Open it up and get a hit?

The general advice on bunker chokes one usually hears is to use Improved Modified (IM) and Full with the 24 gram load in bunker. So what, exactly, is "IM" as a choke dial measurement? Similarly, what, exactly, is "Full" choke? Perazzi, judging by its catalogs, suggest that for Olympic trap chokes, on the choke measurement dial, IM is to equal 28 thou and Full is to equal 40 thou. Considering Perazzi's status in the bunker world, the 28 (0.71 mm) and 40 (1.0 mm) numbers would pretty much be the gold standard.

To test this standard, below are the results of pattern tests with first barrel choking from 0.015 (0.38 mm) to 0.042 (1.07 mm) thousandths. And second barrel choking from 0.042 (1.07 mm) to 0.033 (0.84 mm). Since previous testing had shown that the maximum effective pattern width was 24 (61 cm) inches, and any pellet strikes outside that circle were totally random and could not be relied upon to consistently break a target, no attempt was made to add the 24 to 30" taurus count to the table.

First barrel tests:

30 yard (27.4 m) choke comparison run details: load was Remington STS case with enough Hodgdon TiteWad powder for chronograph-verified 1250'/s, Remington 209P primer, (old/original, softer) Winchester 12L GRAY wad, 24 grams of Black Diamond 8's, 335 pellet count average per shell. Same lot of components used for all 9 pattern runs. Each shell's components were scaled to insure consistency. Barrel cleaned before each session.

Weather very similar for each session, with temperatures in the 80's. Patterns were fired over the course of a month at sea level. Choke tubes were Briley X2's, except 0.030 and 0.043; Seminoles. All tubes were verified to be exact with the same Perazzi barrel used throughout.

Choke Dimension	10" Core	20" Taurus	Total	%	24" Taurus	Total	%	90mm Hole Count
0.015	100	148	248	75	45	293	89	2.125
0.019	118	154	273	83	35	308	93	2
0.023	143	138	280	85	30	311	94	3
0.025	146	142	288	87	25	313	95	5
0.028	135	149	283	86	32	315	95	3.88
0.030	146	147	293	89	28	321	97	3.5
0.033	146	149	295	89	26	320	97	3.75
0.042	155	138	292	89	28	320	97	3

Looking over the table, you will note that opening the choke weakens the 10" circle – the pattern's core – and does little to "juice up" the outer 20" to 24" taurus. The 10" to 20" taurus pellet count shows little change. The 20 to 24" taurus change is practically meaningless as there is so much area to cover here. Yes, some additional pellets clustered around the outside of the 24" circle with the more open chokes, but these cluster locations inconsistently wandered around from sheet to sheet and were not uniformily expanded from the 24" circle. In other words, the more open chokes did not uniformly open up the pattern. With the tighter choke designations, there were virtually no pellets seen outside the 24" circle.

A comment on results of using 7 1/2's in place of the 8's shown above: you'll see about a 12% lower pellet count, depending on shot sizing and grading quality. Expect also an increase in hole count, (poorer pattern uniformity) due to that 12% lower pellet number. Increasing velocity can also be expected to drop pattern percentages by about 8% for a 50 '/s increase, again depending on component quality.

On the bunker, the tighter chokes produced first barrel target ink stains in the sky when properly centered. It seems pretty clear that a target is unlikely to slip through the tighter choke's pattern. As you would expect, the much more open chokes rarely produced smoke and often gave rather "chunky" breaks leaving several larger chips/bits to shoot at with the second barrel. These poor quality breaks also raise the question of whether some targets are occasionally slipping through a now too-weak center pattern.

In summary, it appears that just "opening it up" by screwing in a slightly more open (2 - 4 thousandth) choke may well have no significant effect at all on the pattern. Installing a dramatically more open choke, IC through modified, does put more pellets around the outside of a 24" pattern but only by weakening the center pattern core, possibly eliminating an otherwise guaranteed break in the case of a perfectly centered target.

It appears from the above runs that there may well be a first barrel "get-a-hit" choke and it would be around a modified in the above chart done at 30 yards—or choke on the minimum side for the distance you normally break targets. Carefully kept first-barrel comparison scores will show if this is valid for the individual shooter.

Some patterning absolutely needs to be done to verify this choice with the cartridges to be used, especially when using 7 1/2's at Euro 1350'/s (412m/s) velocities. Enough testing has to be done to confirm pattern consistencies i.e. that the shell and choke combination does not deliver significant, occasional drops in pattern percentage(s) that will result in a lost target. (An occasional 10% dropout—one choke designation—is not unusual with some component combinations.)

However, it should be remembered the real problem with the "open it up and get a hit" approach is that there is little feedback in terms of a smoked target from a dense, center pattern core to indicate that they are doing everything right, and so scoring improvement is slowed.

Second barrel choke dimension tests:

35 yard (32 m) "full" choke comparison run details: load was Federal Gold Medal plastic target with enough Alliant Red Dot powder for chronograph-verified 1315'/s, Federal 209A primer, Bascheri and Pellegri T2 wad, 24 grams (370 grains) West Coast High antimony shot. (This component combination proved to be good, although a very few other combinations have shown better results. . . At least there were no holes in the 10" core circle as is sometimes seen!) Load data from Precision Reloading. Same lot of components used for all 5 pattern runs. Each shell's components were scaled-measured to insure consistency. Barrel cleaned before each session.

