For this Sierra Vista.
Used Gold Euro RT Center Bands, reamed out to 27/64ths and a little CA.
The CB's are the exact OD to match the fittings.
Slimline 7mm Center Bands which I have left over from slimline kits I use to make my Rifle Cartridge pens.
They fit over a 7mm tube with no reaming and match the OD of the slimline fittings perfectly.