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Barrel Bulges

21 Jan 2015
@ 04:13 am (GMT)

Robert Kennedy

This topic comes from guilty feelings about a barrel bulge in my brother's 22LR Brno decades ago. As I recollect it happened on my watch as my brother let me use his rifle while he was away. The bulge was about two and a half inches from the muzzle. It did not seem to affect accuracy at all. I wasn't careless about cleaning and always ran a pull-through or cleaning rod through the barrel before shooting. Another friend who was fanatical about cleaning also had a Brno 22 that had a bulge about 3 inches from the breech. Although it also did not seem to affect accuracy he had a new barrel fitted. (Yes, you know its there and someone may one day look up your barrel!!)I have always wondered why these two events happened. My only theory is that the waxed bullets used then may have been the reason.
This brings me to ask all you experienced shooters to tell me of your experiences. Nathan says he shoots over CRC Longlife applied on a patch through the barrel, and when I read this my paranoid feelings about having any trace of oil or lubricant in a barrel came back to haunt me. Bulges in centrefires do happen but where's the tipping point? Apart from debris falling into the barrel, hence the plastic electrical tape used to stop this, how much and what type of lubrication is safe to use? Come on Nathan, help me get rid of this ghost from the past.


21 Jan 2015
@ 04:50 pm (GMT)

Nathan Foster

Re: Barrel Bulges
Hi Robert, some of the problems can stem from the fact that barrel makers can get away with using mild steel in .22LR barrels or basic 316L stainless in stainless rifles. Manufacturing methods can also be fairly basic without the same quality control as a center fire rifle. I have come across buldges myself b ut mostly due to water. The rifle is used at night, then laid on grass, then moisture (dew) enters the bore. Rain drops will do it every time.

There is also secondary pressure spike theory but this so far has only been postulated for centerfire rifles. The theory goes that the charge sends the projectile off down the bore at say 52,000psi, then the pressure drops, then the projectile loses some speed followed by a catch up of gases, resulting in a secondary pressure of between 70 and 80,000psi towards the muzzle. This may be the cause of some barrel bulges where water or moisture was the initial blame. I have not put this info it the reloading book as I am still learning about this myself. There are many ifs and buts here too.

Catch up pressure theory may be more relative to high capacity cartridges using the slowest burn rate in very long barrels. But if we were to now postulate this same theory here, we could have a situation were the catch up pressure is significantly lower, but because of the soft nature of the steel, we are seeing bulges. keep in mind I am just theorizing.

Regarding center fire:

If catch up pressures are a reality in center fire, it means that we need to be highly vigilant of powder selection in long barreled rifles. We can still work with the slower powders- but not necessarily the slowest possible powders. I have found that the slowest possible powders are often less than ideal anyway for a given cartridge (Perhaps was I seeing catch up spikes effect harmonics during research?). I have made comments within the reloading book regarding powder selection to help avoid problems regardless of whether catch up theory is true or false (I suspect that it is a reality).

Other problems can include bores with a highly polished internal finish as I wrote about in book 3 (Accurizing). The bullet glides along the bore, then all of a sudden binds. In this regard, I have seen a type of catch up first hand during barrel manufacture experimentation and the pressures were extreme. So there are a few variables that are keeping this subject at a theoretical level. It appears that these down-barrel pressure spikes can occur, but we have to be careful how we allocate blame (powder or bore or X). On top of this are the more common causes such as water or heavy oils within the bore.

In the .22:

If catch up pressure spikes are a reality for .22LR and .22 WMR, the only answer is to keep barrels short and or utilize full grade steels during barrel making.

I seldom clean my .22 rifles. I leave the wax to do its job. I have based this on having old farm guns delivered to me for inspection- some vintage, and finding the outer steel rusted from coastal salts while the bores are mint. If I do clean a .22, I make sure I put many fouling shots back through it. Modern suppressed .22's are a little more complex. The suppressors trap carbon which can lead to severe corrosion. Also, semi auto rifles need a good clean up once in a while for smooth function. So in this regard, carbon cleaning is my main concern for the .22's along with routine barrel cleanings at set down time periods. Outer metal is treated with greater care on a regular basis.

Hell of a long winded answer but there you go.
23 Jan 2015
@ 03:38 am (GMT)

Robert Kennedy

Re: Barrel Bulges
Thanks Nathan. You have given me more material for consideration. I can see that having a tighter section at the muzzle end of the barrel may enhance accuracy but if overdone could result in a pressure spike and a resulting effect on the harmonics of barrel vibration also. which could be made worse by hot loads with slow powders. Obviously there's a hell of a lot of unresearched happenings going on after you squeeze the trigger.
I found this on with regard to rimfires, which backs up what you have said and which I've heard from others.

"Not only does the wax coating help prevent the lead from smearing off into the barrel, but it also forms a lubricated surface which reduces friction, slightly fills the pores of the metal, and ultimately makes the bullet’s path down the barrel more uniform in velocity. Thus, there is an accuracy gain from the lubrication.

Digging in deeper, a general rule of thumb I have seen is that it takes on average one bullet for every inch of barrel length to properly fill all the barrels pores and lay an even layer of lubrication from the chamber to the muzzle. For premium hand-lapped match barrels like a Lilja, it takes fewer rounds, maybe about half. For rough or hastily made barrels, it can be much more. There will be more discussion about barrel quality later on.

This is where you start to see the rationale behind shooters not wanting to clean their barrels. It’s because they remove all the wax lubrication and the next series of groups are not very good. That’s the result of the bullets “squeaking” down the bore without the aid of a fully lubricated path. This is where the phrase “Fouling the Bore” comes from. It’s the process of shooting enough rounds down the barrel to reapply the desired amount of lubrication."

04 Feb 2015
@ 02:49 am (GMT)

Michael Lang

Re: Barrel Bulges
I think that the theoretical pressure spike in rifles is just that, theory. A bullet/projectile is only accelerated if the pressure behind it is greater than both the air pressure ahead of it and the friction of the barrel. Therefore, I believe that the only reason for a second spike would be either an obstruction in the barrel or a flaw in the barrel ( i.e. a rough spot in the rifling such as a burr or an unintentional narrowing creating a bottleneck). These are the only explanations I can come up with without deep research.


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