I've had a project in the works for years, and finally picked up a host.  I want to build an integrally suppressed .300 Blackout bolt gun.  It's going to be a hunting rifle, so primarily supersonic but I'd like it to work well with subs as well.

I have a chunk of .30 1:7 twist Rock Creek barrel blank left over from another project.  I'm going to make it into a .300 BO barrel around 11" long.  I'll make a welded baffle stack and pin and weld it to get the barrel length over 16".  I'm also planning to port the barrel, top and bottom, starting at about 9.5" to bleed as much gas off as possible before the bullet exits the barrel.  I'll put a sleeve over that area to keep from flame cutting the tube.

Planning on 17-4 for the baffles and thin Grade 9 Ti for the tube since it won't be threaded and should be low pressure.

My question- I'd like to use a 1.25" OD tube, partly so it looks more like a normal heavy barrel and also because it will help with stock availability.  This limits the internal volume in the baffles but I have no idea how much it will hurt suppression with the .300 BO's low powder charge and the volume over the barrel.  Has anyone tried a really small diameter suppressor on a Blackout?

Here's my initial concept.  The dimensions are rough, it will all be massaged before I start making anything.

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I wouldn’t go smaller than 1.375.
I'd do a p&w tube (to a proximal point) before a welded stack.
Supers require a much larger blast chamber than subs. It'd be easy to pull the stack, move spacers around, and have a perfect stack for either config.
Also, cut the barrel a bit shorter. The difference from 9 to 11in is negligible. Keep a shorter barrel, with a longer stack, and have a better (quieter) system.
I'd recommend more porting, but keep it at the Muzzle. I did 36 (iirc) ports on mine. Much difference vs a previous build with fewer ports.

There's a 30cal build idea a couple posts down. Some more info in there.

There's a reason 1.25" 30 caliber suppressers dont exist.  If they worked well, there would be some available commercially.
Thinking back to high school geometry class, the formula for volume of a cylinder is V=phr^2.  The radius is squared.  For every amount you take off the radius, you have to add that much, plus more to the length to keep the same volume.  Your can is gonna be like 20" long.  

Just spitballing here.  For subs only, consider an 8” barrel, no ports, instead build thread on brake with full diameter angled-back reflector disc to pressurize annular space around barrel.  Close to that at least 8 inches of skinny stack forward to get volume and gas dwell time.  Won’t be ridiculously quiet but should be hearing safe at the muzzle.  If your hearing is intact port pop from blackout subs is still pretty loud.  A long, skinny can could have significant back pressure. If it does then you could consider incremental barrel porting.  But this is all conjecture on my part, something to think about.

Pinning and welding the tube is a possibility. I’ve never welded titanium, but I’m sure I can figure it out.

I was thinking more barrel length after the ports begin would force more gas out, but I like your idea of shortening it and adding more ports.

I saw the thread you’re talking about, it has some good ideas. That one is dealing with a whole lot more gas than mine though.

Most cans are also built around .308 to .300WM. That’s my question, I have a small fraction of the gas so I’m not sure how much volume I need. But yeah, it’s going to be around 15” long.

That’s a good idea as well, I did build another reflex can a while back like that, but it was really short and fat. It works pretty well for what it was designed to do. I’ll have to think about this.

This one is going on a bolt gun, so no worries about port pop or back pressure.