Rules regarding gun magazines (high cap etc.)

There are no limitations on the use and possesion of high-cappa mags in Poland.
Please make sure, you are cognizant of the EU- and Gun-laws if you travel through Germany, or any other country.
https://www.zoll.de/EN/Private-individuals/Travel/Entering-Germany/Restrictions/Weapons-and-ammunition/weapons-and-ammunition_node.html