View Full Version : Choke to Bore Measurement Experiment

June 9, 2012, 09:58 AM
Greetings All,

I bought a Savage/Stevens 9478 Break Barrel 12 Gauge shotgun at a pawn shop recently and I am trying to figure out the choke in this shotgun. From what I have pieced together from various forums, all the 9478s were made with full choke barrels. What I am finding with my measurements is different.

I measured the end of the barrel with my caliper found the diameter to be .729(typical 12 gauge).

Next I fashion a "choke diameter tester" by taking a AA battery and wrapping electrical tape around it very tightly(to minimize any squish of the tape) and kept trying to slide it through the whole barrel until it felt snug, but still passed through. Once it was snug, I took it out and measured it with my calipers and found the diameter size to be about .720.

Now, on all the "choke to bore diameter" charts I can find, a .009 reduction in the bore diameter for a 12 gauge results in an Improved Cylinder choke. This barrel definitely does not have the .035 reduction(.694 dia) that a full choke would be. So I figure that someone had the choke reamed out to Improved Cylinder at some point(or the gun came with an IC instead of a full choke to begin with). Would that be an accurate assumption based on the measurements I am seeing?

I know that the "real" way to test the choke of a gun is to shoot it, but I haven't been able to get to the range yet and I was just curious.


June 9, 2012, 01:58 PM
With a pawn shop gun, you never know what you're getting. Perhaps the previous owner had the choke bored out. Your best bet is to find out how your new gun actually patterns with some trigger time. That will tell you more than all the measurements in the world.

June 9, 2012, 03:41 PM
I think your methodology is flawed. I don't care how tight you wind the electrical tape, isn't it going to expand after being compressed and throw your measurement well off?

June 9, 2012, 04:21 PM
Thanks guys,

I checked my AA battery and electrical tape setup this morning again and it was still at .720 (I did the original test a couple of nights ago). I put the "gauge" down the barrel again and was able to see light around it the whole time I pushed it down the barrel. So it appears that, if anything, the diameter is even slightly bigger that the .720 I am measuring on my rig. Yes, it is not a perfect measurement, but I at least can verify that it is not tighter than a modified choke.

Another strange thing is that according to all the documentation I can find, the shortest barrel that came on these shotguns is 26". Mine has a 20". I think someone did quite a bit of customization on this thing.

I do plan on getting out to the range some time soon.


June 9, 2012, 04:27 PM
What end measured .729"? And if it was cut back from 26 to 20 you would have the same bore diameter as the choke would be a Cylinder bore

Tale it any competent gunsmith and in 5 seconds, with the right tool, they can tell you what you want to know.

It also matters not what any dimension says, it is what it does at the pattern board

June 9, 2012, 04:48 PM
Was always told a dime will not fit down a full choke, but will down any choke more open. Always worked when I tried it.

June 9, 2012, 05:02 PM
The dime routine doesn't always work, simply because there is no longer a standard 12 bore dimension. Cylinder bore dimensions can run from as tight as .725 to as wide open as .750 - quite a difference

June 9, 2012, 05:13 PM
I herd about the dime thing, I was told if a dime goes through you can put a slug through. Just what Ive been told.

June 9, 2012, 05:23 PM
It measures .729 at the muzzle.

It hadn't dawned on me that if they shortened the barrel, they probably would have cut the choke off. That makes more sense.

Well, off to the range I go.

June 9, 2012, 05:37 PM
And, so ends Lesson #1 of Shotgun Detective 101. :)

June 9, 2012, 05:40 PM
Okay Hardy Boys!