This one on my desk is larger than 1/2-28. Either that weird .595-32 thread for the M1A or M15-1 for an HK.
The back opening is .615"DIA but that's pretty easy to open up.
If it's a birdcage style 18 tooth in possible 15x1 I would be interested. I have a 15x1 phantom but that's all.
I also have a 1/2×28 short phantom than indexes over the barrel 7/8" so that the goal is only about 1" longer. I had a similar one with a birdcage on my son's rifle but it got stolen.