A Backboring Diet


bruce,

i`m playing around with 1100 16s..the new ones…i finally found an auto that feels like a kreighoff…….seriously how much weight can i remove from the bbl by back boring.how far can i take it before i lose gas seal.don`t worry about wall thickness, the bbl is made from xx pipe……altho i hate porting,how much more weight would i lose………..if only beretta would make a lite 391 field in 16…….i went to the annual ale and quail shoot. used my 391 superlite 391 field with fiocchi 7/8 x 1200 lite loads,got one more bird than last year with my 28 browning o/u 30″…….carried the rem but its really heavy.

regards ,

fred

ps. i bought another model 50 win with a 26″ polychoked bbl I LOVE IT (SHOWS YOU WHAT I KNOW)

Dear Fred,

I don’t have any idea how much you can backbore a particular gun before it blows up or bends in a strong wind. Depends on the gun and how much wall thickness you feel comfortable with. I’m sure that Remington won’t recommend any backboring at all. You are on your own as to wall thickness.

But if you want to go ahead, use 4.5370369 ounces per cubic inch as the weight of gun steel. At least that’s what I use. When you do the math, make sure to subtract the length of the chamber and forcing cone from the length of the barrel to be backbored. If you don’t have a bore mike, you might consider starting with an inside diameter of .670″ as a general starting dimension for the 16. It will be close enough.

To save you some math, assuming a 28″ single barrel with a combined 3.5″ of chamber and forcing cone and bore of .670″, opening the bore .010″ to .680″ will drop 1.18 oz. Opening to .690″ loses 2.38. To .700″ loses 3.59, to .710″ loses 4.82 and to .720″ loses 6.07.

I really don’t think that backboring is the answer to a heavy gun. I use backboring more to affect gun balance than overall weight. Two ounces or three ounces pulled from the barrels of a gun makes a tremendous difference in how it handles, but I don’t think it’s too noticeable as to carry weight.

Of course, anything you pull from the front will also have to be offset by some amount removed from the rear of the gun to keep balance the same, if that’s what you want to do.

How much can you backbore and still retain a good wad seal? Dunno. Mossberg’s UtiliMag pump and new 935 Magnum auto are 12 gauge guns overbored to .795″ from the nominal 12 gauge bore of .729″. Since .775″ is standard 10 gauge boring, you can see that these are truly overbored. Mossberg claims no bloopers, even in cold hunting conditions. I’m sure that they tested the heck out of this bore size before putting it on guns obviously intended for a lot of waterfowl duty.

Stan Baker used to make a “Big Bore” barrel conversion to .800″ for 12 gauge target guns. He claimed great things, but the ones I tried made my reloads sound a bit bloopy occasionally.

One last thing about overboring- you will have to redo all your chokes. If you pull out too much metal, you’ll risk cutting into the threads of the choke or making the choke too thin. Screw chokes definitely complicate backboring.

Porting? The only thing that will do as far as weight goes is to lighten your wallet.

Bottom line: I don’t really see backboring as a practical way to overall lighten a gun as much as a good way to change the balance of the gun. I think that if I wanted a lighter, faster 1100 for field use, I’d shorten the barrel and hog out the stock. Remember, autos have about an extra 3.5″ of receiver compared to an O/U or SxS, so a 28″ barreled auto is really the equivalent length of a 31.5″ O/U or SxS. A 24″ auto equals a 27.5″ O/U or SxS.

Best regards,

Bruce Buck
The Technoid writing for Shotgun Report, LLC

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