My HK thread protector (left hand thread) was also stuck a while back, I wrapped the protector in some webbing and slide in cloth to protect the finish and get a good grip. I used vise-grip pliers and my bench-mounted vise to remove it pretty easily.
Glad to see they are sending you a wrench for free. Another good trick is using a strap wrench that has a woven strap. The woven strap would grip the anti-rotation checkering on the barrel nut well, where a rubber strap would not bite and slip.
ie...
https://www.google.com/amp/s/www.yellowpages.com/acworth-ga/mip/amp/randy-k-henson-gunsmith-456465745
Here y’all go. The number posted is his cell phone.
Randy Henson, does some excellent park and bluing work. Typically has a long turnaround time, but I don't mind too much. He's currently re-bluing an old rifle of mine, and I might have him park a rem700 after he cuts the barrel down and threads it...
I've seen where 7.5" 5.56 ARs tend to have issues because of their short barrel length. Many are simply over gassed and need to be tamed. Have you tried an adjustable gas block? A different buffer/spring may also help.
My AR10(18") ran like a champ unsuppressed, but when I threw the suppressor...
Post a picture of the lower receiver. Top view, side view, internals as mush as possible, and where the buffer retainer/tube meet.
Edit:
Are you using factory ammo or reloads? Can you cycle through a few rounds by just pulling back the charging handle? Did you have the headspacing checked?
I'm starting my 300 blk pistol (eventual SBR) build with the "Betsy Ross" Aero Precision set and was planning to use Blue and/or Red anodized accessories to make it stand out and a little patriotic. I know that Seekins Precision also offers red and blue anodized parts but I stumbled across...
Have you tried MSI over in Austell? (Metal Specialists Inc)
We do a fair amount of business with them, but I'm not positive if they will have what you need.