Once again I come in search of the truth. This time about chokes. I understand that a specific choke (Full, Mod, etc.) is not a given size for a particular gauge. Rather it is the constriction from the bore diameter.
Therefore, it follows that a barrel backbored .040 would have a .040 larger diameter at the choke than a non backbored barrel for a given choke. Correct?
I also am lead to believe that tighter choking works by presenting a smaller cross section of shot to air resistance at the instant it leaves the barrel. If this is true, shouldn’t smaller gauge guns shoot much tighter patterns for a given choke constriction? Non backbored guns should shoot tighter patterns than backbored ones. This doesn’t seen to be the case in my limited experience. Something does not stack up here.
Taken to extremes, we should all be shooting cylinder bore .410’s. What am I missing?
Thanks for all your help.
I MUST have covered most of this before, but either way, it isn’t your fault. Pull on your boots. Here comes a ton.
You make some very logical assumptions, but the conclusions (as you rightly suspect) are incorrect. The choke in a shotgun is the part of the barrel that has a constriction in bore diameter (unless it is Cylinder Bore, which is no constriction). The amount of constriction influences the pattern. Pattern is generally measured as the percentage of the shot charge that falls within a 30″ circle at a distance of 40 yards (except in 410). The resultant pattern gives the name to the choke, NOT the other way around.
Example: Your 12 gauge bore measures .725″. Your choke measures .705″ or .020″ points of constriction. In YOUR barrel, using YOUR shell, that combination puts 60% of its shot into a 30″ circle at 40 yards. You have produced a Modified pattern and can reasonably say that your choke is “Modified”.
Indeed, .020″ constriction is generally considered a “modified” choke. But it really isn’t. In the overbore barrel you mention it might not be. The designation of .020″ as a “modified” choke is really only for the convenience of the choke makers.
If you take the same setup of barrel and choke as above, but use a different shell and THAT combination throws a 50% pattern (this is quite possible), you are now throwing an Improved Cylinder pattern and have every right to call the choke an “improved cylinder” choke, even though with a different shell it will shoot a “modified” pattern of 60%. With another entirely different shell, perhaps one of special buffered construction, you might be able to squeeze a 70% pattern out of that .020″ choke. That is technically a Full choke performance and would entitle you to call that choke a “Full” choke.
So, here we have a choke of .020″ constriction which is producing Modified, Improved Cylinder and Full performance, depending on the shell. No one ever said that shotgun shooting was a precise game or that the gun makers made it easy for you. I wont’ even get into dram equivalency vs muzzle velocity here.
Again, the bottom line is that a choke’s designation has traditionally depended on its performance, not on its numeric constriction. I know that this sounds funny, but that is the truth. It stems from the days of custom solid chokes (remember those) when many parts of the barrel were known to influence the pattern, not just he choke at the front. Depending what was done with forcing cones, barrel diameters, choke lengths and tapers, you could do a lot of pattern influencing apart from actual constriction changes.
Today, with screw chokes being so popular, a few of the choke makers in the industry are trying very hard to standardize a certain number of points of constriction as being a certain choke, without direct reference to pattern performance. Briley leads the pack in this with the following constrictions down from 12 ga bore size in his nomenclature. It is interesting to note that he uses the same constriction descriptions whether the bore is a standard .725″ or a “backbored” .740″.
.000″ = Cylinder Bore (40%)
.005″ = Skeet or Skeet I (45%)
.010″ = Improved Cylinder (50%)
.015″ = Skeet II or Light Modified (55%)
.020″ = Modified (60%)
.025″ = Improved Modified (65%)
.030″ = Light Full (70%)
.035″ = Full (75%)
.040″ = eXtra Full (80%)
I have also inserted the generally, more or less, sometimes, pattern percentages expected from these chokes with normal shells in normal guns. I am definitely not saying that every gun with an .040″ XF choke stuck in it can toss an 80% pattern (mine won’t) , but some undoubtedly can with the right shell.
The list also has some trouble in that the first .005″ of choke usually has more effect on pattern than the last .005″. Still, we have to sacrifice a bit of truth on the alter of symmetry. As a general rule of thumb the list is sort of in the ball park.
Now as to your second question of “chokes vs gauge”, or perhaps it is better said “chokes vs bore diameter” so that we cover backbored guns too. Chokes are best thought of as a percentage constriction of the bore. That .020″ choke in a .725″ bore is a certain percentage. It WOULD seem consistent to assume that a 28 gauge with its .550″ bore would get the same pattern performance with a constriction of a similar percentage.
So, if dropping from.725″ to .705″ results in a “modified” percentage constriction of 5.44% of the barrel’s interior diameter, then setting up your 28 gauge .550″ barrel with a choked of .535″ or .015″ would also produce about a 5.42% constriction. As a matter of fact, .015″ constriction IS generally considered “modified” for the 28 gauge. Hey, it actually worked!
Again, I want to emphasize that it isn’t points of constriction or % of barrel ID constriction that counts. It is the resultant pattern that names the choke. Whatever choke gives a 60% pattern is technically a Modified choke. Fortunately, for the sanity of us all, certain constriction generally, often, sometimes, produce more or less certain patterns.
410 bore is a different animal. Due to its long shot column it tends to distort a higher percentage of its shot than the squarer shot columns of the larger gauges. I believe (am not absolutely sure and don’t feel like looking it up after all this typing) that 410s are generally patterned at 25 yards into a 20″ circle. A 60% pattern under those criteria would indicate a 410 modified pattern. .020″ is often considered a 410 Full constriction.
And finally, at last (but I am paid by the word), we can address your question as to why smaller bores don’t shoot tighter patterns even though they introduce the shot column to the air in a tighter column to begin with. There are a lot of reasons (many having to do with pellet distortion), but the main one is that pattern size is more a product of the initial vectoring of the pellets than it is of the width of the shot column.
The reason that Full choke constriction produces tighter patterns than Cylinder bore is because the full choke taper changes the direction of some the shot MORE than the open choke does. In stead of leaving the barrel in a straight line, the full choke influences the shot at the outer part of the column to vector slightly inward. Air resistance soon counteracts this and causes the shot to eventually go outward, but it goes outward LESS than it would have it had no choke to begin with. That makes the pattern tighter. That is also why the sky is blue.
Please note that I have intentionally made some mistakes in my math and logic in the above reasoning. This is strictly to provide all you Junior Technoids out there with something to do in your spare time. You all know that I would never, could never, make a common error.
Enough on this. Boots off, beer open.
Shotgun Report’s Technoid
(Often in error, never in doubt.)