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The Best Shotgun Choke: Duckbill Shot Spreader

Fielded by Navy SEALs during the War in Vietnam, the Duckbill Shot Spreader improved the performance of Ithaca pump-action Riot Guns!

The Best Shotgun Choke: Duckbill Shot Spreader

A look at Leroy Thompson’s Ithaca Model 37 equipped with an interesting Duckbill Shot Spreader.

What is the best choke for a shotgun? There really isn’t one, as the optimum choke constriction will vary with the type/size of shot and distance you will be using it. Traditionally, short barrel shotguns intended for use with buckshot and slugs by military and LE, along with guns intended for personal protection, have typically been fitted with Improved Cylinder. While Improved Cylinder is the most common choke used with short-barrel shotguns firing buckshot, there are other options. Most of these, such as switching to a Modified choke, are rather mundane, but there are a few which will grab your attention.

One of the more interesting developments for changing the pattern of a shotgun can be seen here on Leroy Thompson’s Ithaca Model 37 Riot Gun. Leroy is our Machine Guns & Tactics Editor and he has a rather impressive historical firearms collection. This particular Ithaca Model 37 is fitted with a “Duckbill” choke. The original Duckbill was a 1968 US Navy project for use in Vietnam. As the US military became more involved in supporting South Vietnam in the early 1960s it had realized there would be a need for combat shotguns. The “trench” and “riot” guns in US military inventory at that time were leftovers from World War I and World War II. A variety of models from different manufacturers including Winchester, Remington, Savage, Stevens and Ithaca had served during World War II. In the post-war years the Winchester Model 12 “Trench Gun” and Stevens M520-30 and M620A “Trench Guns” remained classified as Standard for the US military.

Additional weapons were expected to be needed though, and so a contract was given to Ithaca Gun Co. in 1962. The US Government purchased a plain barreled “Riot Gun” version of the Model 37 12 gauge slide-action shotgun. These featured a 20-barrel, plain wood stocks, Parkerized finish, plastic buttplates and no sling swivels. These Vietnam-era guns received a special serial number range (approx 1,000 to 23,000) with a letter “S” prefix. Five years later in 1967 the US Government purchased a quantity of Ithaca Model 37s equipped as “Trench Guns” with ventilated metal handguard and bayonet mount. But, what is of interest to our story, is in-between these two purchases, the US Government procured some Model 37 Riot Guns that were not numbered in the unique range with the “S” prefix. Some of these guns were equipped with sling swivels and a number were subsequently fitted with Duckbill Spreaders for use by U.S. Navy SEALs and other Navy personnel. The Navy SEALs then put these to use running various missions in Vietnam.

So, why is this Duckbill shot spreader of interest? Well, it was designed to change the shape of a shotgun’s shot pattern from its normal circular shape to a horizontal spread. The shot pattern changed to a 4:1 ratio horizontal pattern. It was hoped this would increase the hit probability during engagements in the jungle where the target, shooter or both might be moving, partially hidden or hard to see due to low light. In this type of an engagement having a horizontal shot pattern was expected to be much more effective than a typical circular pattern.

The originals were permanently mounted to 12 gauge Ithaca Model 37 pump-action shotguns. With the Duckbill mounted the Ithaca M37 provided a pronounced horizontal pattern. At 12 yards the pattern would be approximately 3 feet wide and approximately 16 inches in height. By turning the shotgun 90 degrees, you could put out a vertical pattern. #4 buckshot was considered the optimum load due to the pattern density and terminal performance. So, in use the Duckbill diverted the shot as intended to provide a horizontal shot spread. Downside was you could not fire slugs through them, and over time some cracked. The cracking/failures were addressed by adding a ring around the end on latter production models. The Duckbill is an interesting piece of military history from our war in Vietnam, and it is also an interesting piece of engineering to drastically change the performance of something as simple as a Riot Gun.

If you have any thoughts or comments on this article, we’d love to hear them. Email us at

About the Author:

David M. Fortier has been covering firearms, ammunition and optics since 1998. He is a recipient of the Carl Zeiss Outdoor Writer of the Year award and his writing has been recognized by the Civil Rights organization JPFO. In 2007 he covered the war in Iraq as an embedded journalist.

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