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Up-Tight Overchoking

Up-Tight Overchoking by Derek Partridge

Most shooters, especially clay busters and live pigeon shooters, are aware of the standard method of assessing choke, but to recapitulate briefly, choke is the degree of constriction between the diameter of the barrel, just beyond the chambers and forcing cones, and the diameter at the muzzle.

The resultant patterns produced are graded in terms of the percentage of pellets from the original shot charge, which find their tortuous way on to a sheet of paper forty yards away and into a thirty-inch circle circumscribed upon it. Full choke has 70 percent, improved modified 65, modified 60, improved cylinder 50 percent and cylinder 40 percent. It often is stated erroneously that cylinder indicates a complete lack of constriction. English barrel-chokers found that a barrel totally devoid of choke produces uncontrolled and widely varying patterns. Therefore, almost surreptitiously, they put .003 to .005-inch constriction into the guns of customers, who specified true cylinder. For new scatter gunners, the simplest analogy to describe the practical effect of choke is to liken it to the attachment on the end of a garden hose. When fully open, it allows the water to spray out in a wide circle over a short distance. The more it is tightened down or constricted, the smaller the circle becomes – but the farther it travels. Being a little more precise, choke allows us to present the target with approximately the same size “circle” filled with approximately the same amount of shot at different distances.

However, there’s a great deal more to choke than the necessarily simplified explanation above. It is fair to say that no one exists, who knows all the factors which can affect choke and produce good and bad patterns from seemingly identical sets of dimensions and loads, in different sets of barrels. This may seem surprising in view of the highly advanced techniques and testing apparatus available to the world’s leading gunmakers.

Slight surgery on your gun barrels can change your way of life – and scores!

A number of today’s expert American gunsmiths have devised various machine methods of improving the performance of standard factory barrels, for the cost of hand-finishing a mass-produced gun barrel would be so prohibitive as to price the gun out of its market. One essential is smoothly polished bores. It could therefore be concluded that an improvement over the factory barrel could be made merely by hand-polishing, or lapping in the professional term, the interior of the barrel. Some gunsmiths have taken the improvements a lot farther than just polishing. Names which spring to mind are Herb Orre, Al Ljutic, Bill Atkinson and Doc Cordaro. The strange thing, which I believe all have in common, is that each discovered his particular method by accident! The accident generally was caused by a broken reamer leaving an unwanted configuration in- side the bores which, for one reason or another, they happened to test fire and found to their amazement that super pattern resulted.

I also recall, with fond amusement, another friend and amateur expert, Franz Rotthinger. He is a well known trap shooter on the West Coast and a capable mechanic, who bought my Browning. We shot a great deal of International trap together, in different parts of the country. Between each round, Rotthinger could always be seen, peering down the barrels of either the Browning or his Krieghoff. Out would come a hand reamer, a few deft turns – to improve the pattern or move it up or down, right or left – then off he’d go to the pattern board, reappearing in time for the next round. We used to pull his leg about it unmercifully, but his search for perfection in patterning obviously paid off, for he recently won the state championship of Upper Austria with 98/100 – and in International trap, that’s a darn good score.

I am not a hand-loader, so any questions in that department, I’ll leave to Dean Grennell. Nor am I a ballistician, because to shoot successfully, there must be only one thought in mind while on the line: the next target, and killing it. But – I do have my own approach to the question of choke. What’s this sixty or seventy percent at forty yards nonsense? How may people shoot at forty yards? Why not the maximum possible number of pellets, evenly distributed in a thirty-inch circle at the precise range each of us shoot?

Up-Tight Overchoking by Derek Partridge

Doc Cordaro displays a 98 percent pattern shot at 40 yards during a test pattern session. Shell was a Remington International round with load composed of 3-1/4 dram 1-1/4oz 7-1/5 hard shot.

Dr. Sal Cordaro of the Bronx, New York, told me he could achieve this. To my amazement, I found myself believing in him to the point where I was prepared to put my Perazzi on the operating table and allow the doctor to carry out the delicate, high-precision surgery required.

The fact that the worthy doctor had been an experimental toolmaker seemed to provide reasonably extenuating circumstances for contemplating the experimental vivisection. Doc Cordaro questioned the point of having pellets outside the killing circle. They wouldn’t do anyone – except a consistently inaccurate shot – much good. His aim was to contain as many as possible inside the thirty-inch circle – expanded to about thirty-five inches for ranges over forty yards (the margin of error increases with distance) – and to concentrate utterly on achieving their absolutely even overall distribution.

I was interested only in the final, pragmatic result on my targets, but was prepared to go through all the ballistic theory involved, so I eventually would go out on the line with complete practical and theoretical confidence in the performance of my gun. Many years ago, I realized that shooting top scores consistently started from having such confidence in my equipment that I would never blame a lost target on it.

