Shotgun patterns made simple - all you need to know in one page!

  • Shotgun patterns (to a first approximation) are really quite simple - on average they follow a 'bell-shaped' distribution as illustrated below.

  • On average the pellet density in the centre is always (see caveat!) greater than at the edges of the pattern. The average pellet density gets progressively less dense as distance from the centre of the pattern increases (see why this is good). At the outermost edges of the pattern the average pellet density is so low the odd pellet that falls there is often labelled as a flier. It isn't! It is a part of the pattern just as much as any other pellet.  

Above: Illustration of theoretical pellet density versus distance from centre of pattern.

Right: Real shotgun pattern showing density of pellets versus distance from centre of the pattern. Bar charts show real pellet density, smooth line shows the theoretical expected density.

 

As a shooter there are only two things you need to know about this bell-shaped pattern:

  1. How wide the 'bell-shape' distribution is: The width, or spread (along with the pellet count) determines the likelihood of a pellet strike on the target versus the position of the target in the pattern. The spread should be matched to the target so that the centre area of the pattern has a very good chance of hitting the target without excessive pellet strikes and the area over which this is achieved is maximised. There are no 'special' distributions that can give an 'even' or uniform pellet coverage. In a given gun and with a given choke, different shells can give different average spreads. 

  2. How repeatable each individual shell is compared to the average: Having found the best pattern width for your needs, you want a shell that delivers this every shot. If the pattern is sometimes wider, gaps in the centre appear. If it sometimes gets tighter you are denied pellet coverage. Repeatability of pattern width in a shotgun is similar to the shot-to-shot accuracy a rifle shooter looks for. Different shells have different levels of repeatability.

That's all there is to it! Determine the pattern width you need, then find the shell that does it best every time. Easy!

Shotgun-Insight helps you by:

  • Detecting and counting the pellets on a target.

  • Calculating the pattern width.

  • Giving the probability of a pellet hitting the target to help decide if the pattern width is too wide or too narrow.

  • Allowing targets to be averaged to allow the variation from shot to shot to be calculated.

  • Storing results for subsequent analysis or comparison.

  • Printing-out the results.

Shotgun Patterns - 2nd order effects.

Most shells measured will appear to follow the "bell-shaped" distribution closely. However, the pellet distributions are actually a "sum of bell shaped distributions". Each component of the shell - the crimp, powder, primer, pellets, and temperature etc will be subject to shell to shell variations and contribute to the overall shot to shot variation. This is a fact of any manufacturing process or manufactured product. 

Some shells behave very differently with temperature variations. You should either pattern at different temperatures and understand how the spread varies, or, use a shell that is affected less by temperature variation.

In practice it is the inherent variation of the cloud of shot that dominates the spread. This is what gives the spread we expect of a shotgun and it is this spread of shot that on average follows the bell shaped distribution.

However, there is a subtlety even to this that may be very pertinent to shotgun shells. Imagine a shell with a very long shot column: Pellets at the top of the column will have one set of pellet to pellet variations causing them to spread following a certain bell-shaped distribution; Pellets at the bottom of the column may get much more deformed to the extent that they follow a different distribution, i.e. a wider spread than those at the top of the column. Careful analysis of ratios of pellet counts between the outer and inner regions of a pattern and between different shells or forcing cones will quantify the effect of pellet deformation at the base of the shot column. To date I have not found any consistent difference between 24g and 28g shells with their different shot column heights, at least not from testing at 25yds. Long range (40yds) testing will take place in due course.

 

Why the bell-shaped distribution of pellets is good for shooters!

People often talk of 'even' pellet coverage. Shotgun patterns are not even, they are bell-shaped! And this is helpful because:

  • For sporting clay shots where the targets vary, having a 'hot-spot' in the centre of the pattern allows the shooter to see if the pattern is in front of or behind the clay. If the pattern was perfectly even then the clay would break the same if it was right in the middle or right on the edge of the pattern and the shooter would get no visual cue as to how to adjust for the next shot. The higher pellet density in the centre of the pattern allows the shooter to home-in on the amount of lead needed to centre the pattern on the target thus giving the maximum margin of error on subsequent shots;

  • As the range increases the pattern opens out but the centre still has enough pellets to give a good chance of a hit. If the pattern was 'even' at close range, then gaps in the middle of the pattern would appear at longer ranges.

Centre density caveat: This may not be true for 'spreader' chokes or when shot spreaders have been added to the wad. 
     
 
 
   

(c) Dr A C Jones