Does it have proof marks? I think if it has proof marks it will be one of their post war rifles, and should be very safe to run to around the 60 000 PSI mark. But i would be confident you could get 2450fps out of and german-made '98, although you may wish to do it with lower pressures and lighter bullets. The 160gr Barnes TTSX would be a great option. I think Heym did do some rifles with surplus military actions, in which quality control may have been a little more variable, but 2350fps with the 160gr would still be very, very conservative. Does it have a thumb slot? Heym did make the "west german boder guard rifle" - these where a weak action.