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This is a pack of twelve (12) extensions that slip over the release valves on VFC magazines, extending the valve enough to cycle properly in a Viper Tech GBBR.
Note that this is NOT the only modification needed for VFC mags to work in VTs. The gun will still need the magazine well enlarged to fit the magazine, and the magazine catch filed down to hold the magazine at the correct height. This product is just to address the differing valve knocker length between VFC and VT.
Simply snip each cap off the sprue and trim flat, then slide onto the valve.