Metric nipple size converted?

[Oregon Bill]

Just got an e-mail from Euroarms USA saying their Whitworth rifles take an 8 X 1.25 mm nipple. Anyone know if this is the same as a 5/16 X 20? I will need a platinum-lined nipple for the rifle, and they are very expensive.

[snapcap14]

in short no. 8mm = .31496063 5/16=.31250 20= 20 thd per inch.
Metric thread sizes are specified differently than USA thread sizes. USA sizes are specified using the nominal major diameter and the number of threads per inch. For example, a 3/8-16 (coarse) thread has a 3/8" nominal major diameter and 16 threads per inch, while a 3/8-24 (fine) thread has the same nominal major diameter and 24 threads per inch.

Metric callouts are fundamentally different because the thread pitch (distance between each thread, in mm) is specified instead of threads per inch, or threads per mm. For example, an M10x1.5 (coarse) thread has a 1.5mm pitch, while an M10x1.25 (fine) thread has a 1.25mm pitch.