By 870 I assume you mean the Model 870 shotgun? If so, there would be no purpose for that type of conversion. Shotshell primers are of the 209 size. The 209 "conversion" is done with muzzle loaders. The idea is to replace the muzzle loader nipple (that holds the cap) with a conversion unit that uses the 209 shotshell primer to get more positive ignition of the black powder.
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