The factors affecting the creation of an optimum pattern include overall trueness or roundness of barrel interior; length, taper and diameter of forcing cones and chokes; powder charge and resultant muzzle velocity; size of shot, relative to choke constriction and whether lead, hard lead or nickel-plated. Other factors to be considered are the type of target: clays, live pigeon, game birds, length of shot column and, when necessary, the effect of wind.

Cordaro claims that, by altering the dimensions of the above factors to tolerances of precision never before attempted (through his experience in experimental tool making), he could customize the pattern of any gun to the 16-yard man, the 27-yard handicap shooter, doubles, live pigeon or International trap shooter. He would do this only by working from dimensions he had worked out over five years of testing to perfect his method, which is known as Cordo choking.

The gun had earned a reputation of absolutely pulverizing International targets – a second barrel kill being equally as dramatic as the first; more so, considering the greater distance.

A previous accident had left a stepped ridge in the end of a barrel. He hadn’t noticed it and, when he patterned the gun, was amazed to find almost double the percentage he had been aiming for – in the region of ninety percent. Discovering the ridge, he ran tests which showed that the wad was being stopped momentarily by the ridge, preventing it from any possibility of breaking up the shot column and subsequent pattern.

Shortly afterwards, also as a result of an error caused by a broken reamer, Al Ljutic came out with a barrel having a spiral in the end, to spin the wad away from the shot column. Then the protective shot-cup wad and the sleeve came out and rendered the spiral choke obsolete for shells so loaded. They would spin both protector and shot and cause too much pellet dispersion.

Cordaro first watched me shoot to ascertain at exactly what distances I consistently broke targets. Then we patterned my gun with various loads to see how it performed. The gun was patterned to give seventy-two percent lower barrel and eighty percent top barrel. The gun had earned a reputation of absolutely pulverizing International targets – a second barrel kill being equally as dramatic as the first; more so, considering the greater distance. So I thought the pellet density might be too concentrated in one area and that distribution could be improved – along with putting the maximum number of pellets in the circle at the distances I was shooting.

International targets travel 77 to 87 yards – and sometimes farther at a speed of nearly one hundred miles per hour through an arc of ninety degrees. Height variations are between three and thirteen feet off the ground at a point eleven yards from the trap. (American targets have a constant height of about nine feet at that distance).

The average experienced American trap shooter breaks his target at around thirty-five yards and takes from nine-tenths of a second to 1-1/10-th. An International trap a shooter will fire his first shot between 36 and 38 yards, his second between 41 and 45. Average time for a straight-away shot is six-tenths of a second and for an acute angle, seven to eight-tenths.

At our test site, we had a Winchester-Western Continental trap, which gave us all the vertical and horizontal variations and angles, but because we didn’t have any of the harder International targets, was only throwing American targets seventy yards, just short of International minimum.

Cordaro set out wooden stakes at five-yard intervals and retired to observe where I broke targets. First barrel kills were consistently broken at 31 yards and 35 to 38 yards was my breaking distance for second barrel shots. We debated whether to add a few yards to the distances he would work to in choking, to allow for the sightly greater distance of true International targets. (Subsequent tests in Europe on International targets established the distances as 34 and 38 to 40 yards.) Out of curiosity, he had me shoot sixteen-yard American targets and found that I consistently broke them just under ten yards from the trap – a distance of twenty-five yards from the gun, or ten yards nearer than the average trap shooter. To further test the effect of my present choke, he had me shoot the seventy-yard modified International targets from the twenty-seven-yard line. The gun still pulverized targets and, when there were any left, pulverized tiny chips with the second barrel! We moved to the pattern board and fired patterns at 30, 35, 40, 45 and 50 yards with Remington, Federal and Winchester International Trap loads in 3-1/4 dram 1-1/4oz 7-1/2s and 8s and also tried some 3-3/4 dram shells. We used both nickel- plated shot, lead and Federal’s hardened lead. In keeping with what Cordaro had expected from previous years of testing, the Remington showed the least variance, with the Federals a close second.

Up-Tight Overchoking by Derek Partridge

Cordaro’s outdoor test range has earthen backstop in back of patterning board. Patterning of shotguns is done from firm, sand-bagged position at the benchrest.

The 7-1/2s gave slightly higher percentages than the 8s, but so slight that the 8s still delivered more pellets in the circle. The hotter 3-3/4 dram load blew the pattern more open. The blown-pattern effect of a high velocity shell will decrease in proportion to the increase in choking.

However, the most striking feature of the patterning procedure was that my gun consistently threw elliptical patterns, crowding the upper right portion of the sheets with pellets and leaving the lower left markedly bare. Could that be the reason for my more than normal amount of misses on low, acute left-angle targets?

