yes there is no problem, you have to register here
http://www.egun.de/market/register.php?PHPSESSID=1c48372950c373727e35fa1e0204049c

thats a good translator if you need
http://www.dict.cc/?s=the
works in both directions

I will send the seller a pm asking if he is willing to ship into the united staates, accept payment with credit cards and correspondent in english.
the gun needs a licence here so he could not send just like a muzzle loader. he needs a paper from you local police that allow you to import the gun into the united staates. the seller have to apply for a german export licence.
needs time and a little money but is not ther biggest problem.

have asked a gun export firm that make shipment inside of europe if they would do it.

the büchsflinte would accept Magtech brass after sizing rim thickness and diameter. 65 mm for the shotgun barrel and 40 mm long for the rifle barrel. in this time spitzer bullets were in vogue like here on the right side.

the later 1906 RWS bullet catalog gives a 14,55mm (.572) diameter for the 28 bore cats head bullet but his early one here may differ a little bit.
you can easy load 70 grain BP into the case and this is the 577 snider load.

its possible that the rifle chamber is only made for a 30-33mm long case. sometimes older bore rifles are made this way like this very early center fire