Separating nipple from Plug

[FOGofWar]

I haven't used my USAK in some years, but decided to hunt with it this Fall. Giving it a checkout, when I went to remove the nipple, the breech plug came out as well. Threads on the plug were clean and greased, and the bore was shinney, so it seems I put it away properly cleaned.
I am reluctant to put vise grips on the plug so I can add more force to unscrewing the nipple. Made me wonder if the design is that the nipple and plug have opposite threads?? If not, it seems this would be a common problem. There's nothing in the manual about this.
If they do have the same thread, ie both counter clockwise for open, then any tips for holding the plug, while i put some torque on the nipple?

Thanks,

[DiscMan]

Protect the threads and put it in a vice to remove the nipple.