Cordaro explained that this was quite typical of any barrel and noted that the barrels – from chambers, through forcing cones and bore to chokes – were out of true. This can give misleading patterning result. The pellets will be on the sheet and so will give a good percentage figure, but, when you divide the pattern sheet into clock-face segments, the bad effect of elliptical distribution is immediately obvious.

Cordaro now knew what my gun did, where I shot and what his experience told him I needed as optimum patterns for my first and second barrels. His intention was to put more pellets in the lower barrel pattern for thirty-five-yard shooting, just a few more in the top one for forty yards and to improve the even distribution and density of both.

Doc Cordaro treated my Perazzi as an emergency case and burned the midnight oil in the operating shed. Next morning we met again at Bob Gegerson’s Mid-Hudson Winchester Club.

…the pellets had been evenly distributed through-out the killing circle… the whole circle now was equally lethal to any target venturing into it!

I squinted nervously down the barrels. I could see the fine polishing marks where he had trued up the bores (later polished out to a mirror finish) and relieved the forcing cones. (Too sharply angled a forcing cone can produce increased recoil.) The chokes had been relieved and there were signs of more activity at the muzzles!

I missed the first target out, a simple straight-away. I turned round, grinned sheepishly and explained. I had forgotten that I had started to use a Perazzi release trigger only the week before and in the excitement of firing the now Cordo-choked gun, had completely overlooked this!

Doc was revived by a still-smoking hull being passed under his nose and sat up in time to see targets breaking with a confidence-inspiring consistency. The difference being that now, instead of smoking most targets, with the occasional chip (from the fringe of the pattern) and the even more occasional miss, they just disintegrated into tiny, evenly sized pieces. This showed that the pellets had been evenly distributed through-out the killing circle and that, instead of having a dense central (or off-center elliptical) killing area and a sparsely filled outer ring, the whole circle now was equally lethal to any target venturing into it!

As an experiment to test the second barrel’s efficacy – without resorting to deliberately missing targets, which can have a slightly detrimental effect on timing – I used some skeet 2-3/4 dram 1-1/8oz No. 9s in the first barrel, erroneously presuming that they would merely dust the targets, instead of breaking them. Not only did they break them perfectly from the first barrel, but even broke them out to forty yards when used in the second barrel. We concluded that 9s could be used effectively for 16-yard trap and the first shot of doubles, except in high wind conditions, which will be referred to later. After conditioning myself to miss the first barrel shots, just before reaching the target, I broke 22/25 with the second shots and proved the equally even breaking capacity of the upper barrel.

We moved to the pattern board for the final confirmation. Every shot, regardless of the shell, afforded even distribution of pellets throughout the clock-face segments of each pattern sheet. Yesterday, Doc Cordaro had brought a new pattern board, which he had built. At the end of the patterning, the top and right wooden frames were spattered with pellets, whereas the left and bottom struts were virtually unmarked. Then we reversed the frame and found, as expected, that each of the four struts had a completely even covering of fringe pellets. The frame was built to hold sheets approximately thirty-six inches square. Doc pronounced himself satisfied, told me he could Cordo the barrels a fraction more, if I wanted it, but suggested I try it first on true International targets in top competition.

Up-Tight Overchoking by Derek Partridge

Author (left) fires same gun from test cradle in the temperature/humidity-controlled underground tunnel at the Perazzi works, where gun was made, in Brescia, Italy.

A few days later, I left for the Interservice International Trap Championships at Fort Benning Georgia. I would be shooting with three major changes to my gun, all untried at real International targets: the release trigger, the Cordo-choked barrels and I had decided to try the rifleman’s principle of a reverse slope on my epoxy Monte Carlo comb, to take the recoil of the stock away from my face instead of up and into it. In applying what I had intended to be a gentle one-sixteenth-inch slope from the back of the comb to the front, the rasp wielder had gotten a little over zealous.

It took a little while to adapt my timing and sight picture, because the release trigger is so much faster and several times I let it off before reaching the target. But the first day, I ran my first-ever 25 straight first barrel only – rare in International trap – and turned it into a 50 straight. By the last day, I seemed to have the feel of the trigger and had adjusted to the comb height. I missed the fourteenth target in the first round, then ran a heart-pounding 75 straight to finish with 99/100.

Jim Beck, who beat me with a magnificent 100 straight (his second and the only other of the three-day match), had just come second to Olympic Gold Medalist Mattarelli in the world championships and was awarded the Army Commendation Medal at the awards ceremony for his outstanding services to U.S. International shooting. (As a comparative note of the relative value of scores between American trap and International: at the 1968 Grand, 44 shooters tied with 200/200; in the whole history of International trap, 200/200 has been fired only four times, one of which was in America.)

Opening the chokes to accommodate continuous use of the smaller pellets will not result in a reduction in percentage, but rather an increase and an attendant improvement in distribution.

