I have a SilencerCo Omega 300 suppressor, obviously direct thread adapter is 5/8X24 threads. How do I thread a barrel that is too small like a M70 FWT, Howa Alpine, or Sporter barrel to run direct threads, has anyone done this? I'd prefer not to use the ASR brake for the Omega. I all so realize that some sort of thread adapter is going to have to be used to get a shoulder large enough for my suppressor to work properly.

Your idea of a double threaded bushing adapter is right on, but I would caution you to have it made by a good machinist who will thread and shoulder it FROM THE SAME SETUP. The can bore must be dead nuts true to the boreline. I can't imagine threading the barrel without it being removed from the action, and set up between centers. Time is money in the machine shop.

Setting up the barrel between centers is asking for trouble. How would one indicate the bore with a center stuffed in it? I’ve threaded piles of barrels and each and every one had the bore dialed in in a 4 jaw chuck.

More than likely you’ll need to thread them 1/2x28 and use a bushing style adaptor like Flintlocke said. For calibers under .277 and really skinny barrels I’ve threaded them 7/16”x 24 or whatever the NF pitch is for 7/16 threads. You would of course need to have that bushing custom made more than likely but it’s an easy enough job.

My hair caught on fire and my head exploded..."setting up the barrel between centers is asking for trouble".
You don't indicate the bore working between centers...the 60 deg centers do it for you, instantly.
So, without this devolving into a discussion of "best practices", which will put everyone else to sleep, I will quote the shortest statement of four respected sources on the subject:

"Whenever practicable, the work is held between centers. This method is usually more accurate and has the advantage of permitting removal and replacement of the work without affecting accuracy." and so on. from the manual of "Lathe Operations and Machinists Tables". And so say Machinery's Handbook, Atlas, South Bend, Machine Shop Practice volume 1 (Moltrecht).
Your preferred method is a very good method IF you indicate BOTH ends of the barrel thru the spindle. To center the outboard end, you need a home built spider/chuck...if this was considered a "best practice" lathe manufacturers would have built lathes with chucks on both ends of the spindle long ago. The only lathes with that feature are "Oilfield Lathes", huge things capable of turning 32 foot drill stem.
Use your preferred method but don't run down an industry standard with 200 years of common practice.

I get time is money, but it has to be cheaper to thread my existing barrels than to replace them. Not to mention all my McMillan Hunter and Bansner stocks that only open up for a #3 or 4 contour barrel I'd have to replace.

OK, I am not good at 'splainin' stuff. Yes, you thread your muzzle of your existing barrel with a size and thread count compatible with whatever OD. It can be any thread your machinist will recommend, as an example let's say your muzzle measures .520". He will turn a true with the bore, major diameter of .500". He will likely cut a 1/2-28 UNEF thread on your muzzle, maybe .600 more or less in length. Then he will take a piece of say, 3/4" round stock, likely 12L14 steel, bore the minor diameter to about .461". Using that same setup, he will thread that internal bored hole to 1/2-28 UNEF that will thread onto your barrel. Under no circumstances will this be anything other than lathe cut threads. Taps can seldom hold close tolerances you want. Using the same setup so threads are perfectly concentric he will lathe cut your 5/8-24 UNEF to mate with your can. From the same setup he will use a parting tool to cut the threaded bushing from the 3/4" round stock. He will make the cuts necessary at any point to ensure perfect parallel fit on the barrel shoulder and the can shoulder. When it is all done you will probably secure the bushing in the can with something like Loc Tite 1C (350 deg F.) That is how I would do it, but many ways to skin the cat.

Call Eric at red oak gunsmithing, he will be albe to help for sure and now owns hughes precision for the adaptors.
989-733-7010

Sells a adaptor 9/16-24 to 5/8-24 that indexes off the muzzle for lack of barrel diameter. This will require the muzzle to be trued not an option between centers.

I called a semi-local guy who has built a bunch of rifles for me. Going to drop off these rifles after hunting season to thread.

I called a semi-local guy who has built a bunch of rifles for me. Going to drop off these rifles after hunting season to thread.

I assume you're aware of these if you don't want an adapter. https://silencerco.com/shop/bravo-direct-thread-mount/