I found a trick that anyone can do to work out their chamber length. The internal dimensions of an 8-bore and any other shotgun chamber can be found online. In the example of the 8-bore, the forward chamber is a nominal 23.2mm diameter. I then found a web-page with the dimensions of our local coinage. By serendipity the New Zealand $1 coin is 23.0mm. Dropping the coin in the bore it isn't likely to fall through and get stuck in the bore for the same reason manhole covers are made round. By tapping it with a Parker-Hale style wooden shotgun cleaning rod it sat nicely against the forcing cone. The depth of the shotgun rod in the chamber plus the width of the coin is just over 3,3/4". So Eley-Kynoch rounds at 3,1/4" length should be entirely safe to shoot, (this gun is nitro-proofed). Ironically I can load 2oz of shot, (the Eley 8-guage load), in my semi-automatic 10 gauge magnum and 1,7/8oz in my 12 gauge magnum semi, but my old Langley and Lewis 3" 12 gauge magnum side by side is only proofed for 1,1/2oz.
The same trick should work for any chamber that has a forcing cone, just got to find the right diameter coin to fit.
