isn't much rotation when you use the Phase 5 tube.


The tube also has a little "lip" that sticks out which locks it into the buffer detent pin, so it does not rotate at the threads (stock photo).

I think you're worrying to much. If you want to wrap it in tape do it. Nobody is going to care.

The main mod the glow boys are concerned about is removing the velcro or other band that secures the brace to your arm, or otherwise making the brace more like a stock.

Many braces have mechanical features to secure the position of the brace and keep it from flipping around the buffer tube.

I see no issue with putting a pin in, wrapping tape under the brace on the tube, etc, as you are just trying to secure the brace for safety.

Exactly.  No need to over-think this.

What current law is there that restricts tape on the buffer tube?

I don't think adding tape is illegal, but I still say that you should get the correct buffer tube.  
While researching braces, I realized that not all (non-adjustable) braces take the same tube.

For instance, the Tailhook Mod1/C states: This version clamps to pistol buffer tubes or adapter spuds ranging from 1.17? to 1.2? in diameter.

The SB SOB states: DIAMETER COMPATIBILITY:1.1”-1.25”

The SB Mini states:DIAMETER COMPATIBILITY:1.2"-1.25"


I went with Phase 5 because they make tubes specifically for the SB Mini and Tailhook Mod 1/C....plus they 7075 aluminum.  

I should have just linked to their site instead of taking my potato pic. Much better view of the ledge, and other features.  
https://phase5wsi.com/hex-2-for-the-sb-mini-arm-brace.html



I see they are saying 11.3 inches for LOP.

Thanks, everyone! Phase 5 tube it is.

For Gods sake, please do not write a letter to the ATF.

Do what you want and don’t worry about it.