I haven’t seen this asked anywhere else. I hope you can help.
In measuring the bore of a 12 ga semi-auto shotgun, I find the bore about .732. when a say, IC choke is in place, the bottom of the choke (toward receiver) measures about .740. As the bore moves toward the muzzle, it goes down to about .722 (Say in 3″ or so).
I know the choke has to be larger at the bottom to enable the wad and shot to exit without blowing the choke out. My question is this, “Is the effective choke .010 (.732-.722=.010) or .018 (.740-.722=.018)”?
I’ve asked quite a few people at the shooting club, and opinions differ. Have you seen any information or done any testing for a definitive answer.
I enjoy your website and check it often for your opinions and knowledge.
Green Bay, WI
Excellent question. It will take me at least two cups of coffee to invent an answer, but I’ll make something up for sure.
Due to doing gun reviews over the years I’ve measured a lot of screw chokes. All, without exception, are wider than the bore at the skirt (thin rear part of the choke). As you state, this is done so as to make sure that the rear of the choke never sticks up into the bore, thereby adding about 1-1/4 oz of steel to the usual lead payload.
The manufacturers could machine the skirt perfectly flush. Teague almost does this. There is just an imperceptible drop in the skirt when Teague (English ex Rolls Royce engineer turned choke maker) installs his custom screw chokes. Perazzi tried this on the early MT-6s with factory screw chokes. Unfortunately for Perazzi, the threading was such that it permitted a build up of carbon between the threads and the choke. Over time, if this was not kept clean, the bulk of the carbon pushed the choke skirt up into the bore and you shot the chokes out. This has never been a problem for the Teague chokes and Perazzi soon fixed their problem.
Today, most manufacturers, add at least .010″ of skirt clearance, sometimes twice that. The purpose of this is to build in an extra measurement of clearance in case of manufacturing “ganging of tolerances”. That’s where the machine boring the barrels might have a range of .727″ to .732″ and the machine cutting the skirts on the chokes might range from . 737″ down to .731″. If you get a “loose” barrel and a “tight” choke, the skirt will protrude into the barrel and you have trouble. So you design in a large drop for the choke skirt so that no matter which process, or both, is off, you can never get a ganging of tolerances sufficient to cause a problem.
Does a big jump from bore down to the skirt recess and back up to the choke constriction affect the choke? I don’t really think so. The big reliefs at skirt that I have measured taper back up to bore diameter in a very short distance. Then they taper into the choke more gradually and finally usually into a parallel. The jump from bore over the skirt can be pretty deep, but it happens so quickly that the shot column doesn’t have time to expand into the gap and thus it has no real effect on choke constriction. If the skirt jump did slowly taper up to the choke, then it might be a wide enough expanse to cause the shot column to expand a bit like a jug choke. But I don’t think that happens to any meaningful extent.
Bottom line: Don’t worry about it. Your choke should be measured from the diameter of the bore to the maximum constriction of the choke. That said, custom installed chokes minimize the drop at skirt and come closest to matching the bore. Over the counter or factory chokes have the largest drop at the skirt. Less is better as the shot gets a smoother ride.
The Technoid writing for Shotgun Report, LLC
(Often in error. Never in doubt.)