The 7.6x25 and 9mm have a smaller cartridge base diameter than a 7.62x39. You need the smaller diameter in the bolt head. There are a couple of ways to get the smaller diameter. An AK74 bolt has the correct diameter. It can be used with an AK74 bolt carrier or if the back stem of the bolt is sleeved to a larger diameter it can be used with an AK47 bolt carrier. You can also weld a small ring into the bolt head recess to reduce its diameter.
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