We had ascertained that a light load of skeet 9s would break targets out at forty yards, but Cordaro had made the qualification of not using them in a strong wind. Experiments had shown him that strong winds could affect the smaller pellets, in their denser shot column, to the point where virtually the whole pattern could be so blown; hardly a pellet would be found on the pattern sheet at forty yards. He had found that the same applied even to 8s, in similar conditions while 7-1/2s would still produce good patterns.

It is obvious that, the more pellets we can put into our patterns, the less chance of a clay slipping through a hole. The four square inches of target area the clay presents to a trap shooter is horrifyingly small, when you think about it in such terms! The smaller pellets must have sufficient energy to break the clay. In the case of the International target, which is deliberately made harder and slightly smaller, to withstand the shock of being thrown at twice the speed of the American target, this is a definite consideration. The best combination from my experience, which Cordaro’s technical knowledge backed up, is 3-1/4 dram 1-1/4oz 8s first barrel and 3-1/4 dram 1-1/4oz 7-1/2s in the second. Both should be nickel plated or of especially hardened lead. Nickel plating gives an edge in protection against pellet deformation.

If you want to open up your first barrel pattern without touching the chokes, use lead shot without a shot collar. Although some International shooters prefer 3-3/4 dram loads, careful patterning tests should be made to ascertain whether the shell used still retains good patterning along with the increased velocity – especially at ranges over 38 to 40 yards.

To use the smaller sizes of shot successfully, you may have to open up your chokes slightly, for the smaller the pellet, the more it is subject to deformation from tightly constricted chokes. Opening the chokes to accommodate continuous use of the smaller pellets will not result in a reduction in percentage, but rather an increase and an attendant improvement in distribution. A number of live pigeon shooters successfully use 9s – except perhaps during the winter months, when the birds are in their heavier plumage.

Some years ago, there was an interesting duel between the Jenkins brothers and Bob Allen at live pigeons. While Bob Allen ordered double extra full chokes and used 7-1/2s, the Jenkins brothers conversely opened up their chokes and used 9s – consistently beating Allen. But a pigeon is not a clay and the final word on that I’ll leave to Winchester-Western pro AI Mosier, who reckoned that one and half times as many 8s striking a target as 7-1/2s were needed to achieve the same breaking effect.

Up-Tight Overchoking by Derek Partridge

Shooting some more patterns, although percentages and distribution were good, the centers seemed a little thin, giving doughnut patterns. Searching for a reason, we recalled noticing the Power Piston was sailing down the range almost to the forty-yard pattern board. It was a cold, clear day and the atmosphere was thin, offering little resistance to the passage of the wad. We shot more shells and checked the fall of the wad each time. They fell within a yard to a yard and a half from the pattern sheet. Undoubtedly, that was the cause of the doughnut patterns.

The importance of a mirror finish is stressed and one way of achieving it is in chrome plating the barrels, but this does have the disadvantage of making any subsequent choke alterations extremely difficult. A more recent idea is the use of Teflon lining. Teflon was developed for space use, but its extremely low coefficient of friction makes it ideal for reducing pellet deformation to an absolute minimum. Naturally, Cordaro is working on a practical application! The final test was shot only for the sake of ballistic record. I shot a series of patterns in the test tunnel at the Perazzi factory in Brescia, Italy. The original percentages, shot in my presence, had been 72 and 80 percent at 38 yards – European standard test distance is 35 meters. Now the figures were 83 and 85 percent, with distinctly improved distribution and a slightly larger number of pellets in the top half of the circle where you look at and therefore normally shoot the clay. At thirty-four yards, my first barrel shooting distance, the figure was eighty-five percent. So I had what must be considered perfection for an International shooter – identical patterns at the two distances I was shooting. It’s the same target, so there’s no point in having different patterns! Doc had achieved what he set out to do: put more pellets in the first barrel pattern (one percent more), just a few more in the top one (five percent more); even the distribution of both and customize them identically for my two shooting distances.

The barrel gauge showed that he had opened the lower barrel only one thousandth and the top the same. I noticed that, in the perfect, still conditions of the tunnel, the bottom barrel also gave identical percentages with 8s and 7-1/2s. This would seem to bear out the theory that wind can affect the smaller 8s, as all our outdoor tests consistently had shown the 7-1/2s to give better percentages. His barrel polishing and other mystery ingredients had definitely reduced pellet deformation of the smaller 8s to give them parity with 7-1/2s under windless conditions.

Cordaro has now become more interested in technicalities and ballistics than in his own shooting – to the point where he is in the process of designing and building a Cordo gun. It probably will be the first gun which will enable the trap shooter to sit back, while the gun does the job for him!


This article was written by Derek Partridge and originally published in Gun World, August 1970 issue. Republished with permission.