Long Story short I was thinking of making a Armalite AR-10 pistol build out of an older Armalite AR-10B lower I have. Is there a pistol buffer tube out there that’s long enough?

One that accommodates a brace, or just a round pistol tube?

I need a round tube.

Any one that works on an AR15 will do the job. You just have to use the correct length buffer and spring for the application. Anything that works on a DPMS style AR10 carbine will work. The bcg moves the same distance on either brand.

That’s not entirely true, my armalite buffer is longer than an ar15 buffer, and the Armalite ar10 carbine buffer tube is longer than an ar15 carbine buffer.

Will this be chambered in .22LR  ....9mm .... .40S&W..... .45ACP ???

Read what I said again. You can use any AR15 carbine/pistol buffer tube, with the corresponding use of the correct buffer and buffer spring. I am well aware of the difference in length of the Armalite tube and buffer. Internally, the Armalite carbine tube is 3/4" deeper than the tube from an AR15. The buffer tube from a DPMS carbine is the exact same depth as an AR15 carbine. That's why Armalite uses a 3.25" buffer, the same size as an AR15 carbine buffer, and DPMS uses a 2.5" buffer. Armalite made the tube longer so they wouldn't have to manufacture a special length buffer to enable the use of an AR15 length tube. Use the 2.5" DPMS length buffer and you can use any standard length AR15 pistol buffer tube. You'll also need to use a .308 DPMS carbine type buffer spring, available from several sources.

I doubt you will find a pistol buffer tube with the same internal depth as the Armalite carbine tube. I can think of no reason to manufacture such an item.