Mike, (long post warning...but please read, I think it will help you a lot).
You really need to read up on how shotguns work...they are WAY more technical than you would ever suspect. More specifically how shotgun shells work, what happens inside the barrel, etc.
If you pick up WW Greener's classic book “The Gun and it’s Development”,
published in 1881, you’ll see listed standard chokes…
They’re the exact same measurements as they are today. Yet the ammunition we use today is like comparing a bicycle to the space shuttle. They were using black powder loads with very soft lead and felt wads. The patters these guns made with that ammunition were just horrible by today’s standards. As a result, the shooters were REALLY FREAKING GOOD, because they completely ignored all but the absolute center of the shot column; counting on it to do all the work. In essence the “usable” shot coming out of a 10 bore is comparable to the usable shot from a bargain basement 28 bore load from today’s ammunition. Yet the “chokes” haven’t changed at all.
Since then we have smokeless powder with a WIDE variety of burn rates that are much easier on the shot at the base of the column. We have collapsible plastic wads with fingers that sometimes extend the full length of the shot column. These wads protect the shot from deformity in ways Greener couldn’t even fathom in his day.
So when you take a modern shell with a long plastic wad, slow burning powder, and hardened shot; there is about 80% less shot deformity vs. shells from the 19th century. But then you shoot that shell down a barrel with the same full choke they used in 1880, and all that beautiful technology gets trashed in the last ½” of the barrel as you squeeze it through that tight constriction. I’m telling you…Anything tighter than a modified choke with modern loads, the only thing it’s doing is trashing your pattern. That’s because American’s when they choose a choke, they chose a constriction measurement, when choke is actually NOT a physical measurement of the bore, but a PERFORMANCE measurement on a pattern board.
The most successful competiton shooter in history, Mike Digweed uses a 34” Perazzi over & under with fixed full choke barrels (yeah, he’s THAT good…shoots ALL shots on ANY sporting clays rand with fixed full chokes). But his “full choke” barrels are “full choke” by PATTERN, not by measurement!!! And that differentiation is VERY important. His barrels are over-bored so by measurement I’ll bet his muzzle diameter is pretty close to what we would “measure” out at Skeet or IC.
So what is stamped on the barrel is rather irrelevant to what's actually happening with your shot pattern. "Choke" when done as a physical measurement isn't really the measurement at the end of the barrel...it's a measure of constriction relative to the diameter of the bore. So if you have a shotgun with a bit of overbore, then ALL of your chokes are going to produce tighter patterns that what the choke says. Many shotguns are over-bored or "backbored" these days, making their choke markings completely meaningless...yet the shooters still think they mean something.
The ONLY true measurement of “choke” is shooting your gun on a pattern board. And from load to load you can have pretty big changes in “choke” (pattern) without ever changing ANYTHING on your gun!
That's why you're going to find (through shooting, not by reading or internet research), that your IC is probably perfect for most jobs you'll ask your shotgun to do. You can shoot skeet just fine, and you can break clays out to 40 yards just fine. I use a skeet choke when hunting phesant when "everyone" says you have to use modified or tighter along with a magnum load to bring down big birds. No, you have to hit them squarely (more pellets into the bird), that's how you bring them down. You get all these different “choke” effects just by changing your load. Out of your IC barrel, a cheap Wal-Mart promotional #7.5-#8 shell may produce something approaching actual IC patterns…or could even be a little closer to skeet choke. This is because those shells are made very cheaply (that’s okay, there’s a place for “cheap” shells). They use a plastic wad that actually have no fingers, just a little plastic “disc” to separate the shot from the powder. And they use very fast burning powder for maximum velocity (ever notice those shells tend to kick a bit?), and finally pretty soft lead. This all adds up to a lot of shot deformity down the barrel, which opens up the pattern a great deal. The pattern will be open and there will be gaps in the pattern. This actually makes a downright decent practice load for skeet shooting, since skeet is pretty short range.
Now choose a Winchester AA competition trap load out of that same barrel. Same gun, barrel, “choke” on the barrel, and you’ll be producing patterns that are tighter than modified, approaching full choke pattern. And it will be a very uniform pattern. Now your “IC” gun is shooting like a trap gun and smoking clays at 40 yards…It’s a miracle!!
When you use a gun that's way over-choked you end up deforming much more shot, because you're squeezing that shot as it exits the barrel. That may have the center of the column going tight, but the outside is blown all to hell, and you have huge gaps in your shot pattern…gaps large enough a big rooster pheasant can fly right through! You have a "tight", but ineffective pattern.
Soft shot will deform as the shot on the outside of the column rubs down the barrel. When it leaves the barrel, it's flat on one side, and it will fly wide. The more shot that deforms down the barrel, the wider and more erratic your pattern will be. Higher quality loads will use plastic wads with longer/thicker fingers to protect the shot for its trip down the barrel, and they use chilled shot that is much harder. With a first rate trap load, you can have near full choke patters from an IC barrel, and the only thing you did was put a different shell in the chamber.
By using a more open choke that's more fitting for today's modern loads, one that deforms less shot, you end up with a much more uniform pattern...one that birds can't fly through. And because of that high quality shot and miraculous plastic wad, your choke (patterned on a board, not measured at the muzzle) is actually much tighter than IC, even though that’s what’s stamped on the barrel.
So your pheasant load shot through your more open IC barrel may have a more open overall pattern than it would be through a “full choke”…., but it's a much more effective pattern because there are no gaps, and more shot finds its way into the bird! Your IC barrel produces a tight, but very high quality choke. (am I making sense, or just losing you?). Where your full choke barrel has a VERY tight center section, but anything outside that center section has a lot of gaps in the pattern, making nearly 50% of the shot almost useless.
I gave up my interchangable choke competition gun, and now I shoot an over/under with fixed Skeet/Skeet chokes. I have shot skeet, sporting clays, trap, doves, and phesants with that same gun, using those same barrels; and never missed because of the gun (always the loose nut behind the trigger). When I shoot trap with my Skeet chokes, I just switch to Winchester AA trap and my patters tighten way up from my normal cheapie promotional loads. I have taken a 2 ¾” 20ga (old Browning Auto 5) with a 28” fixed IC barrel on a pheasant hunt where everyone else were using 3” 12ga magnums. They all laughed at my gun/cartridge choice, but I limited out VERY early in the hunt, and ended up filling tags for two of the other guys who kept “feathering” the birds rather than killing them. Most were shooting copper plated magnum loads (that’s really hard shot) through modified or full chokes. They were WAY over-choked and it was blowing the patterns. Would have worked just fine as a turkey gun/load, but was a nightmare for pheasant.
My recommendation is...Go to a sporting clays range, drop $100.00 to spend a couple of hours with a professional instructor...I promise you, it will be the best $100 you ever spent in your life for your shooting hobby! It will blow your mind how much you learn. What blew my mind was how much I was doing wrong...and I'm a gunsmith...I'm supposed to know it all...yeah right!!
The best book I know of on the more technical aspects of how shot shells work, and behave is Shotgun Technicana by Michael McIntosh...but that book is out of print and EXPENSIVE these days!!
Terry Weiland is pretty good, and you can get this book cheap used. Or pick up an old/used shotshell reloading manual (whether you load or not). You’ll find a treasure trove of information in there about how shot shells work, and all the neat things a reloader can do to get different things to happen with his loads.
https://www.amazon.com/Gun-Digest-Shoot ... 1440234639
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