Just checked three barrels, the averages are 3 1/16" and 6 5/32" to the dovetail centers. Please be advised that I've encountered differing dimensions in the walls, by as much as 1/16" at times in the length, curvature and positioning of the flat mainspring. I usually cut the mainspring screw hole to a slightly elongated shape so that it can be adjusted fore-&-aft to provide/restore a 'relaxed' extractor position for chambering. The factory apparently designed the mainspring to have two 'open' positions, one with the lever at the end of its travel & the extractor fully extended and the other with the lever slightly relaxed & the extractor free to move back into its slot for full easy chambering of the new cartridge. Stacking of tolerances and differing manufacturing standards on replaced parts often result in the absence of the 'relaxed' or 'loading' mainspring positioning but moving the mainspring slightly fore-or-aft will almost always restore the second position.
Just one of the reasons I like the flat mainspring models better.
Regards, Joe
