I am looking at an Italian hammer gun, to maybe purchase. The gun is in almost new condition, a 2001 gun. The shop gunsmith emailed that each barrel measured .631. The right barrel has .010 constriction,the left barrel .020. ( Imp/Mod)
Isn't the bore diameter of a 20g .615 , or is this within acceptable parameters ?
Thanks, terc

Terc,

The bbls should be marked what their original nominal bore dia. was at time of proof in mm. Trust that to be a good number. Do the conversion, is that .621? If so, then the bbls are unmodified, in that regard. If not, I have to ask myself how likely it is that some one would have back bored them. Choke markings had gone to the asterisk(s) by 2001, but they too should be on each bbl. If it is a 3" chambered gun, it will have the higher definative proof marking of two stars inside a circle over 'PFS' & if a 2 & 3/4" chamber may have only one star inside a circle over 'PSF', which would be the normal final proof for 2 & 3/4" .

I'm going to assume that you meant to type that the right bbl. has .010 & the left .020 constriction, which would make it IC & Mod., though it may pattern tighter than that w/today's ammunition.

Easiest way to tell if the bbls are modified is to compare their present weight to that stamped on them. It will be in KG and read something like 1.XX Kg. You can convert it to pounds.If they have been back bored it will be easy enough to tell from weighing them and making a comparison to what is stamped on them.

All the above says that you likely do not have an issue aside from double checking that someone has not indescriminately lengthened the chamber or forcing cones from 2 & 3/4" to 3" or back bored them.

I'd guess that the reason it remains in almost new condition is that it was found to have a rather abrupt 'thump' by its original owner using std target loads. If they shot some 1oz. field loads in it if it is a 2 & 3/4" gun or God forbid, some 1 & 1/4oz. 3" rounds(!)there is little doubt where the incentive may have come from to let somone else enjoy it.

Also, you sometimes have to take other people's measurments w/a grain of salt. They do not all know how to read nor use measuring instruments w/good accuracy. No offense intended toward anyone, just saying.

To add just a bit to TW's excellent post. Yes .615" is nominal 20 gauge bore diameter. Being Italian the bore diameter at proof would me marked in metric. .615" converts to 15.62 mm but I don't recall offhand if the mark to one or two decimal places. .631" = 16.03 mm.

I believe all the countries that mark bore diameter in mm only go to one decimal place. Just checked a Belgian 20 that's marked 15.9. That one is also overbored from the .615 standard by about .010, but that's how it came from the maker.

Thanks all, TW,I corrected my post ,it is Imp/Mod. I'll have the seller give me all of barrel flat markings today. Hopefully the the barrels are unaltered.
terc

I thought .020 constriction in a 20 ga. was tighter than Mod. Briely's lists it as Lt. Full.
Choke Chart

A 20 ga with .006 constriction is improved cyl.and 20 ga. mod is .014.

The constriction of .010 and .020 is for 12 ga ic/mod.

A lot of makers today have 625 bores for 20 ga.

The two charts I have show mod in the 20ga as .013-.014 constriction. Jack O'Connor's "Shotgun Book" shows the constrictions used by various gunmakers. Browning: .018 for 20ga mod. But owners of older Belgian Brownings probably already know that those guns tended to be choked on the tight side.

Thanks again, I talked to the gunsmith this afternoon. The bore diameter stamp is 15.9. That converts to .626 inches, a little closer. He is going to calibrate his gauge and read it agian.
Other stamps are CAM .70 and HE 1.200 . I was on a cell phone in a poor service area, I'll try to get more info later.

I usually use the choke chart on Hallowell's site. It shows 20g, .005-.011 IMP and .012-.019 MOD
terc

Continental 20 bores are usualy.626"rather than the .615 that the British have used as standard . Point is what dose it matter as long as the gun measures at the diameter it is proofed at . In millimetres the increments in proof size are .01 mm which equates to .004" ,with .02mm or .008" enlargement before the gun is out of proof.

Gunman;
I believe you have your decimal point one place off. 1 mm = .03937" (1/25.4) or approximately .040". Thus .1 mm = .004" while .01 mm = .0004" or a bit less than ½ thousandths.
I was thinking the sizes were only stamped to one decimal place but wasn't absolutely certain so didn't say it that way.
In machine shop lingo, although not perhaps technically correct .1 was called 100 thousandths, .01 10 thousandths, .001 one thousandth &.0001 one tenth, which meant it was a tenth of a thousandth. This was universal among all machinists I ever knew & many of them came from widely different locations.

Hallowell is the only one that uses a spread like .005 to .011???? The only thing I can figure is they are talking about an IC choke giving an IC size pattern at a given yardage. I know I have been surprised when patterning guns giving much smaller or larger patterns than they should have, given the restriction. I am convinced how the choke is configured has a lot to do with the size pattern it may give.

The Cam 70 means it has a 70 mm (2 34") chamber, don't recall the HE offhand.

Piper you are correct and sorry for the confusion . This modern metric is not my favourite. It was always claimed the British gun trade worked in "attles" to wit attle do and attle be near enough .

Not to split hairs but, one (1) mm=.3934 inches, not .3937 inches. Obviously I AM splitting hairs!! Sorry.

So now a mm is well over a quarter of an inch long? And where did you get the "hair" to split?

Well not to split hairs "BUT" 1 cm =.3937". 1 mm =.03937". Rounding mm's to four decimal places will give .0394". My convert program shows 1 mm carried to 8 decimal places as .03936996. Rounding to 5 places gives us .03937, not .03934.

Here is my original statement on this, as you can see I did use mm's not cm's which was actually the error I was correcting & to which you have followed suit;

Those guys who talked about the number of angels that could dance on the head of a pin have nothing to us with our hair-splitting!

Not positive, but I believe the HE 1.2 might be the barrel weight in kg.

1.2 kg would convert to around 2 lb 10 oz. This would certainly be a feasible weight for a light set of barrels. I was just not sure what the HE stood for so was hesitant to state this was bbl weight, but you're probably right Larry.