Backboring


Hi Bruce,

I do so enjoy you web site. You’re a wealth of information and a marvelous source of thought stimulation (at my age any stimulation is to be relished).

In re-reading your technical tract on barrel modifications, I got to thinking. I’m not an engineer or any such thing, but a bit of a tinker and mechanic, and was wondering what you might say to following.

All other things being equal, changing to a larger diameter wheel cylinder on a hydraulic brake system of an automobile will increase the required pedal pressure to achieve the same pressure on the brake lining.

Using this line of thinking, my question is; could the larger diameter of the barrel and subsequent wad piston actually cause more perceived recoil, be it ever so small?

I don’t know the technical term, but I think it has to do with the pressure concentration. Kind of like how it hurts a lot more when a 120lb female steps on your toe with her spike heel vs a 175lb guy in street shoes.

This has keep me awake for a while and I certainly don’t know if there is any valid basis for my thoughts, but at least I’m thinking.

Keep up the good work,

Don

Dear Don,

Well, I am glad to hear that other people stay awake at night thinking about really dumb things- just the way I do. I thought I was alone.

Here’s the problem with figuring out recoil. It is easy enough to measure “free recoil”. There are a number of formulae for it, depending on exactly what you want to show. SAAMI has one that I use, but most are about the same. The ONLY ingredients that free recoil takes into account are the weight of the gun plus the weight and speed of the ejecta. Nothing else. Not backboring, slow powders, gas actions, recoil pads, porting, cones- none of that.

“Subjective” or “perceived” recoil is another thing entirely. It is very difficult to measure because it is so personal. What is subjective to one person, may not be perceived by the next.

For example, hold you shotgun from your shoulder and shoot it in the normal way. You will feel a certain amount of recoil. Not hold the same gun with the butt on the end of your nose and fire it. You will feel an entirely different level of recoil. Subjective recoil has changed, free recoil has not.

The shoulder/nose analogy closely parallels the variance of stock fit and goes a long way to explain why a particular gun won’t kick one person, but will pound the next one silly. The stock fits the first guy better. Any time you are taking recoil in the face, you are dealing with stock fit more than free recoil.

Barrel modifications are a different subjective recoil deal because they should produce the same results for everyone. If backboring does indeed reduce or increase recoil, it should do so for everyone regardless of stock fit. Well, there doesn’t seem to be any unanimity of opinion at all.

In theory, the larger the bore, the shorter and broader the shot/wad column, the greater the base area of the wad for gas to push on and the lesser the friction. Result: more shot velocity. More shot velocity equals more free recoil. Stan Baker claimed an extra 50 fps for his “Baker Big Bore” 12 gauge barrels of .800 diameter.

So, if backbored guns kick more in theory because they increase velocity ever so slightly, why are they promoted as reducing recoil? Answer: because backboring really doesn’t do much of either, at least not enough for the average person to notice one way or another, so the ad campaign can say just about anything it wants.

It is the same with porting. In theory, porting works by vectoring gas vertically to push the muzzle down. We know that porting works on high power competition pistol compensators. That means that it must also work on shotguns, opines your Technoidal Mahatma. Right again, but the problem is that shotgun muzzles work at gas pressures very, very much lower than high power pistol muzzles, so the small amount of gas being vectored up by a shotgun barrel does very little, if anything, to keep the muzzle down. A proper left hand grip is far more important. If a shooter has a weak left hand grip, then he might notice some difference with porting. If he has a normal left hand grip, he won’t. Subjective again.

Bottom line, technically I agree with you that backboring should very slightly increase free recoil because it increases velocity. Your sleepless nights were not in vain.

As to your analogy of being stepped on by a 120 pound woman with spike heels vs a 175 pound guy, the girl hurts more when she steps on you, but the guy hurts more when he belts you. What really hurts is when that guy steps on you with those spike heels.

Best regards,

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

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