Whenever shotgun barrels suffer internal damage, the gun’s owner almost immediately leaps to the conclusion that there was something faulty about the ammunition. It is true that certain forms of barrel damage can indeed be caused by faulty ammunition (see this column January and February 2014). However, as we examined in last month’s Part 1 of this series, certain forms of barrel damage can occur from shoddy barrel modifications through no fault of the ammunition whatsoever. Part 1 covered faulty chamber and forcing cone lengthening. In Part 2 here we’ll examine damage that can occur from faulty backboring, porting, and screw-in choke installation.

Backboring
Backboring is the common gunsmithing term for enlarging the diameter of the bore proper of a shotgun barrel. This is accomplished after-market by removing metal via reaming from the entire length of the barrel’s bore to result in increased bore diameter. The claimed virtues of this maneuver depend upon the boldness of the salesman. Let it be known here that removing metal from the bore of a shotgun barrel cannot and does not result in increased velocity levels. Yes, I know there are individuals with their home chronographs that will tell you they got higher velocities from certain shotshell ammunition after backboring. To this I respond that home chronograph systems were never designed to accurately measure shotshell velocities and therefore no velocities measured by such systems can ever be truly trusted. Such systems are also generally incorrectly set up and operated by the owner insofar as shotguns go, but that’s another story.

What backboring definitely does is remove a significant amount of metal from the inside of a shotgun barrel. Because shotgun barrels are tubular structures for which wall thickness is a major factor in their strength, any thinning of the barrel walls results in a weaker barrel. The other thing about backboring is that when done as after-market modification, it ALWAYS voids the barrel’s warranty.

About the only good I’ve ever been able to empirically measure from backboring is that if not done to excess – and I define excess as more than 0.015 inch larger than standard for a given gauge – patterns with some lead loads will indeed improve slightly. In my tests this happens less so with small shot in target loads at clay target shooting distances, but much more so with large shot in hunting loads at long range distances of 50 yards or greater. Backboring greater than 0.015 inch in my tests frequently results in LOWERED velocities due to more gas escaping around the obturating skirt of the wad as the ejecta travels down bore. 

As far as barrel damage is concerned from backboring, if you get enough metal removed, and especially if it is not removed exactly parallel to the axis of the bore, then backboring can cause weak spots at certain points along the barrel. Bulges can then occur at those points when firing perfectly normal shotshell ammunition.

Porting
In simple terms, porting amounts to drilling or etching a series of holes in the barrel walls near the muzzle end with the hoped effect of reducing recoil. Be advised: porting does not and cannot reduce rearward recoil. Porting can only reduce muzzle jump. Now, because reducing muzzle jump is helpful especially for quicker and more accurate second and third shots, porting does have value. 

But with porting comes greatly increased muzzle blast noise. While clay target shooters with all of their ear plugs and ear muff sound protection usually in place may not object to such additional noise, trust me that your hunting friends will hate it especially if they’re right next to you in a duck blind. 

Also be advised that the shape of the ported holes doesn’t matter. What matters is the PLACEMENT of the ports. They should be as close to the top of the barrel as possible to have the greatest influence on reducing muzzle jump. Ports in the side and bottom of the barrel are useless. Also be advised that after making all of those porting holes the barrel at that point has been weakened (metal removal again). And, ports allow additional avenues for rain and snow to migrate into the barrel plus they are notorious for becoming clogged with plastic wad residue. So ported barrels need more cleaning than non-ported barrels. 

As to damage, get too many portholes at a given point in the barrel and you greatly increase the chance for a bulge to occur at that point even from perfectly normal shotshell ammunition. Always remember that when the ejecta reaches the ported zone, it is very near its maximum velocity and is starting to impart another pressure spike as it enters the choke.

Screw-In Chokes
Screw-in choke installation also involves removing wall thickness this time at the muzzle end of the barrel. And, of all reaming that occurs in a shotgun barrel it will result in leaving the very least wall thickness when the reaming is complete.  Therefore, it is imperative that any screw-in choke installation be absolutely parallel to the axis of the bore. If it is done crooked, the screw-in chokes will go in slightly crooked and as a result the shot charge will be forced high, low, left or right in proportion and magnitude to the crookedness of the choke ream job. I’ve never seen any real barrel damage that has resulted from crooked screw-in choke installation reaming, but I have seen plenty of point-of-impact changes caused by such faulty reaming.

Conclusion

You shouldn’t be cavalier about any barrel modifications regarding safety and performance. Be absolutely certain that any vendor you deal with is qualified with experienced personnel and the proper tools.

Copyright 2014 by Tom Roster. All Rights Reserved. This material may not be published, broadcast, rewritten or redistributed.

Tom Roster is an independent ballistics consultant and author specializing in the design and testing of shotshell loads for U.S. shotshell and reloading components manufacturers. He is a court-recognized shotshell/shotgun expert witness. Tom was formerly the Ballistics Research Director at Oregon Institute of Technology and then served as a Ballistics Specialist for the Dept. of the Interior. In these capacities he designed and administered the world’s six most extensive lead versus nontoxic shot duck, goose, pheasant and dove shooting tests ever conducted. He then co-authored their peer-reviewed scientific reports. Roster spends about 100 days afield each year testing  lead and nontoxic hunting and target shotshell loads, then traveling worldwide reporting on his findings to industry and wildlife professionals, hunters and shooters, and in his writings for various shotgunning magazines. Contact him in Oregon at (541) 884-2974,tomroster@charter.net.

Tom Roster

Tom Roster is an independent ballistics consultant and author specializing in the design and testing of shotshell loads for U.S. shotshell and reloading components manufacturers. He is a court-recognized shotshell/shotgun expert witness. Tom was formerly the Ballistics Research Director at Oregon Institute of Technology and then served as a Ballistics Specialist for the Dept. of the Interior. In these capacities he designed and administered the world’s six most extensive lead versus nontoxic shot duck, goose, pheasant and dove shooting tests ever conducted. He then co-authored their peer-reviewed scientific reports. Roster spends about 100 days afield each year testing  lead and nontoxic hunting and target shotshell loads, then traveling worldwide reporting on his findings to industry and wildlife professionals, hunters and shooters, and in his writings for various shotgunning magazines. Contact him in Oregon at (541) 884-2974, tomroster@charter.net.