doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Choke in 20 ga. and small(er) bores

Question: Do the smaller gauge guns - 16, 20, 28 - require less or more choke to achieve a given pattern percentage compared to a 12 ga? I have always been told that in the case of a 20, to achieve IC pattern percentages I would need less than the .010 normally prescribed for a 12 ga., and similarly for other chokings such as MOD, FULL. One internet chart shows IC pattern percentages in the 20 can be achieved with .005-.007 constriction, yet recently a British shooting instructor/game shot well credentialed tells me exactly the opposite - that the smaller the bore diameter MORE choke is needed.

Guidance please.

In my opinion, books give you an idea what the chokes should be. In the case of L.C. Smiths you ordered the gun with chokes you wanted. You told them I am going to be shooting said amount of drams, 7/8 oz shot, with a certain wad, etc. and they would make the chokes to your specifications. This was confirmed on the hang tag.
With today's store bought guns they mark mod/full, etc. Every different shell even with the same oz. shot will give you different patterns.
The only real way to tell is pattern your gun with the type of shell you will be using. Some books specify shooting 10 rounds of each type shell at the range you want and see what patterns the best. Also tells you to bench rest it for consistency.

Nice chart http://www.glencovesportsshop.com/choke.htm

Choke is expressed as the difference between the bore and the constriction at the muzzle, either in thousandths or as a % (12g full choke .040, modified = .020 = half choke, IC = .010 = quarter choke)
The effect of that constriction can be measured as the percentage of pellets in a circle, usually at 40 yards but patterning at the distances one expects to shoot birds is important.
Because small bore guns (usually) have fewer pellets, the percentage will be the same for a given choke but the number of pellets on target/game will be less.
1 1/8 oz 8s = 461 70%= full= 323 pellets
3/4 oz 8s = 308 70%= 216
Therefore, for a clean kill/likely broken target the choke constriction may need to be greater for smallbores.

Of course the only way to determine what any barrel patterns with any given load is to "Pattern" it. A gun throwing a given pattern percentage with todays shells will normally have less constriction than it used to.

All that being said to answer what I believe is the intent of the question choke is generally proportionate to the reduction of the "Area" of the bore.

Thusly a 12ga bore of .729" with a 10% reduction of area would have a choke dia of .6915" or.0375" constriction.

Likewise a 20ga of .615" bore with a 10% reduction of area would have a choke dia of .5835" or .0315" Constriction.

These Should be expected to have a similar spread, ie % in the 30" circle. The equivelent reduction of .010" in 12ga would be acheived with about .0085" in a 20ga. In this line note that a reduction of .010" in the 12ga was truly a 1/4 choke not IC though often so marked. A true IC in 12ga was generally only around .006" constriction & even less in the 20.

I can address the issue of 20 ga. guns, but will have to leave the 28s and .410s to someone else.

As to the twenties, it's kind of a moving target. In broad-brush terms they don't require as much constriction. A 12 ga. gun will have 30-40 mils of constriction for full choke, but I've rarely seen more than 25 mils on a full choke 20 ga. Then there are the exceptions. I have a few 20 ga. barrels in the 15-20 point range that throw legitimate improved cylinder patterns. And is it my imagination of did Winchester seem to get away with less constriction than other manufacturers?

Here is the answer from Ithaca Gun Co. back in the day --



That said, I have a 1933-vintage NID 20-gauge with 30-inch barrels with no choke markings, but it sports .038" choke right and .042" choke left.

They require less constriction.

Choke is a performance - this is definitional. For most choke designations performance is expressed as a percentage of pellets in the 30" circle at 40 yds. Some exceptions: .410s have traditionally been patterned at 25 yards. Modern shotcup loads perform well enough to warrant revisiting this figure. Skeet patterns are usually optimized at 21 yards (skeet out/WS2/SK2 may be optimized another 10 yds out), but in recent years 45%, or so, at 40 yards has been considered "skeet".

Constriction is not choke, though many confuse the terms. Constriction required to deliver a certain pattern percentage varies directly with bore diameter, as you can see from various charts published by Briley, etc. There are other variables: required constriction for a given percentage will vary with payload - as the Purdey website illustrates.

Sam

All my Win Mod 12's seem to have less measured constriction than the barrel stamping would lead one to believe. They work, though.

Here is some data I saved from a WW chart

12 .035 .019 .009
16 .028 .015 .007
20 .025 .014 .006
28 .022 .012 .005

I assume that you know the full from the IC. Hope this adds some light, don't talk to a trap shooter about this data, they live in their own world.

bill

bill

Well put, DH. I, too, saw two questions. You answered both very nicely.