Barrel Fouling: Black Powder vs. Substitutes

[cryout-pullquote align=”left” textalign=”left” width=”100%”] The following text is a message board post by Bill Knight, a retired chemist with black powder industry experience. He is responding to a comment by the late Dick Trenk, who was the factory rep in the US for the Italian manufacturer Pedrisoli.[/cryout-pullquote]

Dick’s Comment:

From:    “dicktrenk”
Date:    Wed Apr 27, 2005  10:20 am
Subject:Pyrodex & 777 corrosion problems clarified

On the BP-L message board there was a very interesting and important thread running due to a report I posted about Hodgdon Pyrodex and 777 powders causing corrosion under certain conditions. The world’s most-recognized powder expert, Bill Knight, has been off the boards for a year and was not aware of this thread so I called him and discussed the subject.

Bill was kind enough to prepare a long report which clarifies this problem very nicely.  …what follows is what Bill Knight has to say about Pyrodex and 777 powders.

Dick T.

Bill’s Response:


According to the original Pyrodex patent, the formula that appears to be the propellant composition contains 17 parts of potassium perchlorate. When the powder burns, the potassium perchlorate gives up its oxygen, leaving potassium chloride. This potassium chloride will be found as tiny crystals scattered over the surfaces of the bore after the projectile exits the muzzle.  These crystals are mixed in with other chemical compounds created by the combustion of the powder.

Potassium chloride is by itself not overly hygroscopic.  If crystals of potassium chloride are scattered over the surface of a steel plate they will not began to attack the metal until the relative humidity reaches a certain point.  Generally fairly high humidity is required to wet the surfaces of the crystals.  Once the surfaces of the crystals become wet, they will then begin to attack the metal via electrolytic corrosion.  Rather than a uniform shallow surface rusting as would be seen with potassium carbonate, the primary solid product of combustion with black powder, the potassium chloride crystals will cause deep pitting of the metal’s surface.

In the case of brass, a film of potassium carbonate on the brass will cause discoloration of the surface of the brass—various shades of brown to black, depending on the humidity.  But, the potassium carbonate will not actively pit the brass.  Attack on the brass is confined to a mild surface leaching of copper from the brass alloy.  Any potassium chloride present in the residue on the brass will result in surface pitting of the brass.  The extent of pitting being largely a factor of how much potassium chloride is present.

According to the MSDS for Triple Seven on Hodgdon’s web site, their Triple Seven does indeed contain potassium perchlorate, but no formula weight is given for it.  Working with open flashing on brass and steel showed that the amount of potassium perchlorate in the Triple Seven is considerably less than that found in Pyrodex.  But, it only takes small amounts to change a powder’s combustion residue from a rusting agent to a pitting agent.  Old chlorate/perchlorate primers exemplified that property.

A rep from Hodgdon has long stated that Pyrodex is not corrosive if the gun in which it is used is properly cleaned.  The issue in this is: what is “proper” cleaning?

Under certain atmospheric conditions the powder residue’s attack of the metal may occur in a few short hours.  The electrolytical reaction is driven by both temperature and humidity.  In moist climates the damage to the bore may be in 3 to 4 hours.  In a less humid climate the damage may take several days.

Recently I have been asked several times to look at various BP subs to see how they compared to a good BP in flintlocks and percussion rifles shooting patched balls.  Anytime I shoot Pyrodex or Triple Seven, I shoot it first and the follow it with my BP rounds to purge the bore.  Then, when I clean the bore at the end of the day’s shooting, I do so with copious amounts of warm water.  Potassium chloride is considered to be poorly soluble in water.  When other water-soluble products of combustion are present in the bore cleaning water, the crystals of potassium chloride do not want to dissolve.  Large amounts of clean water  are required to remove all traces of the potassium chloride, ammonia and chlorides.

The fumes given off by ammonia (in water) are corrosive.  When combined with a chloride salt, such as potassium chloride, the effect on the metal is akin to ammonium chloride.  I have used solutions of ammonium chloride to brown M-1 barrels and iron parts of M-1 accoutrements such as the pick and brush sets that I used to make.  Under the proper conditions, the resulting rust (browning) is rapid.  Roughly 2 to 3 hours per application.  A buddy and I jokingly refer to ammonium chloride as essence of horse urine.  The coarse heavy rust formed by the ammonium chloride browning is difficult to “kill”.  WD-40 and other wonder products don’t begin to slow or kill the rusting process.

I was a bit taken aback by this thread on Pyrodex and Triple Seven, since the subject of BP substitutes has been taboo on [this message board] except during periods when the posting rules are liberalized.  During the past 2 years I have done more work with the subs than with BP.

We are looking at a trend, of sorts.  Smaller gun shops are turning in their explosive licenses.  Stocking BP in the gun shops raises property insurance rates.  If a small shop does not sell enough black powder to defray the additional insurance costs, they usually quit stocking BP and then push the various subs.  If this trend continues for the next few years we expect to see most civilian small-arms BP sales going through distributors, shipping case lots to individual shooters or groups of shooters who pool their money.

So, like it or not, the subs. are becoming more popular as the availability of real BP to the average shooter becomes more limited.  Which is why there is a recent scrambling for more of the market by the various BP sub. producers.

I view this shift in emphasis from black powder to the subs with some degree of sadness.  U.S. shooters now have a choice of three brands of BP.  From the perspective of a patched ball shooter (if it ain’t round it ain’t right!), the Swiss powder simply cannot be beat for performance.  The Schuetzen powder is a great improvement over the WANO powder once imported into the U.S.  Goex has made great strides in getting their act together with a quality powder.  The failure of the only U.S. producer of potassium nitrate in 2000 forced Goex to look to other sources, which proved to be higher in purity.  More recently, they were able to get a handle on a decent charcoal supplier.  So, the quality of the BP’s commonly available has improved dramatically in the past few years.  Now this question of future availability crops up.  That is not to say that it will disappear from the market, it will simply be increasingly more difficult to obtain in anything less than case purchases.  We BP shooters now have more than one brand of BP to select and all three are quality powders at a reasonable price for the technology involved in their manufacture.  Now I get to kick back relax and enjoy life.  So now I get in a few days of shooting mixed in with long bicycle rides on the various rail trails around here.  Retirement is so nice!

Bill Knight

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