20 ga. choke dimensions

338magnut

Can anyone tell me what the choke dimensions are for a 20 ga. full choke and a modified choke? Thanks in advance Terrill.

golferboy426

there ia no set answer. choke is always related to bore diameter. Choke is really the difference between the bore which is larger and the muzzle which is obviously smaller ie constricted. to get a choke you take the bore diamer minus the muzzle diameter in thousandths of an inch. That measurement will give you the actual choke

Bert H.

Cylinder bore = .617
Skeet I = .613
Improved Cyl. = .611
Skeet II = .608
Modified = .603
Improved Mod. = .598
Full = .592
Extra Full = .590

hedgehopper62

The bore dia on 20ga is .615 so full would be -.033 or.582 mod.-.019 is .596 chart lists -.006 and be in tolerance.warning your milage may vary with different mfg.tks hedge[:D]

XXCross

There is also a major difference in the amount of choke (restriction) between lead shot and steel shot. (choke for steel is about half that of lead)

only winchesters

Good morning 338magnut: Well on the surface that sounds like an easy question. But there are so many varibles, such as the vintage and make of the gun. Older fixed choked guns vs. new guns wiith choke tubes.

Yes it is true that "choke" is a relationship between the bore diameter and the choke diameter, in newer guns with choke tubes, the tubes are machined independantly of barrels to a specific diameter. In other words choke tubes are not "match" machined to a specific barrel's bore.

all machined parts have a tolerance period!! Today choke tubes are very easy to control the +/- .002. Older fixed chokes were often hand lapped, such as Winchester, Parker, LC Smith, etc. and had a wider tolerance. Same holds true on the bore diameter. Those were generally held to +/- .005 and today have a total range of .005.

Now from an early choke/bore requirement on the Winchester Md 21, 20ga. bore and choke requirements were:

Bore= .615 +/- .005

Full .590-.595
Imp/Mod .600-.503
Mod .603-.608
Imp/Cyl .609-.611

That does not hold true for all Winchesters, the Md 12 is:
Full .590-.598
Mod. .602-.610
Imp/Cyl .611-.615

The standard for "patterning" any shotgun is a 30in. circle at 40 yards regardless of gauge. Knowing a specicfic load such as 1 oz. of #6 lead is approximately 232 pieces of shot, you count the number of strikes inside the 30in circle. This gives you a percentage IE: 170 hits out of 232 would mean the gun pattern 73.27%. what does this equate too?

Well in the days of old paper hulls and fiber wads, this would be what you would expect from a "FULL" choke gun. Today with plastic cupped wads, or a plastic wrap around the shot colume, it could be Imp/Cyl or a "loose" Modified choke.

Today using good ammo, (cushioned shot cup, chilled hard lead shot) I would expect a "FULL" choke to pattern at 88% or better at 40 yards.
A Modified choke should pattern in the neighborhood about 78-86%.

So in reality it isn't so much what the "machined" demensions are for the bore and choke, it's the results you get. Since there are over laps created by the machine tolerances. Using the numbers from above. Bore of .615 +/- .005 and a Full choke of .590-.595 Mod. choke of .603-.608

You could have a total restriction of .015 on a Full choke that measured .595 with a bore that measured .610 or a Mod. that measured . .603 and a bore that measured .618 again you have a total of .015 restriction, will they both pattern the same HECK NO! the Full choke will give you a denser pattern period, even though the "restriction is the same!

Regards Dave

338magnut

Dave thanks for the info very informative. I should have mentioned I was refering to fixed choke Remington 870 20 ga. Thanks again Terrill.

338magnut

I apologize as I only thanked Dave for his reply but want to thank all who helped me with this question. Terrill.