doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com 12 ga bore dimension ?

I've acquired a few older shotguns over the years and some I shoot well and others not so well. I finally bought a Skeets bore gauge and started to measure some of them to get an idea of what the actual chokes were. Now I have to get out and pattern them and see how they do.

Here are some of the guns I measured. Most of them had the min. wall thickness about 6 inches from the muzzle.

Bore dimensions

My Robust is .725 R & L Minimum wall thickness .029

Husky 51 late production R-.736 L-.735

Husky 51 mid production R-.728 L-.728
Both have min. wall thickness of .031-.034

The Costo Super(MF Ideal with Robust style barrels)

Right .712 Left .712 with min. wall thickness of .036.

Is the .712 bore diameter on the Costo very common in 12 ga shotguns? I know the American norm is .729 and some European shotguns will run .005 smaller.

Thank You

oskar.

To the best of my knowledge the nominal dimensions of the various gauges was first set by the British proof houses. if someone has better knowledge of this please post. 12 gauge was set t .729" (18.5mm). 13 gauge was set @ .710" (18mm). When the incremental steps were added 13/1 was set at .719" (18.3mm).
12 ga guns proofed as a 13/1 are rather common, 13 not so much so but certainly not unheard of. I have one 12 ga proofed in Birmingham as a 14. This gun was proofed prior to the addition of the incremental steps & currently measures just over the 13 size. It was likely very close to the 14 upper limit at proof & has worn enough to bring it into the 13 size now. Would likely have been proofed as a 14/1 had those steps been in use at the time of proof.
Your barrels should have a metric number indicating their size at proof. With a .712" diameter this should be either an 18 or 18.1 depending on whether they simply used the largest gage which would enter or if they rounded the actual measurement. .712" = 18.085mm.

oskar, probably the primary difference in how well you shoot your various guns is how well they fit you. Last year I purchased what I thought would be my ultimate upland gun, a French 12 gauge with 27 inch barrels choked F/M that weighed less than 6lbs and was in like new condition. I took it out for a round of trap and couldn't hit squat with it. A trip to the patterning board revealed that at 50' I was consistently shooting about 18" right of the target center. I examined the stock and realized the gun had an extremely narrow comb. After I put an ugly 1/4" thick foam pad on the gun I was able to shoot the target center. Now I'm in a quandary, the gun is in exceptional condition and I want to keep it but I visually can't stand the modification. One side of me says to have the stock bent, another says to restock the gun, and my gut tells me to sell it and start over again in my search for the ultimate upland gun. What to do, what to do...

Double post... sorry about that

"ultimate upland gun" ????

And style is more important than function?

have a day

Dr.WtS

oskar, the bore sizes will have little effect on choke performance. Choke is pretty much a matter of nominal bore diameter and constriction. Shooting for point of impact makes sense. Shooting for pattern performance doesn't unless you are willing to run an extensive research style project.

DDA