Porting Improves Patterns?


Dear Bruce:

For trap singles I shoot an 870 TC. It is weighted and has a Morgan adjustable pad. I also had a local gunsmith reduce the trigger pull to four pounds. I have about 8500 rounds through the gun, and shoot it reasonably well — 94.5%. However, for the past two years, I have been hot and cold on the idea of porting the barrel. I agree that porting will not reduce recoil, and since I only shoot the gun for singles muzzle jump is not a consideration. However, what do you think of this:

My local gunsmith, who I believe is for the most part an honest guy, theorizes that porting a barrel such as the one on my 870 will minimize the concentration of pellets in the center of the pattern, yet retain the same number of pellets within a 30″ diameter at 40 yards. Furthermore, he says that he has patterned guns before and after porting and the results prove his theory. The obvious benefit is that there are more pellets on the perimeter to bust the birds that I have not covered with the most dense area of my pattern. The only downside, he maintains, is that because the porting will result in a more evenly distributed pattern, I will “dust” far fewer birds. Furthermore, to me it sounds as if I could achieve the same result by simply dropping from full choke to improved modified.

In closing, I respect your opinion, and I am interested in your thoughts before I pay this guy $65 to drill holes in what is probably a perfectly good barrel.

Thanks for you consideration,

Andrew Mechanicsburg, Pa.

Dear Andrew,

Well, I dunno. I’m sure that your gunsmith is honest as the day is long. If he says that he has done the patterning work on before and after porting and found that it “leveled out” a pattern, I certainly don’t doubt him. BUT, that hasn’t been my experience either at the patterning plate, nor in my conversations with numerous ballistic experts who do this stuff for a living.

Let’s look at theory a little bit first because it can be done while sitting down instead of trotting out to the gun club with rolls of paper and Magic Marker.

The main claim for barrel porting is that it reduces muzzle jump. The holes are drilled on or near the top side of the barrel so that the jet of escaping gas pushes the barrel downward. Makes sense. It certainly works on compensated center fire pistols. I’ve never had much faith in this application on a shotgun simply because I don’t think that the amount of gas pressure generated by a target shell 30″ from the chamber is enough to have a meaningful effect on muzzle jump. Others, particularly those who sell porting, disagree with me. Everyone to their own opinion on this score.

As to reducing recoil, again porting works in theory. I’ve used a KDF muzzle brake on a very, very light .338 rifle and was amazed by the recoil reduction. Most military canon also use some type of muzzle brake. A muzzle brake depends on the forward rushing gas hitting a vertical surface that it can push against. That’s why rifle compensators are real thick and why tank canon brakes have sort of a curved metal shield. It gives the gas something to push against. Unfortunately, a shotgun barrel is only .030″ to .050″ thick in this area and doesn’t give the gas much to push against. Again, it works, but it doesn’t work enough to matter in my opinion.

Still, most barrel porting operations claim less muzzle jump and reduced rearward recoil. They are entitled to their opinion, but I think that you are entitled to keep your money. Up to you.

One claim I haven’t heard much of is the claim that barrel porting will “improve” a pattern. I’m not quite sure that I understand how bleeding gas off of one side of the barrel (not all sides equally) can improve things. I guess in theory the porting might possible act as a wad retarder. Anything that delays the wad for a moment and allows the wad and shot to separate a bit, might possibly keep the light plastic wad from pushing through the shot cloud and disrupting it. Lujic has often scribed a few lines in his trap chokes to retard the wad. He feels it helps. Pattern Master chokes uses “nodes” or bumps to strip the wad and hold it back. They claim some superior patterns, especially with big shot. Other than these two, I really don’t know of anyone else who goes with the “rough choke” theory. I’m sure that there are others, but I just can’t recall them now.

Does retarding the wad help? Well….. Look at what kind of choke systems have been setting all the records in all the clay target sports and then you tell me. If this were any kind of miracle cure, everyone would be roughing up their chokes. They aren’t.

Now let’s look at the theory of evening out a pattern that your gunsmith claims porting can do. Many choke borers claim that their magic can reduce the target’s hot center where the pellets aren’t needed and move those pellets out to the fringe where they will actually do some good. Sounds great. So does Ponce deLeon’s Fountain of Youth.

I have absolutely no doubt that barrel work can be made to move pellets from the center of the pattern into the fringe of the pattern, BUT (big BUT) I have a whole bunch of doubt that it can be done while maintaining the same pattern percentage in the 30″ circle. Aye, there’s the rub.

When someone says that they can keep a 60% pattern in a 30″ circle, and yet change the pellet distribution in that 30″ pattern from a hot core and weak fringe to a cool core with a denser fringe- all still with that 60% in the 30″ circle- I say “Prunes!”. I don’t believe it. I do believe that patterns differ on a random basis within a certain range and that if you only shoot a pattern or two you might possibly come up with a 10% to 15% change, but it could just as easily be the other way.

A shotgun pattern is a random event, but it obeys certain mathematical laws in the aggregate. Each shotgun pellet in the load is a random even, but all the shotgun pellets taken together obey the laws of the bell curve (or Gaussian distribution). Don’t take my word for it. Heck, I don’t even take my word for it. I KNOW what my math grade were. But it just so happens that I steal my material from real ballisticians and math PhDs. Ed Lowry (Winchester’s ballistican for years and years), Giblin (commissioned by the British cartridge industry to study the ballistics of non-toxic shells) and our own Warren Johnson, math PhD and developer of “Choke Chooser”, plus the original French General Journee who started the analysis using multiple rifle shots, all agree that the multiple strikes of a shotgun pattern or multiple rifle shots on a target, obey the immutable laws of the Gaussian curve.

What does this mean in words that humans can understand? Simply put, it means that once you set the standard of 60% of the pattern in a 30″ circle, the laws of math take over as to the distribution of the pellets within that circle. The shotgun choke can control whether that 30″ circle has 50% or 70% of the pellets in it. Patterns normally vary 10% from shot to shot as to the pattern percentage in the 30″ circle. But once you get a pattern that puts 60% in that circle, the laws of math will govern how that pattern is distributed in that circle. You don’t have any say in it. You can’t move pellets from the center to the fringe and still keep 60% in the 30″ circle. Of course, the pellet distribution in the circle will vary on a shot to shot basis, but over time and over an average of many, many shots, a certain distribution pattern will emerge based on the bell curve. You can’t influence it unless you change the pattern percentage in the 30″ circle. No amount of magic choking will defeat the laws of mathematics.

The bottom line is that the 60% pattern from your Purdey using solid gold hand molded pellets individually loaded by Vestal virgins will have exactly the same distribution over numerous patterns as the 60% pattern that your rusty old Mossberg shooting promo dove and quail loads gets. Depressing, isn’t it. That’s why I hate math. It’s so cold hearted.

And now the CYA part: All of the above applies to a two dimensional pattern sheet. It does not apply to shotstring. High quality chokes and shells can significantly reduce shotstring by reducing shot deformation. All of this gives you a better chance of achieving that 60% pattern percentage on target, with less loss of pattern due to shotstring. BUT once you do manage to get that 60%, no matter how you do it, the distributions within that pattern will be mathematically equal over time. No amount of barrel porting can change the distribution of the pellets within that 30″ circle. It just ain’t gonna happen unless we rewrite the laws of mathematics and defrock some of these math PhDs.

And now for the short answer…..

Ha! Not on your life. No short answers here.

Best regards,

Bruce Buck
The Technoid writing for Shotgun Report, LLC
(Often in error. Never in doubt.)

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