I have a Factopry Converted (UK) No4 rifle to a 7,62x39 Carbine, using short Norinco 5 round Mags, whicvh has the same configuration as the Neck of the Norinco 75 Round Mag.
Since I have a 75, I will have to try it in the 7,62x39 No4 (It too, is "sporterised" and the Barrel "Carbinised" and set back to utilise the front end of the .303 chamber which is almost the same as a 7,62x39 case Profile (ie, No reaming required).
Rifle shoots quite well with 5 round mag...don't know what the added weight of the full 75 will do to the Mag Locking Mechanism and the Balance of the rifle.
Doc AV