Re: Great, did I buy the wrong ammo......?
Did you mean to say 7.62x56mm? I've never even heard of 55mm so I looked it up and... See below and compare what you have to those listed. PS. Cheaper than dirt doesn't have a listing for 7.62x55
# 6.5x55: Used by Sweden until the adoption of the 7.62x51 NATO cartridge. It was used in the Swedish Mausers, Norwegian Krag-Jorgensen rifles, Lljungman AG-42, and initially was used in Swedish variations of the FN MAG machine gun.
# 7.5x55: Used by Switzerland until the adoption of the 7.62x51 NATO cartridge. It was used in the Schmidt-Rubin M1911 and M1931, and others.
# 7.62x45: Adopted in the postwar period by Czechoslovakia, but discontinued after they succumbed to pressure by the Soviet Union to adopt the Soviet standardised calibres. It was used in earlier versions of the Vz.52 and Vz.58 rifles.
# 7.62x54R: A rimmed cartridge, first adopted by Imperial Russia in 1891, and still in service with the former Soviet Union, Soviet/Russian client states, etc. This is a rimmed cartridge, and is used in the Mosin-Nagant rifle, Simonov AVS-36, Tokarev SVT-1938 and SVT-1940, Dragunov SVD, Romanian PSL, PK machine gun, and others.