doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com ? Bore Dia. and choke constriction 12ga?

Gentlemen, I have been reading about bores and chokes and have measured one of my doubles, an Ithaca NID and found the bores are around .740 and the chokes .20 and .30 (as I measure them). Having always heard 12ga bores were closer to .725-.729 it got me doing research. I discovered Kolar guns are bored .750 and their choke restrictions are standard. For example Kolar's .750 is reduced .35 for full choke. They say the patterns are better. I am thinking that their thinking is that less force is exerted on the shot as it passes the choke? The pattern result is the same- I guess? My guns restriction is .20 and .30 so I'm thinking improved modified and modified, but it is marked 2 and 4 which I thought was modified and full. I know the pattern board is the real answer to chokes, but I wondering about the Kolar theory? I was going to have the chokes opened on this gun, but now better get the paper out. What are your thoughts? Thank-you, Kurt

Hi Kurt,
12 ga., going strictly BY the gauge system, IS .729, for a fact. However, gunmakers don't have to follow the rules. Browning have made their older guns .724-725, Beretta has made their "12 gauges" .721" and the newer magic recoil reducing, pattern improving, physics-defying miracles have many makers using bores of .733-750". Whether or not you believe this crap(my word) is up to you. Constrictions required to produce a given choke effect is generally more for larger gauges and less for smaller gauges. I would suggest your measurements of .020" and .030"(not .20 and .30), are about modified and improved modified, respectively. I don't think the amount of constriction will vary much within an alleged gauge designation. Meaning that .035" or thereabouts will give you a full choke in 12 gauge, regardless of whether the actual bore ID is .721" or .750"
I would not worry about it. Trust the manufacturer to know how to make chokes. If you WANT more open chokes, have them opened a bit but don't go nuts worrying about a couple thousandths difference between one brand and another.

Stan Baker overbored more than .750". The Barnes "Moneymaker" aftermarket on my m12 measures .779". Obviously a nominal 10 ga. bore with 12 ga chamber and there is very little cone between chamber and bore. Relative constriction of the Colonial Arms choke tubes is pretty close to normal 12 ga. numbers.

jack

Happy birthday Jim!!

What is the serial number range of your NID. My earlier NIDs tend to have larger bore diameters and those after 1935 seem to be pretty close to standard. I had a Grade 1E 12-gauge in the 441xxx range and its bore diameter was .744". Maybe these early overbored guns were that magical boring Ithaca advertised by Uncle Bob Edwards?!? The 12-gauge NID I have now is in the 461xxx range, shipped in 1936, and is .730" left and .729" right. My 16-gauge NID is a 1941 gun and is right on the money at .662" bore diameters. My 20-gauge was shipped in 1933 and has a bore diameters of .623" right and .629" left. My .410-bore was shipped in 1934 and has bore diameters of .415". There ia a bore chamber and choke diameters chart produced by Ithaca with dates from 1939 and 1945 reproduced in "Shotguns by Keith" on page 113, and it gives the bore diameters of .775" .729" .662" .615" .550" and .410".

I've measured a couple Parkers that ran about .750. And there was still PLENTY of choke left in those guns, so I don't think they were reamed post-production or anything like that. Just measured a Parker Reproduction today: .735. Seem to recall a couple other 12ga Repros I've measured are about the same.

Thanks for the replies and the zeros Jim, Happy Birthday too! As said I believe the thinking Kolar has is that lead shot is less likely to be deformed by their combination thus yielding better patterns. My NID is a 1926 gun, well taken care of and I'm certain it was not tampered with as I bought it from the 85 year old son of the original owner. Researcher my guns bores would be further evidence as to what you have found in earlier NIDs. L.B. I consider Parkers good company to be with in the overbore group and Jack- your MoneyMaker must be working with the same idea as Kolar. Jim's assessment of Improved modified and modified sounds good in that order. Thanks again for your ideas! Kurt

Thank you for the birthday thoughts! I had a nice old Parker that had .750" bores and plenty of choke, too. Oscar told me that some were made that way to work well with brass shells and the 11 ga. wads that were used in them. The gun shot fine with modern plastic wads and smokeless low pressure loads but I hated the brass shells and got rid of the gun and the brass shells. Too much work for nothing.

Jim in old guns that were over bored to shoot brass shells with over size wads do you think plastic wads designed for a smaller bore diameter can cause pressure problems and or velocity loss.

ben-t, shot does not get deformed in the choke constriction. It flows through the constriction and actually is subjected to a loss of internal pressure. This is what reduces the spring-apart as it exits the muzzle. It also gains a little velocity as it passes through the choke. But, it does not get deformed. Shot deformation is due to acceleration setback at primer ignition to peak pressure.

Kolar has done some excellent work on thier barrels and chokes. With the over sized bores, virtually no forcing cone and porting, the overall recoil is about the same in my perception but the gun jumps less and the patterns are the best I have ever seen. I patterned my Kolar in Kolar's Proof Room while testing for Point of Impact and the resulting pattern's even distribution were impressive.
I don't think that the bore of your Ithaca will really do anything to affect recoil and as you say the pattern board is where you will see the results.-Dick

I traded for the nice old Parker, at the Beinfeld show, thinking it would be good for BP skeet matches. I was told it was IC and IC, nothing about an odd bore size. When I got it home and measured it, I found this overbore size and Full/Full chokes. I bought a case of brass shells and got all rigged up to load them. The smokeless. plastic wad loads I used shot fine but I never chronographed them. To me, it seems likely the pressure and velocity would be somewhat less but that's just a speculation on my part. Anyway, I disliked the ordeal of dealing with the brass shells and sold gun, shells and all the reloading stuff for them.