View Full Version : New Mossberg 12 Gauge Bore Diameter: .733"

December 8, 2010, 12:48 PM
I just bought a new Mossberg 500 12 gauge shotgun, with a 28 inch ported field barrel with screw-in choke tubes. I measured the bore, and it is .733 inch. The advertised bore diameter is .731 inch, which is .002 inch over the so-called "standard" measurement of .729 inch. I checked my gauge, and it appears to be accurate. (For example, I measured a Carlson's modified choke with an advertised inside diameter of .710 inch, and my gauge was right on.)

Also, the factory Mossberg choke tubes measure larger diameters than standard choke tubes, based on Carlson's, Turlock, and other choke charts. The larger choke diameters put them very close to the correct constrictions for their respective choke designations, although a bit more open than they should be. I have not patterned the gun, but it converts clay targets to dust at 15 to 35 yards with IC, Mod, or Full chokes.

I wonder if this is a recent phenomenon, or if Mossberg 500 bores have always been slightly over-sized. Do any of you know? Have any of you measured the bore diameter of a Mossberg 500 12 gauge field barrel? I am especially interested in the bore diameters of older Mossberg 500s. It would be interesting to know the bore diameters of other shotguns, as well.

December 8, 2010, 01:18 PM
Maybe they are using SWO (slap wore out) brand lathe and milling machinery and tools...:D

December 8, 2010, 02:43 PM
Possibly. The tool marks were very prominent. I burnished the bore with 0000 steel wool, and it looks pretty shiny now. (The measurements were taken before I burnished the bore.)

December 8, 2010, 02:59 PM
.002 is an acceptable tolerance for most makers. Several companies "overbore" their barrels to even larger diameters - Browning and Kolar are ones that immediately come to mind

December 8, 2010, 03:46 PM
Some shotgun maker might call what you seem to have a custom light over-bore and charge you a premium. Oneounceload is correct, there is some tolerance to be expected, there's no need to get upset. I don't think I'd use a Carlson choke as a reference standard, since they are subject to an allowable tolerance, too. What may be more important, is the chamber cut to SAAMI specs, have you checked it?

Now that the bore is bright and shiny, have you checked it for straightness? Looking from the chamber end, you should see reflected a series of concentric rings. If the the reflected rings aren't concentric, then your barrel may be bowed.

As far as your screw-in chokes are concerned, it's the constriction that's important, not the actual diameter. Is your IC about 0.010" less than your bore -- your Mod about 0.020" less, and your Full about 0.035"?

I have not patterned the gun, but it converts clay targets to dust…
Thus far, it seems there is no reason to worry.

December 9, 2010, 06:27 AM
The bore appears to be straight, with concentric circles all the way up and down. Here are my choke measurements and constrictions based on a barrel bore of .733":

Improved Cylinder .725” (.008” constriction)
Modified .719” (.014” constriction)
Modified (Carlson) .710” (.023” constriction)
Marked: Full .702” (.031” constriction)
Marked: X Full (Carlson) .690” (.043” constriction).

I realize these choke labels are only a baseline, but this (above) is the way they are marked.

The IC, Mod, and Full choke tubes are Mossberg factory tubes. The others are Carlson flush tubes. I'm going to treat the Carlson modified choke as IM until I have a reason to do otherwise. It breaks clays in this gun like an IM choke in my other guns. Thankfully, it is easy to tell the Carlson from the Mossberg at a glance. When the weather breaks I can take time to pattern all of my chokes in this gun. Then I'll have the full story.

My mind is as ease after hearing from you guys. I'll rest assured that the gun and the choke tubes seem to work great. With the IC choke tube, I can hand-throw a clay target up, and break it at about 10 - 15 yards with ease. All of the chokes listed above seem to break clays at the appropriate distances, except that I have not tried any targets past about 35 yards. That's as far as my little thrower will toss them.

I appreciate the information. Right now I feel pretty good about my purchase. For $196.99 I can't complain. It fits me pretty well. The LOP is 13 7/8", right where it needs to be for me. The DAC is 1 1/2, which is a little low for me. I'm going to raise it by 1/4" and put a tad of cast-off into it. It should make a nice all-purpose (or MOST-purpose) field gun.

December 9, 2010, 08:41 AM
Here is a picture of the bore. I took a couple of dozen pics to get a few that might show the concentric circles. It seems that the focus will not extend for the entire length of the bore, so one end always seems more prominent than the other. This is a 12 gauge ported 28" Mossberg 500 barrel.

December 9, 2010, 10:36 AM
Seems to me, you're ready for more ammo, patterning targets and some serious trigger time. A while at the 40-yard line will give you a good idea how and where your M-500's chokes shoot.

December 9, 2010, 12:16 PM
The .014 constriction - that's one of those "tweeners" you see now and again that many like to call "Light Modified". With IC nominally at .010 and M nominally at .020, LM fits in nominally at .015.

If someone wanted to be persnickety about it, .014 is still IC.

What matters is not what is printed on the tube, or even as much what the constriction is, but how they pattern in YOUR gun with the intended ammo.

Case in point - I have reduced my 1oz reload to 7/8, and now to 3/4. For me in MY gun to replicate the 1oz pattern at distance, I need to choke one .005 increment tighter when shooting 3/4.

Pattern your gun AFTER shooting it a few times. (One friend has a rule - if it is hitting where I'm pointing and the breaks are good, no need to pattern it and give your mind something else to ponder if it isn't exactly like you thought)

Seems to make sense top me...............

Good luck!

December 10, 2010, 08:55 AM
Sounds like sound advice to me, oneounceload. I've been shooting mostly 20 gauge guns until recently. I reduced my 20 gauge loads to 3/4 oz. and tightened up one choke notch. I really don't enjoy the patterning process any more. I used to think it was fun, but now I'd rather expend my energies on shooting clays, birds, and squirrels.

At first, the reason I lightened and tightened was just for practice sessions, to aide in cleaning up sloppy technique and close in on centering the pattern better. I found that using a lighter load with a tighter choke allowed me to better "read the breaks" and find out which way they were going. (That's the only time I ever read breaks.) I discovered that 3/4 oz. of magnum lead was enough, and those loads with a faster powder are a pleasure to shoot--all day long, if the ammo fund holds out. My go-to 12 gauge practice load is also from 7/8 oz. to one ounce of #8 shot @ about 1200 fps, or one ounce of 7 1/2 shot @ 1145 fps. For 20 gauge trap I use 7/8 oz. loads, and for 12 gauge trap I use the full 1 1/8 oz.

Now I need to just get out burn up some powder! Thanks for the info, guys.

Dave McC
December 11, 2010, 04:33 PM
For comparison's sake, here's a few.....

1955 vintage 870 barrel,.725"

870 newer LC barrel, .728"

'Nother LC barrel,.7285".

Both barrels on my Beretta White Onyx, the Optima bore model,.732".