Performance Of Overbore Barrels


Dear Technoid:

In, perhaps, a moment of testosterone poisoning while awaiting the awarding of prizes at our regional Jeep shoot, a fellow sporting shooter pedantically remarked on the evils of overboring.

He stated he had “personally” witnessed a chronographic trial of sundry loads passed through aftermarket .740 bore barrel. The result was an average loss of 200 fps of velocity near the muzzle. The test included several brands of loads, all with similar results. The obvious conclusion to all the testosteronaly-challenged mates guzzling beer was that the charge’s gases had escaped around the wad base of the overbored barrel. Why how can one break a midi at 35 yds. with only 1000 fps pushing your 7 1/2’s?

The pedagog went on to state it was essential to use only straight-walled hulls (Active, Remington, Federal) in overbored guns. (The exact jump here failed to register with a mere tyro regarding the Art and Science of Shotgunning.) Researching Brister and doing a search of past Technoid articles (when I desire help, I go straight to real experts) failed to solve this conundrum. Please help me Obiewan Ka Technoid.

Tom
Columbia, KY

Dear Tom,

Well, you have certainly come to the right place for an opinion. As a matter of fact, I have several. Which one would you like?

Overboring has pluses and minuses. It all depends on how and why it is done. The benefits of overboring are not universally recognized. Browning (Japan) overbores to about .740″ and claims great benefits. Browning (Belgium) uses standard bores of .725″ and claims (perhaps rightfully) to be the standard by which all O/Us are measured. Beretta not only does not backbore, but they have fairly tight bores in many of their guns. My 303s average .723″. Krieghoff M32s started all the large bore craze and are well respected among target shooters who prefer Buick Electras. M-32s and K-80s have always averaged around .735″, BUT the ultra tight patterning Krieghoff Ulm pigeon gun has barrels well under .725″ and patterns like a laser. As you can see, unanimity of opinion on backboring does not exist.

Does backboring cause velocity loss due to gas blow by? Maybe, but that has not been my experience. I believe that backboring actually increases velocity. Stan Baker makes Big Bore barrels. These are .800″ tubes sleeved into standard monoblocs. I spent some time with a Perazzi so outfitted. .800″ is the chamber size for a 12 gauge shell, so these guns basically had 30″ chambers. You cannot get any more overbored than that. Baker claimed a 50 foot per second INCREASE in velocity for this Big Bore barrel. He still sells them and people pay a bunch to get them, so there may be something to it..

I am not so sure that a slight overbore of .740″ from .725 would make any difference at all. I certainly would NOT make a 200 fps difference. That’s a ton. A grossly undersized wad will not make that much difference. A standard 20 gauge wad will obturate (seal) pretty well in a 12 gauge barrel. The plastic skirts are quite flexible. 20 gauge chamber inserts work decently in 12 gauge barrels and this is partly because the 20 gauge wad, meant for a .662 bore, expands and seals fairly well in a .729″ 12 gauge bore. If a wad meant for a .729″ 12 gauge bore can expand and seal in a Baker Big Bore of .800″, the .662″ 20 should have no trouble in the .725″ 12. Both of these are far greater jumps than the overboring your acquaintance is speaking of.

The increased velocity (NOT decrease) in an over bore barrel is theoretically due to two factors:

1) as the bore increases, the shot column decreases and so does friction, and;

2) as the bore increases and the wad obturates to fill it, the base of the wad presents a larger area for the fixed amount of gas to push against. This make the burning powder gasses more efficient and more velocity results.

I am not sure if the differences are great enough to notice, but Baker felt that extreme backbores did increase velocity.

Downside: If the wad does NOT obturate properly, then there will be a loss in velocity and your mouthy knowitall will gain the upper hand. The colder the weather, the harder the plastic wad gets and the more likely it is not to obturate properly. Cold weather will give the same effect on cheap wads in standard bores as it hardens them up.

In my personal experience, it is all really a wash. I have never noticed much increase in velocity from an overbore and I have never experienced failure to properly obturate in a overbored barrel when a standard brand wad was used. The fellow who said that going from a standard .725″ bore to an overbore .740″ bore cost 200 fps was full of prunes. Trust the Technoid. You can take that one to the bank. Tell him to shell out for a better quality chronograph and spend less money on hallucinogenic drugs.

As you have no doubt surmised, straight walled hulls (Reifenhauser or compression formed straight wall) have absolutely no possible effect, plus or minus, in an overbore barrel. The hull stays in the chamber. It is the wad that gets into that overbored barrel. It is the wad that counts. It has nothing to do with the hull.

The argument could be made that the hull dictates the type of wad used. Federal makes a pretty big based wad to go with their Reifenhauser hull. I fool around a lot with the Federal 12S0 one ounce wad. Winchester made a narrow based wad for its now discontinued, tapered compression formed AA hull. I also used a lot of Winchester pink SL one ounce wads. You would think that the “big” Federal would generate higher pressures than the “smaller” Winchester wad. Well, if pigs could fly, I would be eating bacon on the run.

The hulls may differ, but the wads are pretty close. Check any reloading manual (I happen to have Alliant’s open in front of me) and you will see that often the Federal wad runs higher pressures, but often it doesn’t. The size of the base of the wad doesn’t seem to matter as long as it is big enough.

I have backbored a bunch of guns, but the ONLY reason that I do it is to lighten the barrels. My backbore are usually in the .010″ – .015″ area, depending on the weight I want. I have never noticed any change in velocity that my faithful ProChrono could measure.

Bottom line: A backbore from .725″ to .740″ should not cause a 200 fps velocity loss with a standard brand of shell. It should cause a very slight increase in velocity. Anyone who tells you different is, well- different.

There it is. The last word from your Guru of Gunning Gear, The Solon of Shotgunning, The Obergrupenfuhrer of Over and Unders, The Sultan of Sidebys and Your Genial Purveyor of Bovine Remnants.

Best regards,

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

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