So I called YHM yesterday and asked them about shooting the M2 on a fixed barrel. They said it would be fine even with the spring in place. So now I don't know what the heck to do.
Yea, they told me that too, but you should use the fixed barrel spacer because otherwise you are going to be jack-hammering your threads and it will screw you over the long term.
Here is the thread where I asked about that over on Silencertalk.
YHM won't sell fixed barrel adapter. Make my own?
Now that I read the thread I remember that I was given the dimensions in a PM. Here they are.
"1.120 outside diameter...1.120 long (yep they are both the same) .994 inside diameter .062 thick"