i was a gun corporal antitank platoon, american division ww2 in
ardennes, etc.  we used a british armor piercing shell that had a
split in projectile. so that when it struck it spread.  we called it
the doughnut.  "give me a doughnut" crew knew.    i would like to
obtain an original, unarmed of course. in new condition at reasonable
price.  use in discussions in lectures when questions asked about
advanced weaponry today

Hello petroc and thank you for your question.

The technical term for the type of ordnance you are seeking is HESH.
"H.E.S.H. - High Explosive Squash Head.  A thin-walled H.E. shell
which deforms on impact with the target in order to apply maximum
wracking effect against the surface. H.E.S.H. does not penetrate.
Instead, pieces of armour plating are broken loose on the inside of
the vehicle, causing significant internal damage"
http://miniatures.de/html/pzgr/shelltypes.html

I have been unable to find one in 57mm calibre and whilst researching
the types of ammunition used in the British 6 pounder, I came across
several sites which omit the HESH round in the list of ammunition
used.  It seems there were four types of round commonly used in the
57mm 6-Pounder:

AP = Armour Piercing
APDS = Armour Piercing Discarding Sabot
HE = High Explosive
HVAP = High Velocity Armour Piercing

http://www.pmulcahy.com/ammunition/large-caliber_guns.html

That's not to say that a HESH round wasn't used, I have just not been
able to find any reference to it.  Maybe the information I have
provided here will help another researcher in your quest.

Also see:
"57mm 6-Pounder AP: Weight: 3.4 kg; Price: $530 (R/R)
57mm 6-Pounder APDS: Weight: 3.6 kg; Price: $585 (R/R)
57mm 6-Pounder HE: Weight: 3.6 kg; Price: $530 (R/R)
57mm 6-Pounder HVAP: Weight: 3.6 kg; Price: $530 (R/R)"
http://www.pmulcahy.com/ammunition/large-caliber_ammunition.html

Very best regards.

THX1138