Chamber Length and Pattern Efficiency

This subject isn't talked about much, for the results aren't what most people would like to hear. You get the best pattern efficiency when the chamber length of your shotguns matches the unfolded shotshell length. When it doesn't, you have excessive free-bore which hurts pattern efficiency.

Using a Benelli Vinci (3 inch chamber) and Federal 1 oz. #7-1/2 Target Loads, pattern efficiencies with a Modified choke ranged from 58 to 68%. Using a Fabarm L4S (3 inch chamber), average pattern efficiency with Federal 1 oz. 2-3/4 inch unfolded length Target Loads was 74%, using the factory Modified choke.

Using the same shells and the same modified choke tube, through a 2-3/4 chambered Fabarm XLR5 FR, the pattern percentage skyrocketed to 94%. Same exact Exis 5/10 Modified designation choke, same shell, yet 20% more pellets in the 30 inch circle. Counting pellets by hand is a drag, made far faster, more convenient, and more accurate with the Target Telemetrics Turbo Targets and software. Patterning basics are covered here: http://randywakeman.com/PatterningaShotgunforFunandProfit.htm .

What the patterning results show is that with economical 2% antimony target shells, you can expect a 10% difference in pattern efficiency shot to shot, or more. With 6% antimony premium target shells, the variation from pattern to pattern is less: generally within 5%. You can also expect about a 10% better pattern efficiency using a “Modified” choke at 40 yards using 6% antimony shells: http://randywakeman.com/IsIttheChokeortheShell.htm .

It isn't by accident that dedicated clays guns such as the Fabarm XLR5 LR and FR use 2-3/4 inch chambers, as does the Caesar Guerini Invictus and Beretta DT11. Invariably, 2-3/4 inch shells pattern significantly better with matching 2-3/4 inch chambers.

For hunting guns, no, the 3 inch chambers are a negative with 2-3/4 inch shells, 3-1/2 inch chambers being remarkably worse. The 3 inch chambers are necessary, though, as if shooting steel you'll likely need 3 inch shells, and even with lead loads for turkey, you'll probably go the the 3 inch route. Chamber length matching the unfolded hull length is one of the reasons that 3 inch 20 gauge loads don't just pattern as well as 1-1/4 oz. 12 gauge loads, they often pattern noticeably better. If you are wondering why your old A-5 Light Twelve or 2-3/4 inch B-80 seems to pattern better than your newer 3 inch guns, it probably isn't your imagination.

It may not sit well with some that a 20 gauge pheasant gun is ballistically superior to a 3 inch 12 gauge, but the pattern board shows that this is indeed the case, with 1-1/4 oz. lead loads. B & P MB Long Range 12 gauge shells put 107 pellets in the 30 inch circle through a 12 gauge Modified tube, while Remington buffered 3 inch 20 gauge shells (Benelli M2 20 gauge) put 148 pellets in the same 30 inch circle at 40 yards with a Trulock Precision Hunter Modified Choke tube. If you do your own patterning with your own individual gun, choke, and shells, you'll be surprised.

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