Let's Talk Sixgun Pressure

[dunnyboy]

I am an avid collector and shooter of vintage Colt single actions. As you know, there are black powder era Colts, smokeless era Colts (with verified proof markings) and then there is that gray area of single action production during the transition era - after the cross-pin retainer was introduced in the mid-1890s through the point that verified proof stampings were incorporated around 1902. I have revolvers in this transitional period of production and thus my quest for information.

Black powder era Colts were proofed at 12,000 cup to 14,000 cup depending on the source. For sake of argument I am comfortable in assuming that Colts, from the earliest production, were deemed safe for standard black powder 44-40 cartridge loadings at 12,000 cup.

Fast forward to today, and IMR Powder provides loading data including pressure specs for the 44-40 using their great Trail Boss smokeless powder. There recommended minimum loading is 5.5 grains of TB behind a 200 grain, .427 sized cast bullet. Pressure is under 8,000 cup.

There are admonitions everywhere against using smokeless powder in black powder era revolvers, and certainly these are warranted in most instances. However, I am looking for further enlightenment on the comparison of BP pressure vs. smokeless pressure, and whether there is a difference in the velocity of the pressure curve or other factors that would render a max pressure from a smokeless propellant of 8,000 cup, as hazardous in a firearm proofed for black powder pressure of 12,000 cup. Are smokeless pressure and BP pressure comparable from a measurement standpoint or is there more to the story.

I'm not looking to use Trail Boss loads in the early black powder era Colts, only in the smokeless transition era revolvers. I trust IMR loading data to be accurate as to their pressure specs. Allowing for a hefty margin of error, 8,000 cup would still seem to be well under the safety threshold for even the earliest Black Powder design.

Any thoughts to share?

[Outpost75]

Industry practice was generally to proof at 30% overload above "working pressure", but that old term is no longer used. The "maximum product average" is defined as the sample average plus three standard deviations. So your sample average or "working pressure" if you want to use that terminology, should always be substantially less than the max. product average.

Not knowing how uniform the round to round pressure variation is in your unknown sample, a prudent practice would be to keep pressures of smokeless loads, for guns proofed to blackpowder specs., such as 12-14,000 psi or cup, under 10,000 units, by which ever method of pressure measurment is used.

Many modern powders do not give acceptable ballistic uniformity in reduced charges. One which has worked well for over 100 years is Bullseye. I have found that in .45 Colt brass, using 260-grain bullets of 10 BHN, I can load as light as 5 grains, for 700 fps in a 5-1/2" barrel. In Schofield brass you can go as light as 4 grains with a 230-grain bullet for 630 fps. I do not recommend reducing these loads further.

[Dale53]

dunnyboy;
I know that you didn't ask this. However, one actually rather practical practice is to use black powder. In the .45 Colt, FFG delivers noticeably less recoil and pressure for a more pleasurable round. It is still QUITE effective.

I have shot a good deal of black powder (muzzle loaders) and black powder cartridge guns but frankly I have shot hundreds of times more smokeless. I got into BPCR Silhouette years ago and they had side matches for black powder cartridge revolvers. I ended up with a couple of Rugers (a Vaquero and later a Bisley Vaquero). I learned to manage them quite well, as a matter of fact. Clean up became as simple as smokeless clean up (just had to do it more often:mrgreen:).

I had excellent accuracy results with the Lyman 452664 Cowboy bullet (250 gr RF) and discovered I could move it over 900 fps or excellent results at 750 fps (different brand and granulation of black powder). Accuracy was in the neighborhood of 2" at 25 yards. I won more than my share of matches and gained a great deal of respect for the original black powder .45 Colt.

Just something to think about regarding those vintage Colts. My Rugers shot with black powder (thousands of times) are just as good to look at as the first time I shot them. I learned to clean at the range using a good black powder solvent and it was relatively painless.

FWIW
Dale53

[dunnyboy]

Outpost 75, thanks for the input. Your reasoning is very much along the lines of my own thinking, and the information available seems to be consistent with that approach.

Trail Boss is a rather unique smokeless powder that, although loaded to specific grain weight, the optimal charges are determined by volume. IMR says it is safe in all cartridges with a recommended starting (minimum) loading of 55% of usable case volume - defined as case volume up to the base of the bullet being loaded. Maximum loads, absent qualifying factors such as firearm condition or age, is 100% of usable case capacity. It is not to be compressed. Minimum starting loads have all proven to be accurate and clean. Another characteristic of the powder is that increases in charge weight move velocity up ever so slightly relative to increase in pressure. I have found the minimum loads optimal in almost all instances, and have used it in 32-20, 38-40, 44-40 and 45LC. It was designed for mild loads for cowboy action shooters. I am pleased with the velocities it generates in my revolvers at the low end, usually in the 650 -750 fps range. At 8,000 cup it is lower than any proper BP loads,which of course, can't be safely reduced below a full charge. With BP its really all or nothing. So if I go with the factory loading of BP or any of the substitues, I am still at the 12,000 cup rating according to the data I have.

