The Real Answer On Choke Constrictions

Dear Technoid:

Are choke names and their actual dimensions standardized across all manufacturers? I’m trying to determine which Browning Invector Plus choke is closest to the .015″ Light Modified you’ve been recommending as a good all around choice for sporting clays. The tech people at Browning have been very responsive but were not able to provide me with the actual measurements.



Dear Michael:

Good question. The answer is “Choke names and dimensions are not really standardized.” Many shooters tend to think of shotguns in rifle terms- exact measurements of performance and the like. ‘Taint so. A shotgun is an approximate type of thing.

Accepted choke standards in America are governed by SAAMI- an acronym for our private firearms standardization board. Other countries tend to use national proof houses to set standards, but the idea is the same. Someone has to set the accepted measurements on a 30-06 cartridge so that every manufacturer will not make a different sized chamber. It is the same in shotguns.

Here is the key: In the US, choke is based on performance, not on any kind of barrel measurement. Example: Full choke in the US means that whatever choke dimension and shell combination you are using will shoot 70% of its pattern into a 30″ circle at 40 yards. That is it. There is no mention of the amount of constriction or type of shell it takes to do that. The result is called full choke, not the mechanics required to produce it. You do not HAVE a full choke, you PRODUCE a full choke, if you get the point.

As you know, some shells with high quality shot, low velocity and well designed wad, can produce a much tighter pattern than the promotional dove and quail loads sold at K-mart. Shot through a barrel marked “Full” choke, one may meet that standard, one may not. The choke remains the same, but the shell has changed the result. It is always the choke/shell combination that produces the result. The type of shell is every bit as important as the amount of constriction in the choke.

This is not really a very convenient way of measuring chokes. The average shotgunner wants to know what chokes he will get before he buys the gun. To do this you have to market chokes with markings on them. Today, when a manufacturer stamps “full” on a choke, it means that he hopes that it will produce a 70% pattern in a 30″ circle at 40 yards with some sort of shell. When you buy that gun, you are taking his word for it.

Generally, but by no means always, the market has come to assume that certain constrictions will produce certain results with the “average” (what ever that is) shell. The general rule of thumb in America for 12 gauge chokes is that .000″ constriction is Cylinder Bore and will produce a 40% pattern. .005″ is Skeet (or Skeet 1) and will produce a 45% pattern. .010″ constriction is Improved Cylinder and will produce a 50% pattern. .015″ is Light Modified (also called Skeet 2 in earlier Winchesters) at a 55% pattern. .020 is Modified at 60%. .030 is Improved Modified at 65% and .035″- .040″ is full producing a 70% pattern. Having said all this, remember that just by changing shells you can easily go up or down one full choke designation.

Some makers have further muddied the water by coming up with proprietary choke names, such as SC1, SC2, SC3 or U1, U2, U3, U3 or a bunch of stars and notches. No one has the vaguest idea of what this stuff means in numerical constrictions. All that the average guy knows is that the numbers mean “more” or “less” pattern constriction.

Steel shot makes the selection even more confusing. Generally (but not always) steel shells pattern tighter than shells using lead pellets. So some chokes are marked for both, such as “IC/Lead, Mod/Steel”.

Screw chokes also produce another problem: All manufacturers want to leave a slight relief at the rear of the choke to make sure that the thin rear skirt of the choke never sticks up into the barrel bore. Unfortunately, with one Italian maker’s early efforts the choke skirts could move into the barrel bore if the chokes weren’t kept clean and build-up was permitted to accumulate around the choke tube. The result was that your 1 1/8 oz of hard #8s was often joined by 1 1/2 oz of hard steel choke. If things were kept clean, there was no problem. Sloth hath its downside.

At any rate, this relief at the back of the choke varies in tolerance from not too much to quite a bit. The high end hand-fit Teagues are often perfectly fit and show no drop-off. Many commercial chokes show a bunch. When the relief is a lot you are certainly safe, but you are in effect adding a jug choke to the gun. This means that you are effectively increasing your choke by allowing the shot to expand into the relief before going into the choke. You can easily turn a .005″ skeet choke performance into a .010″ IC performance if there is enough relief. Of course, if the relief is short, or not very deep, it will have less effect, but that requires better machining and that costs more.

As you have gathered by now, chokes markings are a very, very approximate way of predicting performance.

Now, finally to your question. Why can’t a company tell you what one of their particular chokes measures? It’s because of “ganging of tolerances”. Let us say that a typical “backbored” target barrel of today has a nominal measurement of .740″. A .015″ choke would probably give somewhere in the area of Light Modified performance of 55%. That is the theory.

Now add in production variances. Even with modern machinery, that nominal .740″ bore could just as easily have come out .738″ or .742″. I doubt if they vary .005″ each way, but they certainly can vary a couple of thou. The chokes are cut on thread machines and some can easily vary by the same amount in either direction, especially when they are mass produced. Ganging tolerances means that what you had hoped to be .015″ Light Mod constriction could end up being .010″ or .020″- from IC to Modified. No wonder exact numbers are hard to come by. Today’s computer driven machinery has certainly improved things, but variances are still there as you will certainly find out after spending some time with a bore mike.

So what do you do? You can take your gun maker at their word and not ask too many questions. Ignorance may indeed by bliss. A few thou certainly won’t matter much on the clay target. You can measure your bore and keep getting new chokes until one measures correctly. Briley or Trulock will custom make chokes cut to exact dimensions if you ask them. You will still be assuming that the correct measurements will produce the pattern you want- a real leap of faith.

Finally, you can do it the right way and actually do the patterning work. Do at least five patterns with each shell and choke combination to get a reliable number. It is a pain in the neck, but at least you will know what you have. There is no easy way if you want to do it right. The additional advantage to patterning is that you will discover a great deal about the shells you are using.

I am sorry that I had to give such a long answer to such a short and sensible question, but it does take a little explaining.

Best regards,
Bruce Buck
Shotgun Report’s Technoid


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