If your eyes & perception are good,, wing it. If not, just pull out a dollar bill & lay it next to barrel. Measures 6 1/8". ALWAYS have a "ruler" in your wallet.
Want to let the LGS know you are serious,,, Use a BENJAMIN!

Don't ever trust a gunshop tag. I bought a sweet Rockola M1 Carbine they had mismarked in .30 Luger. Several passed on it. They figured it was altered so they sold it relatively cheap