Backboring And Recoil


Dear Technoid,

At the last sporting clays tournament that I attended, there was a Browning sales rep there with an array of finely tuned shotguns for our viewing pleasure and test drive. The one thing that Browning was putting alot of emphasis in was the back-boring.

In one of your articles, you found no gain for back-boring. Yet in another article, you mentioned pellet deformation while traveling down the bore as a cause for poor shot patterns. Could back-boring help in less shot deformation, there by, providing a better shot pattern?

The Browning rep stated reduced recoil is the main reason for back-boring. I feel it is a marketing ploy. Have you changed your mind on this subject?

Thanks for you time.

Brian

Dear Brian,

Change my mind? Waffle! Be a woosie? Not the Technoid! “OFTEN IN ERROR, NEVER IN DOUBT!” Well, sorta.

I put factory “backboring” in the same league with porting. They both technically work. They just don’t work enough to matter.

Yes, increasing the bore diameter will technically shorten the shot column and shorter shot columns should deform less shot. Now lets look at the numbers:

.725″ is pretty much the industry standard for actual production 12 gauge bores. Most of my Belgian Brownings are around this ID and so were the older Japanese Browning Citoris. The current Japanese Browning “backbore” is around .740″ (+/- .003″ according to the unusually frank Browning catalogue of two years ago).

.725″ is an area of 0.412991071″, while the larger .740″ has an area of 0.430257143″. This is a difference of about 4%. That’s it. That’s all. I just don’t see how that slight change is going to do much for patterns. Did the Browning rep offer to show you any before and after pattern stats? Uh huh.

As to reducing recoil, I don’t see how enlarging the bore of the shotgun can do that IF you use the same shell before and after. The Free Recoil formula used by SAAMI only cares about gun weight, weight of the ejecta (shot, wad and weight of powder charge) and speed of ejecta. There is also a slight fudge factor for speed of the exit gases.

No where does the formula mention bore size. Stan Baker in Seattle has, for a number of years, sold Big Bore barrel conversions. They have an ID of .800″! That is 12 gauge chamber size. He claims that they increase shot speed by about 50 fps. That could be. BUT when ejecta speed goes up, Free Recoil goes up. In Baker’s case, ultra maximum extreme backboring technically INCREASES free recoil. Go figure.

So, if backboring increases ejecta speed and therefore recoil, how does the Browning rep get away with telling you that it decreases it. Well, that’s why he is in sales.

Having said all that and made my shaky technical case, I do have a bit of confessing to do. I don’t believe that backboring and porting do anything noticeable to recoil when using standard target loads. I do believe that long forcing cones help reduct felt recoil (not mathematical free recoil) very, very slightly. I also have said that I don’t think very much of the “improvements” performed on Beretta 390s by some of the after market machine shops.

That said, one day I was shooting my 30″ Beretta 303 alongside a pal who had exactly the same gun, but with a fully tricked out Ballistic Specialties barrel. The barrel was backbored from the original .722″ to .735″. Cones had been lengthend from the original Beretta long length to super long. The barrel had all sorts of holes in it and even the chokes had radial ports. I put his barrel on my gun and he put mine on his. About four or five of us shot the guns and then we switched the barrels back and shot again. Universal opinion was that the Ballistics Specialties barrel kicked less than the stock one. It really did and you could really notice it. You could not notice any less muzzle jump, but you could notice less subjective rearward recoil. I grumbled and whined, but there it was. General opinion was also that the Ballistics barrel had also been made too light to swing properly. He ended up selling it, but that is another story.

So, I still believe that backboring doesn’t do a darn thing. Sort of.

Best regards,

Bruce Buck
Shotgun Report’s Technoid

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1 Response to Backboring And Recoil

  1. Gerry says:

    Well known English cartridge, kicks like a mule in my 18.4 bore 8lb 7oz Italian clay breaking Olympic medal winning cannon, noticeably different in my 18.8 bore 8lb 9oz Japanese made American brand Grand Tourer International of 1992 vintage. I wouldn’t have believed it but it’s there.

    Like

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