Double check the thread form as well as the pitch. Some of the older Enfield threads were so close to metric that there was some (sloppy) interchangability.
I'm NOT suggesting you booger fit a metric pipe in there, just that you eliminate the possiblity of there being a coincidental overlap. I've had this once or twice with things like ejector & safety screws being almost
metric, close enough that a casual "run a bolt in" test looks good at first.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”