Ammo ID for McClean automatic gun

Some people may know the 37mm McClean automatic gun which was made in small numbers during WW1.
Recently I came across this image showing an essentially larger McClean gun which also is automatic (note the chute for the clip). Caliber might be 47mm or 57mm.
Is anybody able to ID the caliber/cartridge that was used in this gun?
Of course details on the gun are also highly welcome.

Image source: internet.

Alex, this is a 1909 picture of an experimental 3 pdr model (I think it’s the only known image). No information about the charger used.

Regards,

Fede

Fede, very interesting, thanks!
So it is a 47mm of some sort.

In the book “The Machine Gun”, Volume 1, by George M.Chinn there is a description of this gun on pages 232-237. The exact same photo as the one you show is presented in the book, with the caption: Samuel M. McClean demonstrating his 37-mm automatic cannon. The ammunition is explained as the standard sevice fixed one-pounder or 37-millimeter with a muzzle velocity of 2,100 feet per second and maximum chamber pressure of 25,000 punds. The tests were carried out in 1904. Due to problems with the gun it was rejected, and the stockholders of the company lost interest in it, and instead a .30 caliber version was built and tested.

Vidar, thanks a lot! Very interesting to see as Chinn must have gotten something seriously wrong there.
The 37mm is well known and way smaller.
Here the 37mm in an AA mount and in the commonly known wheeled AT variant.
Compare the size of the grip to the gun and then look at the gun in question, it is at least 2 times the size of the 37mm.

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Image source: internet.

Well, that just goes to show that we can’t always trust historians. Which is nothing new.

I assume there is no single publication in this world that has no errors.
And admittedly back in the time there was no internet and research was painstaking. In particular with things like automatic guns and lots of classified material etc.
Maybe we shall not judge Chinn too hard for this.

Here the 37mm in a Russian Museum and some more info on the whole:

An interesting article, although it omits the final time the McClean guns were offered for sale - by the American Armament Corporation, who marketed the gun in the 1930s as one of theirs! This corporation was notorious for “borrowing” designs without acknowledgement.

A McClean exists in the UK, in the Heugh Battery Museum, Hartlepool: http://www.heughbattery.co.uk
A picture of it: