i use a 690 barrel for fs
688 is about perfect
686 is getting to the point where you will lose reliability with them breaking
lapco makes a 683 barrel for tiberius... BUT it's rifled, and i haven't gottent ot handle one to actually meausre across the rifling and the wells of the rfiling.
you get a nice clean and FAST round down the chambe using a 690 barrel anything above that and it bounces around.
the 688 id just catches those fs that are slightly out of round and hold them just a hair less then snug.
686 needs to be hand feed and not bolt pushed into the breech or you'll shave the fins on oblondged ones or possible crack the shell
all barrels used where checked with plug/pin gauges +.0002 and -.0002 i think ZZ class right?
did't have a bore gauge that was available.
so my final statment would be get a 688-690 barrel for your fs
flasc did a similiar stuy and came up with 686 as perfect if i remember correctly, but the article is is posted around