Actual choke as compared to tapered brass gauge reading for some 12 gauges

[Londonshooter]

Gun *Choke (Gauge Reading) i.e. choke as measured by muzzle constriction vs. choke read off gauge

Thomas Bland L ½ (¾)
R Cyl (¼)

WW Greener L ¼ (¼)
R cyl (I.C)

MacNaughton L cyl (C)
R cyl (C)

Masu L (Jug) IC (I.C.)
R (Jug) IC (I.C.)

Powell L Cyl (I.C.)
R IC (I.C.)

Thullier L ¼ (½)
R IC (¼)

Roberts L ½ (½)
R cyl (½)

Daw L Cyl (Cyl)
R Cyl (Cyl)

Smythe L ½ (full)
R IC (½)

Merkel L F (>F)
R F (>F)

Woodward L (12/1) ¼ (½)
R (13/1) cyl (¾)

BSS L ½ (¾)
R IC (½)

 

[Londonshooter]

Sorry, the formatting did not carry through properly. Today I went through most of my 12 gauge guns to document bore diameters and chokes. Thought while I was at it that I would measure chokes with a tapered gauge just to see how frequently the methods differed (and why). In the other thread the consensus seemed to be that the gauges are not worth the bother and I'm not trying to make a point one way or the other - just looking to see how they compare in this small sample. If the gauge reads IC or full, then there is a good chance it is right. Checking the proof marks gives further clues and exceptions to what you might assume (for instance if one barrel is proved 13/1 and the other 12/1)

 

[Longwalker]

Of course there is great variability in bore diameter between makes and models and also whether the bore is a modern " back bored" oversize type or not. Measuring the muzzle is not very accurate other than as an approximation. I use a Skeet's bore gauge and measure the difference between bore and the tightest part of the choke. That yields some useful numbers that can be a reasonably accurate prediction of pattern performance.

 

[Ashcroft]

As previously discussed, the tapered brass “gauge” is reasonably accurate if your bore is standard size ( or very close) for the gauge - eg, standard bore dimension for 12 gauge is .729”. This gauge is accurate enough on unaltered mass produced barrels as found on most American guns and some European guns but as mentioned, there are exceptions like undersized Beretta barrels prior to about 2010 and oversized barrels on Browning Invectors and the even larger Invector Plus. British guns traditionally danced to their own tune and the farther back in time you go the more they varied but if you can read the coded proof marks you will know what the bore size was originally but over the last 75, 100, 125 years many of them have been altered. Amount of measured choke is the result of subtracting the measured choke diameter from the measured bore diameter and this decimal difference is given a name through use of a choke table. Anything else is guessing but really, the only truly useful measure is done at the pattern board.