I finally got a response from the folks at the RAF museum, and here is the gist of it:
From: Guy Revell [mailto:
guy.revell@rafmuseum.org]
To:
rshubert@excite.comDate: Wed, 28 May 2003 14:46:22 +0100
Subject: Reply from RAF Museum Re: Hispano 20mm cannon AP properties
RAFM/32/6/GR
Mr. R. Shubert
Newburgh
Indiana
47630
USA
28th May 2003
Dear Mr. Shubert,
Thank you for your enquiry which was received on 14th April 2003. You wished
to receive information relating to the armour piercing properties of the
20mm Hispano-Suiza cannon as fitted to RAF aircraft of the Second World War.
The penetrative effects of the gun were obviously dependent on the
ammunition type used, but the thickest armour that I can find reference to
as being pierced by a 20mm round is 18mm. This is obviously short of the
25mm mentioned in your enquiry.
The 20mm cannon was originally intended for air-to-air combat work, and
ground strafing was a secondary consideration. Indeed it was not until June
1940 that the Air Staff started to look at the potential of attacking tanks
with aircraft and 20mm was declared to be too lightweight. It was this which
led to the development of the 40mm Vickers 'S' gun for use on the Hurricane
IID which entered operational service in June 1942. I hope that I have been
able to assist you with your research.
Yours sincerely,
GUY REVELL