Hello Tav,
Tanio Koba's .223" cartridges are expensive so £3.25 each is about right. Last time I bought packs of 30 cartridges which is a few years ago now they cost me £100.
For more info have a look at https://mp40modelguns.forumotion.net/t3354-tanio-koba-223-5-56mm-m4-aluminium-alloy-cartridge#20704.htm
You may need to modify the MGC M4A1 Detonator Pin to suit the different internals of the Tanio cartridges. The magazine will possibly need slight mods to allow the Tanio cartridges to stack correctly too.
Note also that Tanio .223 carts should only be cleaned and washed with warm soapy water. Don't use any chemical cleaners or Ultrasonic cleaners as the brass coloured coating will come off.
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Cerwyn
Hobby collector of Replica model guns and Militaria.
also member of Living History Reenactment Groups.