Screw Chokes Vs Fixed Chokes


Technoid,

My exposure to choke tubes is limited to a Beretta with both factory and after-market tubes, and a second gun with Briley thin-walls. What I see in these tubes is that regardless of the amount of constriction, the tubes seem to get it over with in about an inch of barrel (tube) length. That’s seems to be a pretty brutal transition. Fixed chokes, on the other hand, almost universally take a lazy approach, using upwards of two inches of barrel length to apply the choke. Some of the barrels, particularly a battery of old Parkers, stretch the taper out to nearly three inches.

I have not done much pattern work on the tubed guns, but if you asked me quickly, I would probably say that my impression of their patterns is that they tend to be patchier than from the fixed choke barrels.

Is my observation of the rapid taper in choke tubes typical of the genre, and can it be honestly said that fixed chokes generally deliver better patterns?

In advance, thanks for your enlightenment.

Bill
Dear Bill,

Fixed chokes better than screw-ins? Long chokes better than short ones? Good questions. I don’t have any hard answers, but I do have some soft, wishy-washy ones.

Common wisdom (now there’s an oxymoron for you) has it that tighter chokes require longer tapers and parallels. Maybe so. The fixed chokes in my Belgian Brownings and Perazzis are quite long- over five inches, with very gradual tapers and longish parallels.

If we don’t concern ourselves with jug chokes and Tulas, there are basically two types of choke design. One is just a taper. The other, and more common one today, is a taper to a parallel section. The theory is that this latter design squeezes the shot down to the correct constriction and then allows it to stabilize before exiting. A taper only just squeezes it down without giving it a chance to realign.

In the real world, the general feeling is that open chokes like Skeet and IC can be made to perform quite nicely with minimal taper and parallel. It’s only when you get into tighter constrictions that things become more critical. I’m sure that various choke designers have favorite ratios of taper to parallel that they use depending on constriction and shot load. I’ve heard it said that the parallel ought to equal the height of the shot load, but if the shot load strings out in the barrel, as I’ve seen it do in some tests, I don’t see how that sort of parallel measurement would matter. As I said, I have some wishy-washy answers.

In addition to choke length, another thing to consider when comparing screw chokes to fixed ones is that drop off at the rear of the screw choke. Mass produced choke makers want to make darn sure that their choke skirts don’t stick up into the bore, so they cut in plenty of clearance. This makes a noticeable jump for the shot and gives it a rougher ride as it whooshes down the barrel. Custom screw chokes like some I’ve seen from Nigel Teague, cut the tolerances very close and avoid most of this. Of course, fixed chokes avoid all of it.

One of the usual criteria for a “good” pattern is lack of patchiness. I’ve never fully understood the argument of patchy patterns. If shotgun patterns are Gaussian in nature (according to Ed Lowry, a Winchester ballistician in whom I have a lot of faith), then over a number of interations all 60% patterns will have the same amount of patchiness if you take enough samples. I know that Oberfell and Thompson rated their patterns on patchiness, but I never understood how this jibed with the bell curve. I can certainly understand how one choke/shell combination could produce a different pattern percentage than another or a different shot string length, but I just don’t see how one series of two dimensional 60% patterns could be more or less patchy than another if you take enough samples. Than again, I don’t understand how electricity works or why the grass is green either.

One thing that I have noticed is that gun with screw chokes sometimes need more constriction to achieve a very tight pattern compared to a gun with a long fixed choke. Also, if I were looking for the tightest choke possible, I’d experiment with fixed chokes first as I think that the long taper and long parallel give them the most potential. That said, I’m sure that there are plenty of people who have gotten tight patterns with screw chokes. I guess that the only way you could tell for sure would be to optimize a fixed choke gun for the tightest possible pattern, test it, and then have it screw choked and test it again. It’s really not apples to apples unless you test the same barrel. Very few people will bother doing this. I certainly haven’t, but don’t let me stop you.

There it is. Another dose of salts from your fearless Technoid. “Often in error. Never in doubt.” was never truer.

Best regards,

Bruce Buck
The Technoid writing for Shotgun Report, LLC
http://www.ShotgunReport.com

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