[dunnyboy]

Dale53, thanks for the input. I have considered using BP loads, and may still do so as a change of pace. However, for pure simplicity, it would be ideal if I could use the same basic loads in all my Colts. I already use - and really like - Trail Boss powder due to its rather unique characteristics, and the Colts really like it as well. I get fine accuracy with all of them. Many of them are later Colts of smokeless persuasion, so the issue was really with the transitional production models, and thought I would explore further whether there were any inherent characteristics of smokeless powder that I needed to consider beyond the max pressure specs for the selected loadings. I understand from my research that BP loads really must be full case loadings to ensure proper function, in other words, no reduced loads in black. The published data I can find shows 44-40 BP full factory loads at approx. 12,000 cup. My assumption is that 12,000 cup is the minimum pressure for BP loads in 44-40. That given, it appears that it is less stressful on the old Colts to go with the starting loads of Trail Boss at just under 8,000 cup, unless I am missing some other pertinent information.

[Silver Jack Hammer]

dunnyboy, Your approach at considering chamber pressures when loading smokeless in bp rated firearms is interesting, I have never heard anyone else consider this. As I'm sure you already know, Colt did not proof their firearms for smokeless until 1898. The cylinder base pin retaining latch appeared on some SAA's as early as 1892 so this feature is not a good indication of if the Colt is a black powder only model or not. Colt labeled the boxes the guns were shipped in as safe for smokeless powder or black powder only but those boxes seldom survived shipping and were usually separated from the guns shortly after purchase.

I have not heard of anyone using Trail Boss in modern bp guns and I have not heard of IMR suggesting Trail Boss be used in any bp guns. I would preserve any old Colt for posterity. New guns are half the cost and steels are not questionable. Accidently ruining a new gun does not remove an artifact that history had preserved.

Personally I'm more comfortable with the 192,000 serial number as it is this point where Colt began to warranty their guns for use with smokeless powder. The vp proof verified marks started appearing in 1902 put Colt put them on the trigger guard.

When I shoot bp it is readily apparent that it is a horse of a different color. The pressures are very low yet the velocities are very high. Bp transfers it's energy with a high byproduct of heat. I have never seen or heard of bp blowing up a gun.

I have thought it odd when the frame is referred to as proofed or not, but it is the cylinder that is the weak point. Would installing a smokeless cylinder and barrel in a steel, heat treated bp Colt SAA be safe with smokeless powder? What if the barrel and cylinder were .38 Special?

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[Silver Jack Hammer]

I already have my copy of Dave Scovill's new book. It is excellent.

[dunnyboy]

Silver jack, yes, I generally agree with the Colt history, but there is more detailed information now available. The smokeless transition began in 1896 with frame modifications and more notably, a standardization to .006" for the cylinder / barrel gap. At the same time the composition of the Colt steel was evolving. and most probably heat treating as well. It was a new era, with new challenges, and all the firearms manufacturers were busy experimenting with solutions to the pressure question. examination of Colt factory records now placed the actual transition to "certified for smokeless powder" to serial number 180000, in mid 1898 production. Kopec has documented that the 1898 output between 175000 (beginning) and 180000 were not fully certified in all regards although Colt advertising in their 1898 catalog proclaimed the single action as smokeless ready. From 180000 forward, Colt was selling the revolvers as smokeless models. No one is sure what thresholds were not met before 180000, perhaps there were still some BP parts flowing onto the line. We'll never know. Between mid 1898 and 1902 the "Viewed and proved" stamping was not used, but apparently it was added more as an afterthought than one signifying some significant milestone, as it was only variably employed until 1905 when it is thought to be finally uniform in use. Further complicating the picture for all Colts of the era was the lack of serial numbering of cylinders during most all of the transition period. There is no way to know, even on VP stamped frames, whether the cylinders were BP, transitional, or smokeless era. They were and are still interchangeable and many have been over the years. With all this in mind, I am still mulling the original question. Is 8000 cup chamber pressure generated by a smokeless propellant of lesser stress on steel, regardless of its metallurgy, than 12000 cup generated by BP? I have not yet received any evidence from the powder manufacturers or the shooting community that documents why smokeless chamber pressure is not the sole determining factor. I'm interested to know if someone has such evidence, scientifically derived, of course.

[Outpost75]

It would be interesting to see time-pressure plots from a modern piezoelectric transducer setup using black powder vs. smokeless.

My "gut" instinct is that blackpowder probably has a more rapid rise to peak pressure, and a more rapid pressure drop-off as a function of time and bullet travel, whereas progressive burning smokeless powders have a more gradual rise and drop-off, for even after all the powder has burned, the gases continue adiabatic expansion.

The result is that smokeless loads of the same maximum pressure will have "more area under the curve."

[dunnyboy]

Good point, Outpost.

