You didn't ask for advice, but I'm going to jump in anyway.
You shouldn't be shooting a 70 year old bolt. Use an MG3 bolt instead. Old steel that's been under stress of any type can crystalize and shatter as yours did. The ammo may have contributed to it, but it could have just as easily happened anyway. 70 year old steel, made under wartime conditions with bombing and sabotage going on, and perhaps improper heat treatment and steel/nickel/chrome shortages, is not going to be the best choice for use when it's not necessary.
MG3 (modern) bolts are available from the usual parts dealers for reasonable money. Glad you didn't have it shatter and blow the gun.
Save some grief and shoot with an MG3 bolt (or bolt head for a semi). Save the antiques for show and tell.