This is a very informative and succinct guide to shotgun chokes.Do take time to read it.
http://www.thefield.co.uk/features/4334 ... choke.html
Gun barrels
The Field guide to shotgun choke
By Mike Yardley of The Field
Tuesday, 15 December 2009
How to make the most of shotgun choke...a how-to guide from The Field
Choke is the constriction at the muzzle end of the gun that tightens the pattern of pellets (of which there are about 300 in the average cartridge). Choke gets people thinking. One should never forget, though, that most misses in the field are not down to the wrong choke (or cartridge), but attributable to the gun's not pointing in the right direction. It is possible, indeed, quite common, to become neurotic about choke. Sufferers of such "ballistic thrombosis" may become paralysed by considering it obsessively.
Choke is one of those things, like gunfit, that should be visited occasionally and put out of mind once an informed decision has been reached concerning what best suits your needs.
With that on record, let us move forward. You should take your gun to a pattern plate (or improvise one with paper or card sheets and a suitable frame and safe back-drop) and shoot it at different ranges - 20yd, 30yd and 40yd - using the cartridge you prefer. You hope to see an even pattern without too many clusters, gaps or excessive central concentration.
If there are holes a bird could fly through - a 5in circle test is sometimes applied - or if the pattern is obviously too tight, your gun and its chokes may be working against you. Once you have tested with your usual ammunition, experiment with different cartridges. You might, for example, try to observe the terminal effects of switching between fibre and plastic wads (the former often throw more open patterns) or increasing pellet payload (which may be an alternative to increasing choke). If your gun has multi-chokes, try different tubes.
REVERSE CHOKE
Sportsmen develop strange prejudices concerning choke. My approach, and I happily admit to passing through the stage of confusion, is practical. I have discovered what works for me in different situations and now stick with it. For general game-shooting, I like a bit of choke in the first barrel but not too much - it is the first few thou that makes the most obvious difference. A slightly choked barrel is much more efficient than a true cylinder and inspires confidence, too.
Many 12- and 20-bore game guns are over-choked for their task. Tight patterns may be a means to cleaner kills at longer range but they are an impediment at shorter distances because they demand more accuracy.
There seems to be something in many sportsman's psychology that erroneously suggests more choke good, less choke bad. If you are going out on the average driven day or walking-up, you do not need much choke in a 12-bore. The first few thou does make a real difference; thereafter the law of diminishing returns takes effect. Those who can see shot will confirm this. You can often observe what looks like a tennis ball-sized cluster of shot moving past the bird at short range. I have seen this many times and thought: "It's much tighter than I expected, one might as well be using a rifle."Some years ago, I put together what came to be called my "duffer's gun" based on an old, plain-Jane Beretta Essential over-and-under.
The initial idea was to create a workhorse without regard to aesthetics that would be as forgiving to shoot as possible on normal days. It was based on an over-and-under because, though I love side-by sides, over-and-unders are usually easier to control and easier to point. The Beretta action is supremely reliable, moreover, and the Essential, though a budget gun, had livelier barrels than the average because it dispensed with side ribs.
The gun was a multi-choked model and this allowed for much experiment with chokes at the pattern plates and, later, in hides and on the shooting field. After some months of ex-periment, I determined that I had the most consistent first-shot success with something called a Seminole spreader choke. This device is made in the USA. It might be described as a reverse choke: it has a section that extends from the muzzles and trumpets out to a greater size than the bore.
The form of this section is conical. The concept of reverse constriction is not new. In the muzzle-loading era, before the general adoption of choke boring, many guns were "relieved" at the muzzles because it was found that they shot better than a true cylinder. My experience would seem to confirm this; the Seminole choke still works on clay birds 50yd out, yet it is very forgiving close in.
The second choke that worked really well in the field - in that it was effective and forgiving in use - was a standard Beretta Improved Cylinder Mobilchoke tube. This is a conventional choke with about five thou of constriction. With the duffer's gun, I once accounted for 18 average pheasants for 17 shots. They were not testing, but it really was quite difficult to miss with it. I have since lent it to friends in distress and they have always shot it better than other, more traditional weapons. I have had similar unnatural success with another open-choked Beretta over-and-under using mid-velocity, heavy payload cartridges (11⁄4oz, No 6).
The gun and the cartridges were lent to me in Italy. It was extremely effective on easy birds but the experience was notable because the 36g cartridges had a lot of shot in them but did not recoil excessively (the lower velocity, heavy-payload cartridge was explored by the wildfowler Dr Charles Heath years ago).
LET'S GET TECHNICAL
Does this mean that everyone should open up their chokes? No, not unless one is shooting at close- to mid-range birds routinely. Choke can certainly be useful when shooting at longer range though its effects break down at extreme range and if birds are especially tough - such as wild guineafowl in Africa. A bit more choke than is really required may also in-crease confidence - no small factor in shooting - and give one the sense if not the actual ability to pick one's birds better. If your confidence slips because of concerns about choke, or anything else, your focus may come off the bird and your movements may be hesitant (resulting in misses behind).
