What do you look for when you buy an old military mauser for building a custom magnum rifle(DG)?
I don't know the difference between the zillion different actions you find at gun shows

German or Belgian made from 1930-1943 are going to be good and strong with no issues. If it has Fabrique Nationale or something similar it will be Belgian made and good to go. Mod 98 on the siderail with BYF, BNZ etc and a date i.e. 1942 are good or codes like 42, 237 from a 98k are going to be good to go. Modelo 1909 with German or Belgian address on the rail, NOT FRGA or similar on the left rail.

Dear Mr. Chick:

I have had good luck with an old BRNO made in Czechoslovakia (7x57 Ackley) and with a 1908 Brazilian Mauser made at DWM in Berlin (6.5-06).

Presently, I will be building on a 1940 98k made in Germany (I forget which arsenal), and will also use various 1908 Brazilians and 1909 Argentines made in Berlin and Oberndorf, Germany.

Sincerely,

Chris Bemis

Argentine made 1909's were marked DGFM (FMAP). Properly handled, they are as good as the DWM's, I've built customs on three of them. Unless of course, you've just gotta have the German billboard.