Backbored Chokes


Hello O Master of Ether Errata,

I’m in the process of having my 32″ trap barrels backbored from .729 to .740. They should swing quite smartly for the coming Sporting season, but for now I have more to do. Most folks I talked to didn’t even want to do a straight backbore in a choke tubed barrel set. Some wanted to do a tapered backbore, but the point was to remove weight, the more the better.

At any rate, the issue now is to make certain the shot charge passes safely, without obstruction, ALL the way through barrel AND choke tube. How much clearance should I give the “old style” Browning choke tubes at the entrance to the tube. They currently all mike .740 but if I recall correctly Perazzi had problems with insufficient clearance at the junction of barrel and tube. How much should I open up the entrance to these tubes .003 – .005 or more?

Another option would be to rethread my barrels for the Invector Plus choke, but I haven’t been convinced they pattern any better than my current tubes. What’s your bet, open up the existing choke entrances and remark for the tighter constrictions, or spend the bucks for rethreading and 8 to 10 new choke tubes?

Thanks,
Jim

Dear Jim,

Good questions. You are one of the first guys I have heard from who wanted to backbore for the right reason- to lower barrel weight. Backboring a 32″ O/U from .729″ to .740″ will pull out 3.28 oz. For what it is worth, most of the older Japanese Brownings I miked had bore IDs closer to .725″, but they all vary.

Before you replace the original (generally grossly mis-labeled) short Browning Invectors with the new longer Invector Plus ones, weigh one of each in the same constriction. Then remember that the added weight of TWO Invector Pluses will go right at the very front of your barrel where it is most noticeable. Those big chokes are one of the main reasons why the new Japanese Brownings are so heavy up front.

Unlike all the hype about short chokes vs long chokes, I never found any difference between my standard flush Beretta chokes and the Briley X2 extended ones. If they miked the same, they shot the same. In theory, long chokes can do more for you, but in the real world of mass production screw chokes, there is no difference that I could find. It is interesting to note that British screw choke Guru Teague changed from short screw chokes to long chokes, not because the longer ones patterned better, but because his customers had read all the magazine hype and demanded them. It is a business, after all.

The reason that the machine shops are touting “taper” boring is so that they don’t have to fool with the screw chokes. No one has yet shown me written proof or studies that taper boring does anything except lighten the barrel and the pockets of the customer. The problem is that it lightens the barrel at the rear, where it usually doesn’t matter much. It is loss of weight in front that turns that pig on a snow shovel feeling into something more responsive. You were right to skip the taper boring if you are trying to give your gun a more lively feel.

As to the “step” between the skirt of the screw choke and the bore- Teague’s work has an almost invisible seam with very, very little step. The chokes have a good reputation for staying put too. It is just a matter of keeping things clean. The Perazzi MT-6 did shoot out its chokes on a regular basis, but that may have been as much due to choke design as it was to clearances. Regular Japanese Browning and Beretta chokes have a chasm-like .015″ or so step to allow for ganging of tolerances during manufacture and to be ultra safe in the hands of every possible customer.

I can’t guarantee that any particular clearance will be safe because I have never done what you are doing. You are on you own in this, but reason would dictate that a very modest step along with vigilant cleaning, especially UNDER the choke tube, should work. The people who ran into trouble with the Perazzis let carbon build up under the choke skirt until the skirt pushed into the bore. Proper cleaning should have avoided that.

I have a few pounds of the short Invectors kicking around in the shop. On mine the skirt end IDs are just a touch over .740″ and the skirts are around .021″ thick at the very end. There ought to be room to thin that skirt out just a touch to give you a little breathing room. If it were my gun, that’s what I would do, but you are making the decision and should consult with the local gunsmith who is going to end up thinning your chokes down on his lathe. Briley makes replacement chokes for everything. You might want to talk to them too and see what the dimensions of their short Invectors are. They may know of someone who has gone through this before and can fill you in.

Pulling over 3 oz out of those barrels is going to transform the gun. I think that you will be very pleased with the way it feels. I would be interested in hearing what you decide to do as to rechoking and how it goes.

Best regards,

Bruce Buck
Shotgun Report’s Technoid
(Often in error, never in doubt.)

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1 Response to Backbored Chokes

  1. Edward M Sybert says:

    It is amusing to read that someone opted for 32″ O/U barrels, and then wants to reduce their weight by back boring. Your response that the back boring would remove 3.28 ounces was illuminating. I wonder how much weight at the very end of the barrels would have been saved by purchasing a 30″ set rather than the 32″? -Ed

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