Chamber Length Vs Shell Length


Dear Bruce,

Just for the sake of the never ending quest for ballistic knowledge, here is my question: Does a 3″ chamber have a negative effect in the pattern of 2 3/4″ shells? How about the 2 2/3″ shells? Now, before you jump to conclusions on me, I’ve done my part, and patterned my favorite load with two different guns. The reason I am seeking your advise is simply that the guns were not different only in chamber length, but in a numerous other parts as well, not really a scientific way of yielding the truth. To be more precise, the load was the B&P “Nike” 1 1/8 oz cartridge with # 9 shot loaded in a 2 2/3″ shell. It patterns like a dream in my 30″ .729 bored SKB O/U with 2 3/4″ chambers, but not so in my 24″ Fabarm auto with a tribore barrel (it runs from .740 to .723) with a 3″ chamber. The tribore barrel supposedly produces better patterns than traditional bored or overbored barrels and I have been able to verify that with a number of loads, but with this little shell the situation was reversed.

I hope my question will provoke you to open your magic books once more and share your wisdom.

Leo

Dear Leo,

I’ve written about the 3″ vs 2-3/4″ chamber thing before, but I’ll touch on it again.

First of all, in order to do a meaningful test, you’d have to take a single 2-3/4″ chambered barrel and pattern it. Then elongate the chamber to 3″, with no other changes, and pattern it again. Comparing gun “A” with 2-3/4″ chambers to gun “B” with 3″ chambers really isn’t going to tell you anything. Chamber length is only one of many things that may/may not affect pattern quality.

I think that “in theory”, 2-3/4″ shells used in a “standard” 3″ chamber do have poorer performance. By “standard” 3″ chamber I mean one with a short forcing cone. When a short shell is shot in a long chamber, “in theory” the shot can expand slightly in the 1/4″ gap between the end of the hull and the abrupt forcing cone. That makes the transition into the forcing cone even more abrupt and damaging to the shot than it would be in a shorter chamber. Using a 2-3/4″ shell in a 2-3/4″ chamber, the thickness of the lips of the hull sort of reduce the abruptness of the forcing cone. The thicker the hull (paper vs plastic) the less the violence of the transition from hull to cone.

I keep saying “in theory” because I’m not sure if it matters much in the real world. There are so many other variables in shell and in barrel design. Besides, I’ve never taken a 3″ chamber reamer to a barrel and done the tests.

Shotgun makers are divided into both camps. Browning believes that using 2-3/4″ shells in 3″ chambers denigrates performance. (Ed. Note: Browning HAD a FAQ comparing 2-3/4″ chambers vs. 3″ chambers on their web site.) Browning is clearly saying that there is more pattern variance (and the variance is on the downside) when a 2-3/4″ shell is used in a 3″ chamber. The use of a 3″ chamber with a standard shell causes Browning’s average patterns to open considerably. You should also remember that Browning, both Japan (Miroku) and Belgium, have short forcing cones. It’s also interesting to note that Perazzi doesn’t sell any target guns with 3″ chambers. All chambers are fitted correctly to the shell.

In the other camp is Beretta. ALL of their standard production guns, target and field, have 3″ chambers. BUT, in spite of what the aftermarket machine shops tell you, Beretta factory guns in the 68X and auto series, come from the factory with fairly long cones. Krieghoff target guns also tend to have long chambers and long cones. One of their 28 gauge barrels I tested on a K-20 had 3″ chambers. 28 gauge 3″?! Yup, that’s what it was marked.

But then, waffling around again, the high end Beretta SO series do not have 3″ chambers in their target models. Ditto the European market Krieghoff “Ulm” pigeon gun series.

Bottom line: Obviously the manufacturers can’t make up their minds, so how can you expect me to? I would say that if you are using 2-3/4″ shells in a 3″ chamber, it would be best to have longish cones. Most gun gurus feel that cones about 1-1/2″ long are about all you need. More doesn’t do more and it can cause blowby with certain wads. I can personally attest to that.

Remember one thing also, it doesn’t cost a maker one red cent more to cut long forcing cones or short cones when they make the barrel. Those who choose to cut short cones do so for a reason. Short cones provide a better gas seal during the wad’s transition into the bore. There is less chance of gas blowby with certain wads and in certain temperature conditions. If long cones were best in all circumstances, everyone would use them. They don’t because they aren’t. I’m not saying that longer cones aren’t often a good idea, but not always. In gun theory, very few things are in the “always” category.

Best regards,

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

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