doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Choke Constriction Charts - Comparing

Like others, I think in thousandths. But that often elicits the deer-in-the-headlights look from people less knowledgeable than those who post here. So I often find myself using the "nominal" terms of SK, IC, etc. and explaining how to equate the two conventions....which is what these cahrts attempt to do.

AND, in that regard, I don't like ANY of the posted charts.
I find it much easier to simply think in terms of 5 thou increments for a 12ga, which covers 90% of the choke conversations I have.

.005 = SK
.010 = IC
.015 = LM
.020 = MOD
etc.

I can recite those in my sleep.

Likewise, I like a nice round .025 for full in the 16/20ga, .020 for IMOD, .015 for MOD. I mean, c'mon, who thinks the 16ga and 20ga should be .001" apart? and that .019 and .014 are better estimates than .020 and .015?

There is an interesting thing about those charts, tho'. If we accept that, let's say full, delivers a certain percentage of its' load inside a 30" circle at 40 yards, then look at what constriction is required to make that happen in each gauge.

As the bore size decreases so does the amount of constriction necessary to make that gauge deliver the desired choke pattern. For example, look back at the chart posted above by JayCee and you will see that for full choke patterns .035" constriction is required in a 12 gauge, but for a 20 gauge only .025" is needed. Thus, the old axiom that, as a general rule, 20s shoot tighter than 12s.

As we know, each gun is it's own rule, but the general consensus is that the smaller the bore the tighter it shoots.

SRH

If you look at the full choke constriction as a percentage of the cross-sectional area of the unconstricted bore, you find that the "amount of constriction necessary to make that gauge deliver the desired choke pattern" is the same for the 12 and 20ga.

In other words, the area of a .695" muzzle is 92% of the area of the .729" cylinder bore. The area of the .592" 20ga full choke muzzle is 92% of the area of the .617" cylinder bore. You could think of full choke as an 8% reduction in bore area. From that perspective, the 12 and 20ga constrictions are actually identical....is that coincidence?

Maybe, but it's true for the 28ga as well. Using .550 for the bore and 22 points for a full choke gives 93%. Use 23 points and it comes out to 92%.

Found this 1949 Winchester 21 Choke Sheet. Appears to be copyrighted at http://bluepike.tripod.com/Model21/Model21Tables.html - maybe the web site contents are copyrighted.

Some of their values show less constriction for the choke than I am used to seeing.

WINCHESTER Model 21
Bore and Choke Specifications

GAUGE
BORE DIAMETER CHOKE % in 30" Circle @ 40 yards*

..........CHOKE DIAMETER CONSTRICTION

12 .730" + .005"
Full
65% - 75% .694" - .699" .031" - .036"
Imp. Mod.
55% - 65% .712" - .715" .015" - .018"
Mod.
45% - 55% .715" - .720" .010" - .015"
Imp. Cyl.
35% - 45% .724" - .726" .004" - .006"

16 .670" + .005"
Full
65% - 75% .640" - .645" .025" - .035"
Imp. Mod.
55% - 65% .652" - .655" .015" - .018"
Mod.
45% - 55% .655" - .660" .010" - .015"
Imp. Cyl.
35% - 45% .664" - .666" .004" - .006"

20 .615" + .005"
Full
65% - 75% .590" - .595" .020" - .030"
Imp. Mod.
55% - 65% .600" - .603" .012" - .015"
Mod.
45% - 55% .603" - .608" .007" - .012"
Imp. Cyl.
35% - 45% .609" - .611" .004" - .006"


* Figures from "Winchester Gun Salesman's Hand Book" �1949

�2010, M.C. Manges

For whatever reason, with the smaller bores, you'll sometimes find makers who think they're boring 12ga guns. My all-time favorite, consistently, has to be the Japanese back when they were making the 28ga Parker Reproductions. Not at all unusual to find full choke on those guns well in excess of .030. I have a 12ga Repro with one barrel choked full, and it only measures .034.

Mike - very interesting - thanks - Paul

What are the patterning differences between the old, longer chokes and the newer short compression measurement chokes? Wouldn't the longer chokes be commensurate with the longer forcing cones out of the chamber in gently transitioning the shot column? Steve

http://luckyweasel.com/

As a side note the all the discussion about chokes,
A friend at my club developed this computer aided pattern evaluation device. He's actually sold a couple to shotgun tube tube companies. As for me, I tend to lean toward Wonko's comment the other day..."It's a shotgun...get over it."

Your mileage may vary....