[Silver Jack Hammer]

dunnyboy, your inquiry is very interesting. I've got a couple of Kopec's books, and I have met and spoken to the man. As I'm sure you are aware Colt started experimenting with steel frames in 1893. The new steel frames were adopted in the 95,000 to 96,000 serial number range. I don't know what serial number range or date or condition your guns are, or if they have been altered since leaving the factory. Kopec writes "Caution should be exercised in the firing of these old revolvers, and only black powder ammunition should be used to keep well within the safety zone."

If no manufacturer is going to publish pressure levels of Trail Boss for the purpose of loading in antique Colt pistols, I'd suggest starting off with an Italian thin cylindered revolver and experimenting with it. I've seen guns blow up and I've checked out a gun ordered destroyed by a court and pressure tested it to failure under controlled circumstances. When it comes to a black powder frame Colts, personally I'd just steer my stage well away from the edge of that cliff.

[BCRider]

.....My "gut" instinct is that blackpowder probably has a more rapid rise to peak pressure, and a more rapid pressure drop-off as a function of time and bullet travel, whereas progressive burning smokeless powders have a more gradual rise and drop-off, for even after all the powder has burned, the gases continue adiabatic expansion........

My own gut feeling is that black powder CHARGES burn in a more progressive manner from the cap or primer end to the front end by the bullet. The result being that it takes longer which leads to the longer and softer "THUMP" to the hand instead of the typical smokeless powder's "BANG!"

Granted in open air BP darn near explodes. But that's because more of the particles are exposed to the flame front. The key to controlling this is to pack the particles into a compressed stick form. By doing so we can control the way the powder burns.

This is borne out by the fact that BP has to be compacted anywhere from slightly to firmly in order to avoid a pressure spike. Even lighter than typical BP charges can produce dangerous pressure spikes if there's an air gap and the powder can move around. But when compressed into more of a stick like form from the proper compression it would appear that the powder burns in a more linear manner from rear to front.

This also suggests why we see fairly high velocity particles of the powder still burning after exiting the muzzle.

So all in all, and keeping in mind that I've not seen a graph of the chamber pressure obtained from a transducer, I'd have to say that with BP the peak pressure could actually be lower than many smokeless loads for the same bullet weight and muzzle velocity.

EDIT- I did a google search for "black powder pressure graph" and looked at the images. There's a bunch of them that seem to suggest that this "lower pressure for longer" seems to apply to actual black powder loads.

I also noticed an image that sort of confirmed what I've seen in the loading data for Trail Boss that suggests that it's less than ideal in larger rifle rounds. The loading data suggests high pressures for relatively low muzzle velocities. And that is supported by one image of the pressure tests that shows a short base with high peak compared to some other powders.

Google for "black powder pressure graph images" and you'll see what I mean rather than try to post a bunch of the images here.

[Groo]

Groo here
A LLLOOONNNGGG time age I recall a piece in one of the gun rags on gust this thing...
Black powder has a pressure limit above which it cant go, 14000 to 18000 psi depending on the F rating and quality.
Smokeless will continue to increase in pressure.
Where as smokeless will burn at a steady state [ flamable solid] BP is a low grade explosive and hits fast then the pressure drops.
The problem with BP steels is that BP goes to 16000psi average fast but not for long and smokeless mantains the pressure for a time.
That time is what stresses the metal , The sharp hit is what causes the boolet base to swell and seal the bore better than we think it might.

[W.R.Buchanan]

Dunnyboy: why don't you call one of the powder manufactures, like Hodgdons, and get an answer to the question from someone who knows exactly what they are talking about. They could probably suggest or refer you to safe loads for your guns or confirm that BP is the only safe propellant.

All we can offer is Anecdotal information, and no one so far has definitively answered your question, so why not go directly to the "Horses Mouth" so to speak, and then come back and tell us what they said.

Randy

[bearbud]

Dunnyboy: very interesting subject.
I had the exact same question in regard to a Webley WG 455/476, same vintage (1895-1900) as your Colt. I posted last month and I received very useful information from jethunter that you may want to look at.
Also very good reading is by Sherman Bell at the Double Gun Journal, p.19, Summer of 2002. "finding out for myself smokeless vs. black"
His work is the comparison of pressure between smokeless and black that you may find relevant to your question.

[james6600]

Groo Hit the answer, I cant verify his numbers but somewhere 20 years ago I read the same thing, black can only build pressure to (blank) then it stops. The book did not explain what happens at that pressure, a flame out, a smolder, I don't know. Smokless has a way different pressure curve and can build way higher pressure before it goes linear like black. But TB is a whole nother animal, it's low pressure (I don't know the curve) but when compressed or kernels broke it spikes like crazy. Where the problem lies is I doubt IMR/Hodgdon will OK it's use even if it would work at min. charges because these weapons are 115 years old. Like you I think at 8000 it looks good on paper but is the steel soft and malleable, is it work hardened, or is it brittle and porous like cast and how will this pressure curve change the metal if at all. Short of doing the test properly yourself and bearing the outcome good or bad I dont think you can obtain a safe answer. Remember even if your Colts are proven 100% original the next persons may not be and how many times have they already been (proofed) with a stray smokeless round or box after 115 years.