Before accepting a barrel for machining to accept screw-in chokes, you must measure the outside diameter (O.D.) to determine
if there will be sufficient wall thickness after machining. “Mike” the O.D. of the barrel and the O.D. of the tap for the particular
gauge.
Subtract the O.D. of the tap from that of the barrel, divide by 2, and this will give the wall thickness after machining,
providing the O.D. of the barrel is concentric with the inside diameter (I.D.). A sample calculation appears below.
Barrel O.D. (12 Gauge) .850"
Tap O.D. (12 Gauge) Win-Choke™ – .814"
.036" ÷ 2 = . 018" Wall Thickness
We DO NOT recommend installations in barrels where the resultant wall thickness will be less than .015", providing the I.D. of
the barrel is concentric with the O.D. If the I.D. is not concentric with the O.D., you will have to make your own decision as to
whether the installation will be safe.
In addition, you CANNOT install screw-in chokes in barrels whose inside diameters (I.D.)
exceed the following dimensions:
10 ga. - .780";
12 ga. - .735";
12 ga. Thinwall - .728";
16 ga. - .666";
20 ga. - .624".
Exceeding these dimensions WILL CAUSE DAMAGED CHOKE TUBES and there is a GREAT POSSIBILITY OF CAUSING A BARREL BLOW-OUT! Check EVERY installation before firing to make sure the tube does not protrude into the bore. Back-bored or jug-choked barrels
are usually NOT suitable for screw-in choke installation.
http://www.brownells.com/UserDocs/Miscellaneous/Screwinchokeinstallation.pdf
