Jani,
I inherited a nice interwar Emil Eckoldt drilling 12/12/6,3-52R (Suhl). It has been firing 25-35 Win cartridges for decades, original and reloaded, with no trouble whatsoever. The rifled barrel is marked 8 gr, that would mean a 123 grain bullet. If you doubt the proofing of your gun go for a 6,5-52R S&B that gives 2200 bar. Another way is to reproof it for 25-35 Win with your local proof house.