Weather very similar for each session, with temperatures in the 80's. Patterns were fired over the course of a month, wind level permitting. All choke tubes were Seminoles. All tubes were bore-mic verified to be exact with the same Perazzi barrel used throughout.

In comparing the second barrel runs, there is little core core count or total pattern percentage difference as the choke is opened up. (The results can be expected to change around on retest and with different components and choke contours/brands.) What does change is the uniformity or quality of the pattern in the 20 inch core. It noticibly improves as the choke is opened. Again, these results will vary with choke contours and load selection.

The 90 mm hole counts, H1 and the new H2, are an attempt to learn how uniform the patterns are for comparison between chokes and loads. The second, holes within the 20" circle, has been added to compare the circles. As you can see, there's quite a difference. Perhaps the 24 gram load should be considered to have only an effective 20" pattern. . .

Complete data from the five runs summarized above is here (PDF). Additional explanation is also included.

The above picture shows a oft-seen extreme artifact from the 40 thousandth choking level: row stringing.  Notice the open areas to the upper right. The 10" circle of this particular pattern is to the lower left. This stringing effect goes away with the pellets distributed more evenly giving the patterns better uniformity as the choking is opened to the 33 to 36 (0.8 to 0.9 mm) thousands region. 

The above illustrates the reason to have dense, uniform patterns. The three dome hits obviously didn't break this NASTA target. If the background was clear blue sky and you were fortunate enough to have a very experienced referee, he/she would have called the target dead from the chip(s?) that flew off from the center-left hole. To be sure of getting a target on the board, you pretty much have to put pellets into the rim ring area and out as shown by the arrow. Having that high quality, dense/tight and uniform pattern increases the odds of pellets hitting the rim area when you've done your job.

In summary, it appears, as expected, that the Perazzi recommendations of 28 and 40 thou (0.7 and 1.0 mm or Perazzi #7 and #10) choking are excellent ones. They will serve you well, especially if you favor fixed chokes for their simplicity.

However, for tournament level performance, the above pattern runs suggest that it is worth while to investigate opening up the choke very slightly. Probably the result will be a set of chokes in the area of 25 and 33 thou; the actual final numbers will depend on your cartridges and the barrel itself. If you are testing with factory-made cartridges, tests should confirm that the lot used for adjustment is of expected standards of component quality and velocity consistency and not a one-off lot. A change in components at the reloading bench (or a switch to a different shell brand) will require careful re-testing to confirm that tournament level performance has followed the switch.

In consideration of achieving tournement level performance, a barrelmaster will be needed to tune a fixed choke gun. A gun using choke tubes only requires screwing in choke tubes of the desired dial measurement (choke tube suppliers will confirm the dimension upon request before shipment), then verifying the tube delivers the desired pattern. 

To expand on the above: the following is a comparison between the virtues of fixed vs. choke tubes:

Fixed chokes:

1. The simplest, no fuss, no worry approach.

2. Maintenance minimal; clean them when you feel guilty. Well, at least once a month. . . And definitely before a tournament.

3. No worries about a choke tube loosening (or whatever), then shooting out and causing permanent barrel damage. Or getting stuck in the barrel, preventing a needed change.

4. No additional expense of buying choke tubes.

5. No choke tubes to keep track of or forget to pack.

6. No simple way to fine tune – and $$$ to have a barrel man do his magic (you'll want more than just a reamer run down the barrels: see Derek's article Up Tight on Choking) – for your one preferred shell. A tune-up optimization done with one brand of shells should work well enough for most quality shells.

7. Can be damaged accidently, requiring a barrelmaster to repair, if repairable.
   

8. Balance adjustments can only be done by adding weights to gun.

Choke Tubes:

1. Practically an infinite number of pattern quality adjustment/changes possible. Many different brands, types, weights and styles to select from. Different brands of shells or component combinations likely can be optimized using different brands and dimensions of choke tubes.

2. Give ability to subtly fine-tune the gun's balance and smoothness vs. speed to the target by selection of choke type: Extended or flush, standard vs. titanium weight (Examples: 2 regular steel flush chokes, weigh about 1.3 oz (36.5 grams). Titanium is noticibly even lighter. 2 extended chokes weigh about 3 oz (83 grams).

3. Instant ability to adjust patterns for altitude, temperatures, field conditions, target hardness and different shells. Plus, makes gun very usable for occasional use in other disciplines, as sporting clays or skeet.

4. If damaged, easy and economical to replace. No $$$ barrelmaster service needed.

5. Can quickly and easily go tighter for training purposes.

6. More maintenance and care needed.

7. Constant need to confirm tightness in the barrel, some brands better than others.

8. Probably need cleaning more than fixed. On the other hand, it's easy and quite fast to pop a clean one in.

9. They are an additional expense and hassle.

As you can see, there are good reasons both pro and con for fixed chokes vs. choke tubes when it's time to choose chokes for your next bunker gun. For bunker use only and for the majority of non-seriously competitive shooters, the simplist, no fuss, no worry path is fixed chokes. The Perazzi recommendations of 28 and 40 thou (0.7 and 1.0 mm or Perazzi #7 and #10) choking are excellent ones and leave the shooter with the future option to optimize.

On patterning frames and plates

Patterning detail chart:

Left arrow points to the 10" core circle

Upper arrow: 20" circle/ 10 to 20" taurus

Lower arrow: 24" circle/20 to 24" taurus

Revised 10/2011