well............here goes
as far as I can tell there are no 14 ga. empty's so you will have to make them. I take .50 cal. bmg fired brass (de-prime), turn the body from the shoulder dia. straight to the rim and cut off . them take a .30/06 empty and turn the base to 5/16" (this will leave the primer pocket) and press fit this into the .50 cal primer pocket (I it solder also). turn the base end of the .50 cal (now 14ga. with the smaller primer pocket) so that a large pistol primer will sit flush. and you are ready to load. I use lower end 12ga. data and cut down 12ga. wads. seating the wads and the over shot wads with an old 12 ga. press, seal the over shot wad with some high temp. bullet lube or bees wax. you will have to make your own resizing and crimping dies. the 14ga. eg's only had the single firing pin (and no nose cap) and as to "I'd love to use this for skeet shooting" those doubles will be hard.

 

Proper Greener Police guns
Have a bottle-necked chamber, from 12 gauge (head and rim) down to 14 ga. Part of body and neck.
And just to improve the spread of the pellets, the barrel is 13 gauge!!! ( barrel gauge marking from proof house)
Shells can be made from 12 ga
CBC-Magteck brass by necking down, and removing the two
Lateral pins of the breech face.

Doc AV

 

"Proper Greener Police guns
Have a bottle-necked chamber"

Doc; the 'early' police guns did not have the 'bottle-neck' chamber but have a straight wall. someone back a ways did a lot of write-up on these. while not a true 14 ga. they are very close. also they did not have the '3' pronged firing pin on the later guns with the 'necked' chamber.

 

there is a realy 14 ga with a thick rim, thicker than "common" 14 ga shells.
know it because years ago I change a lot of 14 ga greener brass for a very old manton 14 ga double gun.
and then there is the 12/14 greener police cartridge.

 

"there is a realy 14 ga with a thick rim, thicker than "common" 14 ga shells. "...........the .50 cal. bmg rim (as above) is too thick and the bases must be trimmed for proper headspace. also Winchester (and maybe others) loaded the 14 ga. way back when. I suppose you could use standard .50 cal. bmg primers to load this and then thin the inside or the rim, but they are expensive.

 

I know some Greener police guns (though I don't know about their being for Egypt) used the standard 12 gauge case and had a single firing-pin. It might be that these had a 14ga bore. It would probably be safe in a gun as strong in action and barrel as the Greener. I know of a break-open damascus single designed by the unidentified maker to fire the 3in. 12ga through a 20ga barrel. It stood modern nitro proof and fired a lot of such ammunition without incident (and probably with no advantage whatever over a standard 12ga, but what does that have to do with anything?

So I would fire a 12ga chamber and 14ga bore Greener, if such a thing exists, with ammunition for which Greener intended it, but not at higher pressures. That means black powder if proved only for black,for smokeless is peculiar stuff. Anything that tends to run up pressure a little, runs it up rather a lot. For conventional 12ga and the special 14ga I would use cautiously loaded smokeless anyway. I believe some were made when there was no British nitro proof, but Greener knew smokeless would sometimes be used.

I have seen a sporting and entirely conventional 12ga stamped EG. I may be wrong, but I think it came from the factory that way, before they used the name GP, and wasn't bored from 14 to 12.

Yes, the common 14ga "Special Bore" cartridge was made from 12ga cases. You might be able to do it with plastic cases and a heat gun. The .BMG route sounds good, but you might get better brass life if you reduced them slightly in a die before turning down. I don't think there is any shortage of internal space to argue against this.

 

Not sure if this will help...

 

SG is the shot size, a little closer to 00 than 000 in the American buckshot sizes.

Jeff Cooper used to say that in the Third World, where you had to arm low-grade security men who might be suborned or robbed of his weapon, a single barrelled shotgun is a good choice. It's a far easier weapon to use well than a pistol, and one nobody really wants to face. But it isn't really tempting to steal, and if a guard wanted to assassinate the client he would have to trust the others to have a chance of getting off the property afterwards.