20mm Ordnance.
Good evening.
Not my area, but by chance I have to hand a copy of the US Army's "Small-Calibre Ammunition Identification Guide. Volume 2. Small Arms Cartridges. 20mm to 40mm". R.T. Huntington. May 1978. (Doesn't everyone?)
Don't you just love a good book?
It lists some 21 different 20mm calibres. Projectile weights vary, as do users, dates of manufacture and use. It would assit identification to have details of weight of remaining projectile component, original location, and any markings.
In the absence of such information, it is really a case of "take your pick".
That said, our Member' s comment above is generally sound advice: leave well alone!
Enjoy.
S.T.
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