In a 20ga, what would you call a gun with 0.007 and 0.013 constriction? Several charts I’ve looked up give several different answers. Best I can tell, it is between Sk/IC and IC/Mod.

I'd call those constrictions IC and LM, but they may well shoot tighter than that with certain loads. What they really are can only be determined by the patterns from them.

SRH

A chart published by Brownells in 2001 gives constrictions for each gauge. They vary as smaller bores take less constriction to make choke.

The 20 Gauge is shown as:

Skeet .004
IC .006
Skeet 2 .009
Modified .014
Imp Mod .019
Full .025

Other charts seem to use the twelve gauge constrictions for every gauge, which is misleading.

My experience with several 20 bores supports the Brownell's chart.

As Stan stated the true choke is determined by flattering. As for bore constrictions, this should relate to a percentage of restriction, rather than a certain number of thousandths of restriction. Thus under old standards for card & filler wads, a full choke had about a 10% reduction. This amounted to around .040" for a full choke 12, but only around 032" for a full choke 20. Note these percentages have t be worked using the Square of the bore diameter, not the actual size.

I have two charts (one from Mike Orlen and another from the late barrel guru Stan Baker) that match quite closely with Little Creek's numbers for the 20ga.

However, there was significant variation from one maker to another. Belgian Brownings, for example, tended to be choked tighter than guns from other makers. In his "Shotgun Book", Jack O'Connor listed choke constrictions used by Browning, Winchester, and Remington. Browning's 20ga full choke constriction was .033, while Winchester's was .027. Browning's 20ga full choke actually had more choke than Winchester's 12ga full (.031). And anyone who has measured constrictions on a Parker Reproduction 28ga will find that the Japanese must have been using full choke numbers on those guns. WAY tighter than standard.

and the french really believe in tight choking....typical field guns are full choke on right,.. and full and then some on the left....

And the early fixed choke Citori 12s were very tight. My first Citori was marked M/F and shot more like Xtra Full and Turkey Full.

Yet you'll also find French doubles--usually made with woodcock hunting in mind--that have one rifled barrel. The purpose of which isn't to shoot a slug, but rather to produce a pattern that's even more open than cylinder.

Thanks guys. Completely agree with patterning being the final determinant. Looking at acquiring a gun for hunting out west and chokes less than IC/Mod start to lose utility pretty quick. I suppose I have the choke tube option as a last resort.