It's not really a "know better" situation it's more of a how to make it work situation. People thread undersized barrels and make them work, just not my personal preference. I have a rifle I want to eventually cut down and thread and as soon as it's cut the muzzle OD will be .650" as well ... so I'll be facing the same issue eventually.
The main problem with not having a "proper" shoulder is there isn't much to square the suppressor on.
Class 3 machining in Dallas is a place I trust. If you give him a .650 barrel to thread 5/8x24 he will thread it if you really want him to but "no warranty".
I'm pretty sure you don't need to worry about warranty.
My biggest worry at this point would be cocentricity or whether or not the threads are accurately aligned with the path of the bore.
Getting that barrel recrowned would be the least of my worries but one of my biggest annoyances.
This is one solution for threading a thin barrel. When I'm ready to thread that thin barrel of mine I'll be trying to find someone who can do this.
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Nice info! I see now about the shoulder issue.