Barrels And Velocity


Dear Technoid,

As the holder of two JTOP(3) pocket protectors, I find it necessary to pull on my boots and challenge the opinions of some the local club experts. The latest topic for discussion has to do with velocity reduction of the shot due to the bleeding off the gas to operate the bolt on a semi-auto. This was brought up by someone who just sold his 12 ga. semi-auto for a 20 ga. O/U. the underlying contention being that the O/U is superior to the semi-auto because the shot has a higher exit velocity. (I haven’t yet asked him about the effect of pellet count. I recall well the Technoid chant: “More is More!”)

In looking at this situation as a Junior Technoid, it seems to me that the answer lies in the timing relationships between the pressure peak, the time that the shot emerges from the barrel and the inertia of the bolt mechanism and the relationship of the gas bleed hole area to the cross sectional area of the barrel. But then, by the time the shot reaches the clay, we are talking about energy differences on the right hand side of the decimal point, so who really cares, anyway.

As an avid semi-auto user, for the sake of discussion, what do you think is the impact of the gas bled off into the cylinder and why?

And as a related question, what impact does barrel length have on the exit velocity. There is a claim that every inch of reduction from 30″ reduces velocity by 28fps.

So, boots off, beer open, feet up while I await your response.

Logan
Morgan Hill, CA

Dear Logan,

Two JTOP(3) plastic pocket protectors? You are distinguished indeed and dwell in rare company. Most of those who have earned their second JTOP(3) are confined by the state for their own protection. It’s stressful being genius material. Or maybe it’s the required Technoidal diet of Twinkies and Jolt cola that does it.

Lemme get this right- some guy at your club swapped his 12 gas auto for a 20 O/U because he felt he was loosing too much velocity from the 12 due to gas bleed-off? And here I thought that Junior Technoids were loony! Geez, waddadunce! Don’t tell him I said that though. My image as a nice guy is fragile enough.

You are 100% right when you say that any difference is to the right of the decimal point. It’s probably also to the right of a couple of zeros after the decimal point. There are many other factors that make up muzzle velocity: bore diameter, bore condition, forcing cone taper, choke taper, ambient temperature, shell to shell variation, barrel cleanliness, wad design, primer brisance, powder burn rate, etc. The little tiny weeny bit of gas bled off half way down the barrel simply is not significant. Most modern powders develop their velocity in the first foot or so anyway. The rest is mostly friction.

The amount of velocity lost with each inch of shorter barrel length varies with the shell used. Large or fast payloads using slow burning powders can lose slight amounts of velocity, but loads using fast burning powder shouldn’t lose significant amounts. 28 fps per inch sounds more like what is lost in a rifle, not a shotgun. Needless to say, I don’t have the figures and am not about to start circumcising one of my barrels to find out. I’m sure that someone’s done it and will rise up to set me straight.

Want to talk about really short barrels? Look at the velocities generated by the Little Skeeters and Chambermates chamber inserts. These “barrels” are only 3″ long or less! In 12>20 and 12>28, they either lose very little velocity compared to a standard barrel dedicated to the gauge, or they actually gain velocity. Depends on whom you believe. Point is that chamber inserts are basically NO barrel and the velocities are close to normal. What does that tell you about burn rates and friction? I admit that the chamber inserts in 12>410 do show significant velocity drops in some test (including mine), but the 12>20 and 12>28 don’t. Go figure.

Still, I don’t think it’s all that important. As I said above, there are so many other variables affecting muzzle velocity. Did you know that changing the choke from cylinder bore to full increased velocity? As I wrote in February ’00 in my “Ask the Technoid” column in “The Clay Pigeon” magazine I was discussing the Venturi effect (pressure and thus velocity increase as the orifice decreases): “The shot charge has many of the same properties as a fluid as it goes through the choke. I tested a Remington STS 2-3/4 dram 1-1/8 oz load of #8s through the 30″ barrel on my Beretta 303. With a zero constriction cylinder bore choke installed, that shell chronographed an average of 1,147 feet per second. When I put in an Extra Full choke of .040″ constriction, the same shell averaged 1203 fps. That’s a 56 fps difference. The only change was the choke.”

Who woulda thunk it.

Best regards,
Bruce Buck
The Technoid writing for Shotgun Report, LLC
(Often in error. Never in doubt.)

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