The Bullshooter

Wednesday, March 08, 2006

Bulged Barrels

Experienced hands may not have anything to learn here, but it surprises me how many shooters are unaware of the pitfalls that can cause damage to their firearm.

A bulge, or pressure ring, is generally caused by an obstruction in the barrel. The bullet, as it encounters this obstruction, has immense pressure buildup behind it until something gives. In most cases this is the inside surface of the barrel (the bore). Of course in more extreme cases the barrel will burst or the action will fly apart.

Typically a bulge, when looking through the barrel, looks like a dark concentric ring. It may be a few millimeters long, and its depth depends on several of the circumstances; the severity of the blockage, the wall thickness and the quality of the barrel metal. Some will show external evidence in the form of a raised bulge (obviously a shotgun is prone to this with its thin walls).

So, what can cause this to happen?

What Constitutes a Blockage

1. Oil. The coating of rust-prevention you may apply to the inside of the bore after cleaning. This should always be removed by pushing through a dry patch, before shooting. Most oils will not compress, and as the tight-fitting bullet accelerates down the barrel it pushes the oil in front of it... there is no way it can pass. Lower power firearms such as 22LR may seldom build up enough pressure to cause a bulge (or as I like to call it, a "passing bay"), but it's better to be safe than sorry. Certainly I have seen a lot of center fire rifles that have been ringed from this very thing.

2. The skirt from a hollow based lead projectile. In most cases this will happen to a handloader, but not always. If there is too much pressure for a hollow based bullet, the skirt can separate from the main body of the projectile, and lodge somewhere in the barrel. The next shot, when fired, will clear the obstruction, but most likely will also leave a bulge.

Hollow based target wadcutters are the most common culprits here, shot widely in both target revolvers and semi autos. It would be easy to say that the loads that cause separation are too hot, and in many cases this is the end of the story. But some loads, that are safely worked up in mild temperatures, can become too "hot" when the temperature climbs. The fast burning powders typically used for target loads have a very steep pressure curve, which is why they use such a piddly charge. By their very nature, pressures don't gradually increase, they go up in a rush. If the ammunition gets hot (for example from sitting in the trunk of your car on a summer's day), a load that was mild and sedate can become punchy and downright dangerous. You may be lucky and see some evidence of skirt separation before any damage is done. I once witnessed a 5-shot target with 10 bullet holes. Needless to say the shooter in question put his gear away and headed home to spend some quality time with his kinetic bullet puller.

Even factory wadcutter ammo should be treated with respect, and we may discuss this further in an upcoming installment.

3. A bullet. Most often from a reload, but even factory loads have been known to have no powder. The firing pin hits the primer, and the blast from the primer may or may not have enough oomph to clear the bullet from the barrel. If not, the next shot can cause a bulge.

It appears luck has a lot to do with outcome here. I've witnessed one instance, and have been guilty of another, when a bullet has been shunted out with no visible sign of damage. My lucky escape came in a 4-second string of Service Match (using a revolver of course) - and for me to get six shots away in 4 seconds means I'm squeezing as fast as I can - the third shot was, I thought , a click. But I had 6 empty cases, and 6 holes in the target (only 10 yards away). My "click" was obviously a primer shot that lodged the bullet just inside the barrel, and the next shot cleared it out. If it had have been further down the barrel I'm certain the shunter would have gained much more speed and done damage to the barrel when connecting with the shuntee.

4. The famous "Clean the Lead Out with a Jacketed Round". I have no proof that this has ever bulged as much as one barrel in the past. But I figure it should have. I'd advise going the traditional route of actually cleaning the barrel first. I know it's painfully slow, even with modern solvents, but there's something about the alternative that smacks of swinging a sledgehammer to kill a fly.

Consequences

What will happen to the accuracy if you have a bulge in the barrel of your target pistol? Does it mean you immediately have to rebarrel?

The most important part of any barrel as regards to accuracy is the last inch or so. If the bulge is before that area, chances are the accuracy will still be acceptable. There may be a tendency to lead a little more quickly.

If you bulge a center fire rifle barrel, I would certainly seek advice from a gunsmith. The higher pressures that rifles are subject to make it nothing to be trifled with. In theory, pressures would be lessened by the bypass of gas at the point of the pressure ring, but there is also more chance that it has created a weakness in the metal.

4 Comments:

  • At 10/3/06 12:26 PM, Blogger Ed Skinner said…

    After cleaning, I oil my barrels and run a dry patch immediately to remove *most* of the oil but I haven't done anything else thereafter. (I will check all my barrels before the 2700 this Sunday.)
    Do you think running a snake through before a competition would be sufficient to remove any excess oil?
    Thanks for the good information!

     
  • At 10/3/06 1:36 PM, Blogger Warren said…

    I think you have no problem; running the dry patch through (as long as it's reasonably tight) will remove any chance of there being enough oil to constitute a buildup that could become an obstruction.

     
  • At 10/3/06 2:29 PM, Anonymous Anonymous said…

    I once witnessed someone pump 6 rounds at the 7 yard line, in Unrestricted I believe. I was next to her. Nothing came out. None of the rounds had powder in it. Hell of a job to knock out those 6 projectiles out of the revolver. At least she was consistent, if the final round did have powder we'd all probably have been in hospital

    And of course we've never let her forget it :)

     
  • At 26/3/06 3:48 AM, Anonymous Snag said…

    Had the bulge thing happen many years back with gun oil in a .22 LR auto. Nearly deafened me and left a visible bulge 6" from the end of the barrel. Machined the tip off (hard going) and made a 'gas shield' to restore aesthetics. worked fine and I learned to clean the oil out.

     

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