this projectile just sold for $1225 on ebay - what is it?

[Spaceinvader]

That explains the 37mm Aircobra casing I have, the primer rattles around!

 

[Bellifortis]

Thank you for this additional background info. When did this french design first appear ? In all the, up to now, mentioned designs, the floating pin, inside percussion cap, obturation sleeve and the primer-tube seem to be made as one integral assembly. There are more modern US percussion caps, for other uses, like for instance the M39A1, that look like small blanks, but none of the parts move. Is this "floating pin design" only used in high pressure applications and does this design exist just as a percussion cap without being integrated with a long primer tube ?
Regards,
Bellifortis.

as an addition to Hazords words: pictures of an US primer and it's roots an original French primer. The firing pin of the gun was simply round and flat shaped. I think the firing pin of such a gun would have lasted much longer then a conventional firing pin as the wear shifted from the guns firing pin into the primers firing pin. In most cases ~3 mm round impressions are slightly visible on such primers after firing and sometimes the primers "firing pin" is loose and rattles.

 

[Alpini]

Thank you for this additional background info. When did this french design first appear ?

The French guys may correct me but I think it was in 1897 with introduction of the Field Gun Mle. 1897

 

[jeff w]

http://www.ebay.com/itm/400775503985?_trksid=p2060778.m1438.l2649&ssPageName=STRK:MEBIDX:IT

So it looks like eBay removed it 3 days after the sale.
That has happened to me before. I bought some ordnance, paid for it, then eBay canceled the listing.
Always pay the seller fast!

 

[M8owner]

I have attached a few more photos of the guts of the US priming system used in many WWII cannons. The heart of the system is a standard .50 cal. Browning Machine gun primer.