[Fish Springs]

Measuring a few Remchoke 20 Ga tubes and found .615 is Cyl from on vendor, I/C from another and Skeet from a third.

What should one consider 20 ga bore diameter and how does one get: Skeet, IC and LM choke tubes and what would the diameter be for each. The gun(s) are Remington 1100.

 

[TexasTon]

.615-.617 is pretty much considered bore diameter for a 20, IIRC, just from previous experience you need to use slightly looser calculations to achieve the same pattern as you do with a 12. What I mean by that, is a .015" choke that would be considered a mod, will shoot tighter at that constriction in a 20 than a 12.

Disclaimer: I only own one 20 and it is a dandy Grade Classics Doubles 30", a .025 choke is one tight shooting sucker in that gun, skeet is a .003" and shoots at least as tight as an IC in a 12.

 

[MrBen]

The SAAMI spec on 20 ga bore diameter is .615-.635.

Ralph Walker listed the following constrictions in The Gun Digest Book of Shotgun Gunsmithing, 1983:
20 GAUGE Sk .004, IC .007, Sk2 .009, Mod .014, IMod .019, F .025

My Beretta 686 measures .625 bottom barrel, .626 top barrel. Briley appears to make their choke tubes on the assumption that Beretta bores are about .625, because their Skeet is .620 which would be .005 constriction, and their Mod is .610, which would be .015 constriction.

Other gun brands will vary in bore diameter. Some might even be outside the SAAMI specs, as that is only a recommendation and only applies in the USA.

There is a lot of variation in bore diameters even among guns of the same make and model. I don't know how true it is, but I have read that manufacturers use a barrel reamer made to a specific diameter, and when it gets dull it is resharpened, which reduces its diameter. When the reamer gets below a certain minimum diameter, it is replaced.

A choke tube manufacturer has to guess what the bore diameter will be on guns in which his tubes will be used. That is especially difficult when several different manufacturers use the same choke tube barrel thread design. For example, Winchester, Browning, Mossberg, Weatherby, Thompson Center, Stoeger, Smith & Wesson, SKB, Ruger, NEF, Miroku, Ithaca, and Charles Daly use the same chokes on at least some of their models, but their bore diameters may vary a lot.

Trulock told me they assume a bore diamter of .725 for 12 ga Beretta MobilChoke tubes, based on actual measurement. Briley said they use .723 - that is what Beretta claims it is, but I don't know whether Briley took Beretta's word for it or measured some guns.

The bottom line is, you can't assume any bore diameter for a specific gun - it has to be measured. Once you have that, you can ask the choke tube manufacturers what average bore diameter they assume for their tubes, or what the actual diameter of their tubes is.

 

[Fish Springs]

Thank you for the information--off to the patterning board.

Briley offered .617-618

Remington's measurement was .617 for current production barrels.

 

[Bill M.]

Mr Ben of course has a lot of good information. My experience has been that 20 gauge is very sensitive to small variations in bore and choke diameter if you are trying to get big, open patterns for skeet shooting.
I have an invector choked Citori. The generic skeet chokes shot a much tighter pattern than the skeet chokes I bought from Browning that say they are only to be used in Browning guns. I think Browning knows what diameter they make their barrels, while the aftermarket guys have to guess what gun they will be used in and thus error on the large side.

I sent an Italian made Charles Daly in and had it fitted with screw in chokes. I never got a pattern any more open than modified. (one barrel was .620, the other .615). The choke company even made me one that was more open than cylinder but it still shot modified. Sold it to a rabbit hunter that wanted a light, tightly choked gun. He got what he was looking for.