Joined: 30 Nov 2009
I picked up a digital caliper in hopes of measuring bore diameter. Never used one before.
What's the trick? I get a chaotic range of values everywhere I measure around the circumference, and also depending how deep the jaws intrude. The first 2 decimal places hold still, but it'll go 0.6340 - 0.6355 -0.6370 - 0.6335 - 0.6380 - 0.6345
Even holding still and trying to re-measure the same spot, values will jump around. Appears to be a decent tool and it does hold zero, but I'm having a hell of a time figuring out how to get a solid reading from it.
Measuring outside diameter is easier, more stable. But inside diameter won't hold still.
Maybe a dial caliper wouldn't be so jumpy? Is there a technique to using these things?
The tool you bought isn't the best tool for the job.
What you need is a dial bore gauge. Brownells sells them as "Shotgun Bore Gages". They are self centering and read directly on a nice dial indicator. They have a long shaft that allows you to measure the entire bore and choke of the barrel. You need to know both the bore diameter and the choke diameter, to know the choke constriction.
Calipers are handy for a lot of measuring chores but are ill suited for choke and bore measurements.
A little tip. Don't get too wrapped up with "digital" measuring tools, they are nice but they aren't any more accurate, than the mechanical gages. For bore and chokes you are only looking for a comparative reading, not an absolute diameter reading.
The largest variation in your measurements is only 4/1000 of an inch. It could be dirt, powder residue or even a choke or barrel that is not "perfect" but will function fine anyway. You could try cleaning the area you're measuring thoroughly and "wiggling" the caliper inside the bore, but you still might not be satisfied with the readings. A bore gauge that is set inside the tube and read with the caliper tends to give more consistent readings, but is really only necessary when you want to extend the depth of your measurement (e.g. chamber, forcing cone or bore) or need greater accuracy or consistency.
The outside of the barrel is a finished surface and probably gives more consistent readings because it is physically easier to see and measure with a caliper. Looks to me like you're measuring a 16 gauge full/extra full choke and for that purpose I wouldn't worry about measurement variations as small as the ones you describe. Throw out the high and the low numbers and use the average of the remaining numbers unless you can see or feel something wrong in the tube,
Joined: 30 Nov 2009
Yes in this case it's a full choke. The greater frustration is trying to measure the "modified" choke underneath it. With all the digital wiggle I can't say how these over-under bores compare, except that they seem to be larger & smaller...
Wrong tool for the job. Not to mention these bores are 60 years old and not perfectly round anymore, if they ever were.
Time would be better spent at a pattern board, but where I live there's no good place to do that. Public range has one board set up right alongside the clay field, which is rarely vacant more than 10 minutes. On public hunting grounds we're not allowed to "target practice" as of this year. Grumble.
Joined: 28 Oct 2005
mcs, I have a variety of 'gadgets' to measure bore/choke diameters. Nice toys.
Your comment regarding the patterning board is spot on. Although gauges, particularly the internal diameter variety Mark mentioned, can give you some information, nothing beats an afternoon at the patterning board for discovering how your gun performs with a particular load.
If'n you're gonna' shoot (at)them snipes yew better find a place to do that.
Measuring ID is difficult with the blades of a caliper, you have to be vertical and square and use just the tips of the blades. The cheapest solution is to get a telescoping gauge, sets can be had for under $20. Or you can go for an inside micrometer like this one;
but depth is the issue with that. I got lucky with a custom Starret guage, which is more like the Brownell's type. Depends on how much you want to know, id at the muzzle doesn't tell you much IMO, as rerun points out, it's the relationship of bore to choke that matters. Iffen you're technically minded, and have a bunch of shotguns, a purpose built, and pricey, guage is worth it. Otherwise just find a place to pattern and keep practicing with the calipers.
Here's a good blurb on the whys, wherefores and uses of a good guage;
Joined: 31 Oct 2005
Measurement of choke is correctly done by measuring the tightest point of the choke and subtracting the measured barrel inside diameter.
This can be done using any number of measuring tools but it is not elegant and may in fact introduce errors.
CSMC makes a Pro Choke Gauge http://www.csmcspecials.com/Professional_Choke_Barrel_Gauge_p/t0170.htm which has three fingers and a dial indicator. It is inserted in to the barrel past the choke area to a nominal area of the barrel and the fingers expanded and the dial indicator set to zero. As you withdraw the fingers you must close the fingers to allow passage thru the choke area. That closure in thousands of an inch is indicated on the dial indicator and is the resulting actual choke.
The CSMC gauge is not cheap but contrast its cost to purchasing expensive doubles and the cost is quickly amortized.-Dick
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