What about high birds?
Nigel Teague, a man who has ex-perimented more with choke than perhaps anyone else in Britain today, advocates 7⁄8ths of choke - about 35 thou - in both barrels for the really tall stuff. This concurs with my high-bird experience where I have found three-quarters and three-quarters works well in a 12, better than full and full. With many modern cartridges optimum pattern performance requires less than full constriction; excessive choke can blow a pattern.
Many foreign guns, especially small bores, may be ridiculously over-choked. This stated, I think 20s, and, especially, 28s perform a bit better with a little more choke than I would advocate for a 12. My 30in Beretta EELL 28-bore, for example, shoots particularly well with two three-quarter chokes fitted (about 20 thou constriction in a 28).
Although, one can try to state general principles concerning choke, I find that some guns just seem to shoot well with a particular constriction and there is no real science - none that is available at least - to support why this should be.
Shot-gun ballistics are much more complex than one might think because there are so many variables: atmospheric con-ditions; shot size; shot density; shot coating; wad, primer, powder and case type; barrel diameter (a nominal 12 might be anything from .710 to .740 in internal diameter) and internal geometric form; barrel steel and wall thickness; and, not least, the length and form of the choke constrictions themselves. Some chokes are short, others long. Some are simple conic constrictions, others have a cone that leads into a parallel section, yet others have complex forms, including features such as radiused walls, relieved sections or expansion chambers.
While we are getting technical, let me note that tight chokes increase pressures, and hence velocity. A point of choke is worth about 1ft per second on velocity.
As barrel length has a small effect on velocity too - about 5fps per inch in a 12-bore - this may become more significant when extremes of choke and barrel length are combined. For ex-ample, it is interesting to note that a 32in full-choke gun, might have a velocity as much as 100fps faster than a 25in open-bored one, all other things being equal.
Most intriguingly, constriction of the muzzles also has the effect of reducing the stringing of shot once it is significantly forward of the muzzles (just forward of the muzzles there may some elongation of the shot column, but the terminal effect of choke is to reduce the length of the shot string and thus improve its efficiency). This may seem counter intuitive but it was neatly demonstrated by Mr Griffiths of the Schultz Powder Company more than a hundred years ago by means of shooting choked and unchoked guns at a spinning disc. The results were published in The Field, like much else concerning choke and shotgun ballistics in the Golden Age.
CHOOSE, THEN FORGET
Cutting to the chase and avoiding the danger of getting too complicated, my all-round choice in a 12-bore game-gun would usually be improved and half or improved and three-quarters (a useful choking if combined with the instant selection of a double trigger). I would not argue with those, such as my friend and former Olympian Kevin Gill, who advocate quarter and half for all-round shooting. (Kevin shifts to half and three-quarters for higher birds.) My rationale is that I like to engage average birds instinctively but it is also good to have the option of a more precise approach at range.
For high birds, two tight but not extreme chokes are in order (teamed with a high-performance cartridge; the choke may never be separated from the cartridge used with it).
For pigeon, quarter and quarter or half and half usually works well. For smaller bores my preference is a bit more choke than commonly advised. I have to say, though, that I have not a clue what is in my 32in Guerini 20s, the guns I use most for game. I put the chokes in some while back after playing at the plates and have not looked at them since. They work.
Measuring Choke
Commonly, one refers to the choke in a barrel as being true cylinder, improved, quarter, half, three-quarter or full. Gunmakers talk about "points" of choke. They measure choke relative to the bore diameter (which may vary considerably within any designated bore size rather than at the muzzle alone).
One point equates to a constriction of one thousandth of an inch. Below is what one would expect in a 12-bore gun.
True Cylinder 0-1 points
Improved cylinder 3-6
Quarter (American Improved) 8-12
Half (American Modified) 17-23
Three-quarters (Improved Modified) 25-30
Full 35-40
Super full 40+
These descriptions should not be appraised in isolation of their observed effects, though. Properly considered, choke concerns the number of pellets any given barrel/constriction throws into a 30in circle at40yd. The quality of shot, the type of wad and other factors such as precise bore diameter and the form of choke - short or long, simple conic or conic cone plus parallel section (the favourite of British gunmakers) may all be significant.
Percentage of pellets inside
30in circle at 40yd
True Cylinder 30-40
Improved 50
Quarter 55
Half 60
Three-quarters 65
Full 70-75
Super full 76+
Choke can be definitively determined only at the pattern plates and in relation to a specific cartridge. Measurement of constriction alone can be misleading. In days past, a gunmaker would always ask his client what cartridges he intended to use and then regulate the chokes according to the desired percentage. If the client opted for the gunsmith's own brand, he would have to continue to use the gunmaker's cartridges to ensure consistency of performance.
Best-
Vikram
