Shotgun patterning is finding the spread of a specific load in a specific shotgun and has been described as being akin to zeroing a rifle. It's important to know how your shotgun will behave at various distances with the loads you use. Assuming I own a single shotgun the practical application of shotgun pattern makes a lot of sense to me. I would map out the longest distance inside my home (say 15 yards) and pick out a handful of the best Dentist-Approved loads such as FC Buck 1. I'd set a target to 15 yards and pattern with each and see how my particular shotgun works with that load. Then I'd stock more of that load and keep my single shotgun loaded with that specific ammo.
Where I struggle is how to apply this concept to owning multiple shotguns and possibly using them outside a pre-determined location, where your max range is unknown. This post has various thoughts that go through my head about this topic. Please pick any individual points that you'd like to discuss.
If I own a dozen AR15s, each with different barrel lengths and barrel twists, I can zero them all at 50 yards and be pretty confident I will hit my target out to 220 yards or so. I wouldn't have to memorize which gun acts in which way with a specific load because the zero of the gun takes care of that. I know at 50 yards all dozen ARs will hit the same point and I know at closer ranges all dozen ARs will need to be held a bit high to compensate for height-above-bore dynamics, and I know the general trajectory will be an arc with the first zero being at 50 and the second zero being somewhere in the range of 200. There's no complex math I have to remember for real-world carbine distances. If I was shooting targets at 300+ yards then it would require some math, but those situations are unheard of for a civilian outside of a marksmanship shooting drill, where you will have time to perform holdover calculations.
Load Selection
With shotguns, I don't have a good understand of the practical nature of patterning when you own multiple and may use them at unknown locations/distances. Suppose I own a 14" SBS, a few 18.5" pump guns, and a 20" pump gun. One strategy could be that I buy a few dentist-approve loads and fire them through each gun, and then pick the load that has the tightest possible pattern for that gun, and that's what I use for that gun. This takes care of the unknown distance outside-the-home situation because I know I've optimized each gun for maximum distance. Hopefully, all guns will pattern tightest with FC Buck 1 for simplicity reasons and because as of 2019 it's considered the most effective load.
Suppose FC 00 patterns tightest with a particular gun, then I have to decide if the tighter pattern compared to FC Buck 1 is worth the decrease in effectiveness. The decrease in effectiveness is probably marginal, so if 00 patterned tighter out to an extra 5 yards, then it might be worth the tradeoff. I don't have enough real world experience to know how to perform this calculation.
Shotgun Use Factors
Another factor to consider is if the gun is meant to be a dedicated home gun, and it's a short distance, such as an apartment, perhaps you want a load that isn't as tight. For example, suppose your apartment max distance is 10 yards for a straight shot. And suppose FC Buck 1 patterns to a 1" hole at 10 yards in your particular gun. Perhaps switching to a load that patterns out to 3" might be more effective in this situation. Question for the SMEs: is there a pattern size that is considered optimal? I believe everyone agrees a pattern that's too big is suboptimal, and I believe at least some people argue that a pattern too small is also suboptimal if it doesn't expand at all. Is there a happy medium, perhaps 3" where if you were sure the distance was relatively short, then might pick a load with extra spread so it opens up at least a little bit? Or is it always better to have the smallest spread possible, even if that spread is zero and it's a sub-1" pattern?
If patterning optimization isn't just for smallest possible pattern, do you stock multiple loads for each shotgun, such that Load X would give you a nice 3" pattern at 10 yards (max apartment range) and Load Y gives you only a 1" pattern at 10 yards but stays tighter out to 25. So you keep the gun loaded with Load X for apartment defense, but still have some Load Y on hand in case you make a conscious decision to go outside with it for whatever reason and if you have time to swap loads (let's ignore the reasons to go outside because as a civilian defender I think those reasons are extremely limited).
Do you have to remember or keep a notebook of how each of your different shotguns patterned with your specific load of choice? Do you make a note on the buttstock or anywhere on the gun itself? Suppose each of your half dozen 18.5" pump guns patterned the same load differently. Some of them kept FC Buck 1 tight to 25 yards. Some were only good to 20 yards. Do you make a note on the gun itself "this one is good to 25" or do you go with lowest common denominator and say "I will never shoot any of my 18.5" pump guns past 20 yards because even though some can pattern tight to 25, a few are only good to 20, and I can't expect myself to remember under stress which is which, so I'll max out at 20."
Does patterning even matter that much because if you need to take a life-or-death shot and the threat is at 25 yards, are you (a) going to be able to estimate distance under stress and (b) going to care if you might throw one or two fliers given the only other alternative is surrendering and possibly getting murdered? Assuming you don't have slugs on board for a slug select drill. Or is patterning only meant to be considered in advance, such that you will only choose a gun and load for defensive purposes that patterns out to the proper distance of your home? So no real-time decision making is necessary? This doesn't apply to LE who might be engaging suspects in unknown locations at unknown distances.
Training Considerations
It seems that a distance drill might be useful for LE where you are outside on a course of fire at unknown distances (to the shooter) and you have to determine if you can make the shot and pattern appropriately on target. Perhaps with an extra layer that different targets have different acceptable pattern misses. For example if the backstop is a preschool, then you might be less willing to take the 30 yard shot than if the backstop is a secluded mountain. The COF would have to take that into account. Is there any drills like this that exist so I don't have to reinvent the wheel?
Final Thoughts / Questions
In sum, I understand that patterning is useful to select the proper load for your specific shotgun, because I hear SMEs say that even the same shotgun length barrel from the same manufacturer will shoot loads differently. What I don't understand is:
Given the most ballistically optimal dentist-approved load (currently FC Buck 1), how do you decide on using a ballistically suboptimal load to get a better pattern?
If short ranges are guaranteed, due to being an apartment defense situation, do you ever want to use a load with a bit more spread under the limited ranges of that home?
How do you track patterning all of the shotguns you own and how do you use that information? Just to select the load and forget about it? Or do you remember max distances you can shoot each particular shotgun? Or do you take the lowest common denominator and never shoot any of your shotguns past that distance to be safe?
How do you drill distance recognition and whether your known pattern will be effective at that distance?
Are slugs always necessary to have on board in a side saddle or buttstock pouch to allow for slug select drills if the distance to threat is further than the pattern allows? Possibly different answer if you live in a home with fixed distances (maybe no slugs needed) versus if you live on a farm and might be outside or are LE and serving high risk warrants (maybe always need slugs on board)?
Perhaps I need to analyze why I own multiple shotguns to better assess how to handle patterning. I only need one home defense shotgun. The rest are spares. So perhaps it only really matters how the primary home defense shotgun patterns within my current home, and then I pattern the rest to keep on file in case they become the primary shotgun.
What other practical application uses of shotgun pattern didn't I cover?