I don't know what happened to my last link to an article. But here's what we sometimes see.
I have the RSI Pressure Trace equipment. I like it because it gives me real world results in the weapon at hand as opposed to a Pressure Rifle, that gives very accurate results and is used by SAAMI, but the results in a production rifle are only calculated, to be sure, the calculations are very good, I just think the Pressure Trace is more accurate for the given rifle I have the equipment installed on, which is on my bench, not your results, just mine, get it?
There are many reasons for the secondary wave, most of them a WAG. But you can see here that the normal pressure went up to about 48,000 psi and fell off to 8-9 thou and then a event occurred, sometimes called a "Detonation", with a spike that approaches 120,000 psi. We can discuss the many possibilities to the cause, but it is only important that you understand that a Pressure Wave Event is very much possible with reduced loads.
Though I am about to quote someone else, it speaks as if I wrote it, so here goes..
.."Indeed powder formulators and powder manufacturers have known about this phenomenon for some time. I first heard about it more than 20 years ago, before good instrumentation was readily available, and in reference to ball powders. A friend who worked at one of the powder companies once told me, "If consistency of performance where the only issue in powder design, ball powders would not exist. Ball powders are simply less expensive to manufacture and make it easier to produce ammo with consistent charges." He then went on to explain, " The three primary formulation features of powder is nitrocellulose content (or base material composition), granule shape and granule coatings. If the granule shape is spherical, then coatings become far more important to maintain a desired burn rate. Unfortunately coatings can burn off and are not the most reliable way to maintain a burn rate for every circumstance."..
.."Power companies have no control over how a particular powder will be used. They must rely on ammo manufacturers and those who produce load manuals to keep things safe. Ammo manufacturers and compilers of load manuals do a tremendous job, but there is no way they can anticipate every possible combination that will cause secondary pressures. Given the litigious nature of our society, this is a real touchy subject, and most in the industry would prefer shooters remain ignorant of the phenomenon."..
.."The above chart is typical of what can be seen in some hand loads and factory ammo. The following is a more severe example of secondary pressures provided by a member of a Midwest Highpower club. It is the club's "reference" ammo tested in a commercial lab to around 63,000 PSI but actually produced nearly 120,000 PSI in his old competition AR!.."
.."The "Catch Up Theory"...."We do not know if the above load suffered from an accelerated burn rate (problematic with some ball powders), but agree with ballistic engineers about the probable cause."..
.."The area under the pressure curve directly relates to the energy imparted to the bullet. The rise to peak pressure engraves the bullet into the rifling and establishes its initial acceleration down the barrel. The highest rate of acceleration occurs just past the point of maximum pressure. As the bullet travels toward the muzzle, lower pressure coupled with bore friction allows the rate of acceleration (not speed) to fall."..
.."If there is insufficient gas produced by the powder (burn rate too slow), pressure behind the bullet will drop excessively. Then, as the bullet's rate of acceleration falls due to bore friction, gases may "catch up" to the bullet before it exits the barrel and produce a secondary pressure event. In the above load we believe the heat generated from initial ignition coupled with a secondary pressure event increased the burn rate of residual ball powder to near detonation."..
.."Note: Secondary pressures readings taken at the chamber are lower and of longer duration then the actual event due to compression of gases behind the bullet and the time required for expansion and contraction of barrel steel. The above event may have spanned only .1 milliseconds of time but could have reached 150,000 PSI!"..
.."Ball powders do not create the phenomenon of secondary pressures but the resultant pressures can be more severe. Indeed, secondary pressures can even occur when using large extruded powder. When using ball powders it is simply more critical that a powder with the proper burn rate be used to avoid secondary pressures entirely."..
.."In every instance when secondary pressures are detected they can be eliminated by using a faster power, heavier bullet or a bullet with more bore contact area. Normal "tweaking" of loads may change the peak of secondary pressures but will not eliminate the problem. Below is the list of factors we now know can cause secondary pressures."..
1. Powder burn rate too slow for the bullet.
2. Bullet weight too light for the powder's burn rate.
3. Bullet bore contact area less then normal for the bullet weight
4. Barrel longer then normal
5. Bore severely worn or incorrectly lapped (loose/worn toward the muzzle)
6. Moly in bore or moly coated bullets that reduce bore friction
.."We are often asked when secondary pressures are too high. Obviously secondary pressures more than 25,000 or 30,000 PSI at the weakest part of a barrel represent a safety issue. On a more practical note, loads that exhibit secondary pressures often show significant variation in barrel timing (when the bullet exits at the muzzle). Even if the timing does not vary shot to shot, it certainly will when the temperature changes so these loads rarely shoot well. Our advice is simply to avoid all loads that produce secondary pressures and keep peak pressures where they are supposed to be, in the chamber. If you shoot factory ammo, try a different brand. If you reload, use a slightly faster powder or heavier bullet."..
.."I recently had the occasion to work with a friend's .223 Douglas barrel that had shot perhaps 6,000 rounds of an off brand ball powder sold as "Data 2200". My understanding is the powder was actually reject 2230, a powder we know produces secondary pressures in small calibers. The owner shot around 2,000 rounds of the load through the barrel each season, then re chamber his good ol' shooter as the throat washed out. When I looked down the barrel with a bore scope I could see rings just down from the muzzle spaced exactly the amount that was removed from the chamber end each time the barrel was re chambered. This convinced me secondary pressures will eventually damage a good barrel."..
.."No doubt there are shooters who will debate our conclusions. We welcome anyone to pose other possible explanations. The "catch up" theory is the only one we have found both fits the evidence and can be used to eliminate these problems. To date, only shooters who do not have access to pressure testing equipment argue adamantly against the "catch up" theory. Professional Ballisticians we have talked to whose job it is to formulate powder seem to be in full agreement. Even if the theory is not the "entire" explanation, it is certainly useful."..
The preceding quotes were from the RSI Sight, but as I said, I could have written them and was just easier for me to steel them, than rewrite them, and besides, for your edification, it is only important that you know this malady actually exist.
BigBore ask a very important question, I have given the most accurate answer I can, at the moment, for there is much more to learn about this topic.
I have given all this to you, because I have been reading some startling data, about .."Going The Other Way".. and indeed, this can and should be done, but I fear it more than ..Going Up" and little is understood about this subject. I'd wager that many are learning about this for the first time, having never heard or read of it, ever. Now you know why the starting loads are listed at 80%, the problem is they don't tell us 80% of what?..
So Guys, let us not under load our cartridges, unless we actually determine how much powder we can get under a given bullet, at our chosen OAL (Dedicate a case and drill those holes and tap the sides of the case to settle the powder) and do not go under 80% of capacity or what can be described as a 20% reduction, by most Powder Manufacturers. BUT, I like 90% of capacity much better and don't get caught up with overly compressing the loads either, the increased results are marginal and the powder tends to clump up and is difficult to light.
One more thing, we need to be more like BigBore, who seems to know allot about reloading and shooting, but WAS NOT afraid to ask the questions. If we continue to be respectful of the fact that we all have different amounts of experiences and answer those questions reverently, then more questions will be asked and more learning will occur..t
by Hoot » Mon Jun 07, 2010 4:15 am 
