Hi Peter Larsson,
I agree with Spooksar that the Highwall action is not quite as strong as the Ruger No1 but, few rifle actions are.
The No1 is one of the strongest non-custom rifle actions ever produced.
Furthermore, I will say that the Highwall is plenty strong enough for the warmest loads intended for the Marlin and 1886 Browning/Winchester actions.
Very likely your shoulder will begin to complain before your rifle will.
But as Spooksar said, watch for pressure signs when working up your hand-loads, as you would with any self-loaded ammunition.
I once owned a Merkel rifle that showed very high pressure with the exact ammunition, (factory loaded) that Merkel claimed it had been regulated with - Federal brand 300 grain round nose.
Too much pressure is too much pressure.
No matter how strong the rifle action is, you can loose an eye or damage some internal parts of any rifle with "hot loads".
I really like the Browning Highwall rifles (also the Lowwall in smaller calibers) and I am surprised that they are not more popular than they are, even here in the US.
One of my friends in Wyoming has a Browning Lowwall in .22 Hornet and I have tried to get it from him, when he lived up here where I live but, he will not sell it.
Perhaps the answer to where they are made is contained in some rifle forum outside of this one but it for sure that I do not know the answer.
I had always thought they were made by Miroku but I do know if that is a fact or not.
Once I owned a Browning/Miroku Model 1895 lever action in .30-40 Krag and it was very well made, as well as quite accurate.
My brother in law owns a Miroku (not imported by Browning) over-under shotgun and it is very well made also.
Cheers,
Velo Dog.