Muzzle Diameter Question - GH vs Repro
 

I have a Parker GH 12 ga that I acquired this year as well as a 12 ga Trojan and a Repro SS 12 ga. I was measuring the muzzle diameters of the them tonight and noticed that the Repro measures .723 and .726 and is marked MOD and IMP. When I measure the others they are .698 / .703 and .690/.706. The Trojan, according to the letter is Full/Mod but I am not sure about the GH. The actual measurements are not my concern necessarily but the difference between the older guns and the Repro is significant. Why would there be this much difference (i.e. Trojan MOD=.706 and Repro MOD=.723)?

I measured using an internal telescoping gauge and measured with micrometer so there could be some error in the actual numbers.

By comparison, I have a 20ga Repro and an Ithaca NID 20ga that have muzzle diameters that are almost identical.

There are a lot of variables that could come into play here, but most importantly the choke is not determined by the inside diameter at the muzzle, but rather by the amount of constriction from the bore diameter to the tightest part of the choke. Your measurements seem odd on the reproduction for the chokes listed. On the GH and Trojan, the actual bore diameters could be quite different between the two guns depending on the time period they were made. And of course chokes on older guns quite often have been opened up.

Further, the way a modern shotgun (Repro) patterns and tends to shoot tighter patterns with the modern loads with a plastic shot cup is vastly different from how early shotguns patterned with nothing but lead pellets travelling down the barrel with no protection and nothing to maintain the integrity of the charge. Shotgun chokes are designed for the shells/loads they are most likely to use.





.

Thank you, gentlemen. I understand about the choke being the amount of constriction from bore to minimum choke diameter and the GH and Trojan are close enough to account for my inaccuracy or a variation of the bore/constriction formula. I don't have a way to measure the actual bore diameter. I suppose I was just surprised that the Repro was so different. Dean, I hadn't considered the modern vs early differences in the way shells were constructed. Further, since this is a Steel Shot version it could be even further skewed in that regard.

Thank you for the responses. I am just getting my head wrapped around all of the intricacies of these guns since I acquired them earlier in 2020 along with an LC Smith and two Ithaca NIDs. Previously, I was only familiar with my O/U with screw in chokes.

A Skeets type bore micrometer would be a good investment for the curious mind at about $140.00. Search on shotgunworld.com forums for a source.

So would a couple of hours at a decent patterning board at measured yardages. I have gotten back into it lately after acquiring more guns than I owned at the time I was really spending a lot of time doing it (c. 1970s-80s). I can tell you, the results are really eye-opening and there are some real surprises. For instance, as Dean notes, plastic shells with poly shot cups tend to show tighter patterns, but the modern soft-wadded loads (felt-wad RST, Eley, etc.) consistently show better patterns with more even distribution at given ranges.