Depends upon your definition of "low pressure rounds".
The Thompson Encore has barrels available for it in:
- .300 Win Mag -- 64,000 psi
- .500 S&W -- 50,000 psi
- .30-06 -- 60,200 psi
- 7mm Rem Mag -- 61,000 psi
I suspect that there is more to it than just pressure though. A caliber that pushes a heavy bullet down the barrel is going to generate more recoil than a caliber that pushes a lighter bullet, even if they are both at the same pressure. That recoil affects the linkages.
For example, a .50 BMG has a pressure of only 54,800 psi, but I know of no break action designs for it. You could *physically* chamber a barrel for .50 BMG on an H&R since their "UltraSlug" comes in 12-gauge and the 12-gauge chamber dimensions are *slightly* larger than the base portion of the .50 BMG cartridge, but I suspect it is questionable whether that would leave enough steel around the chamber to handle 54,800 psi, much less whether the action could survive many rounds of that.