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Rifled Barrels and Shot Patterns

posted Sep 3, 2012, 7:11 PM by Peter Lucas   [ updated Oct 11, 2012, 1:58 AM ]
    A frequently asked question relates to whether bird shot can be successfully used in a muzzle loading rifle. Conventional wisdom is that rifle barrels will produce very wide patterns with a gaping  doughnut hole in the middle of the pattern.  However, there is very little quantitative evidence which supports this conventional wisdom with respect to the impact of riling on shot patterns.

    Interestingly, many shooters (including myself) routinely shoot a form of rifling in their modern shotguns.  My modern skeet gun, which was purchased second hand, came with a pair of Briley diffusion chokes.  I rarely take those chokes out of the gun.  The Briley diffusion choke is a rifled choke designed to spin the wad as it exits the barrel. The diffusion chokes have a .005" constriction which is the same as a standard Skeet choke. Briley claims that the rifling spins the wad/shot package a bit which causes the pattern to "open up" faster.  However, Briley's use of the description of "opening up" the pattern is a little nuanced. As was described in  Mysteries of Shotgun Patterns by George G. Oberfell & Charles E. Thompson, Oklahoma State University Press, 1957, the pellet density in the center of a shotgun pattern is generally greater than at the edges of the pattern. The average pellet density gets progressively less dense as distance from the center of the pattern increases. Oberfell and Thompson concluded that the the distribution pellets approximates a typical bell shaped curve.  

    For most types of shooting the bell shaped distribution of pellets works to the shooter's advantage.  With a bell shaped pellet distribution, the largest number of pellets will be delivered to the center or core of the pattern giving the shooter the best opportunity to hit the target with multiple pellet strikes.    There are situations, however, where the bell shaped pellet distribution can be a disadvantage.  An example is found on the skeet range.  Many shots on the skeet range are very close and the longest shot likely to be encountered is approximately 32 yards.  (The second bird on doubles from stations 3 and 5.)  For virtually all shots on the skeet range the center core of a 12 gauge pattern is significantly denser than is required to consistently  break targets. Under these conditions, it would be advantageous to move some of the pellets out of the core of the pattern and toward the pattern edges.   Prior to development of the diffusion choke, the only way to move pellets from the inner core of the pattern to the outer edges was with less choke constriction.  However, experience has shown that reducing the constriction can lead to spotty patterns.   After significant experimentation, Briely found that shallow rifling moved pellets to the outer edges of the pattern while still maintaining adequate density in the core of the pattern. 

    My Briley diffusion chokes produce consistent patterns at the patterning board.  The diffusion chokes also stabilize modern plastic shotgun wads which can usually be found embedded in the patterning paper. Given my positive experience with the Briley chokes, I decided to do some testing to see if I could get decent patterns from a rifle barrel.  Thet test gun was a 58 caliber Hopkins and Allen Underhammer.  The H&A was designed to shoot round balls and has a slow rate of twist.   The Briley diffusion chokes use a much faster 1 in 14 inch twist. My theory was that the H&A had a decent chance of producing respectable patterns with modern plastic wads. 

    The first tests were conducted with a "square" 7/8 ounce load of number nine shot.  That is 7/8 ounce of shot and an equal volume of powder. That works out to 62 grains of black powder.  The over powder wads consisted of a 20 gauge shot card followed by Winchester AA 28 gauge plastic wad. A 20 gauge shot card was used to hold the shot in place.   The first two shots produced doughnut shaped patterns with a gapping hole in the middle of the pattern.   The pattern shown at the top of this article was the first shot with the Winchsteer AA wads.    After two shots, the pastic wads were abandoned.  So much for the therory that  the rifle barrel would preform like a Briley defusion choke. 

    The next load tested consisted of a "square" 7/8 ounce load of number nine shot with paper wads.   The closest fitting wads which I had were a mix of 20 gauge nitro cards and over shot cards.  The nitro cards were difficult to get into the bore of the 58 caliber gun. So, initially four over shot cards were used as the overpower wads instead.  A single 20 gauge shot card was used over the shot.  Testing was done at a distance of twenty yards from the patterning board.  Much better patterns were produced with the paper wads.   Here are the patterns:

10"        79
10-20'   127
20-30"  136

10"        82
10-20"  146
20-30'   104
10"        71
10-20'   142
20-30"  128
10"        65
10-20'   142
20-30"  112

    Encouraged by the results of the  7/8 ounce test, a square one ounce load (68 grains of powder and one ounce of shot) was tested.  Because I was running low on over shot cards, two nitro cards were used as the over the powder wads in place of the four over shot cards. The twenty gauge wads measure approximately .635 inches in diameter. The fifty eight caliber bore is obviously .58 inches in diameter.   In order to get the 20 gauge nitro cards into the bore, it was necessary to fold the nitro cards into a  "V" shape and force them into the bore one at a time.  Even with the over sized wads, fairly good patterns were produced with the "square"  one ounce loads.  Here are the patterns:

10"       90 
10-20'  178 
20-30" 160  
10"        94
10-20'   174
20-30"  158
10"        74
10-20'   144
20-30"  157
10"        56
10-20'   177
20-30"  125

    The patterns produced by both the 7/8 and 1 ounce loads were quite acceptable for a cylinder bore gun.  Either load would produce perfectly acceptable patterns for shooting skeet.  The velocity of these loads should be approximately 1050 feet per second, which would be adauate for shooting small game like doves.  

    Admittedly, the results from the Hopkins and Allen underhammer may not be indicative of other rifles.  This gun was selected for testing because of its shallow rifling and slow rate of twist.  A muzzle loader with deeper groves may well produce different results.

   A final thought about using a rifle with shot loads. Rifles are generally designed to shoot at stationary targets. They tend to be front end heavy and swing like a fence post.  Just because you can shoot shot charges in the guns does not mean that the gun is particularly well suited for shooting at flying targets. On the other hand if you already have a muzzle loading rifle and want to try shooting a muzzle loading shotgun, drop so shot down the bore of the rifle and give it a try.