Maximum pressure

That Steve, is a very tricky question? I guess quick loads can give you some what of a ball park but then the loads are built upon a bullet that is "grove" size and has to "force" its way into the lands. Muzzle loaders on the other hand, for the most part , are dealing with a bullet that is already sitting on top of the lands and by using enough pressure, we are hoping to get it to "fatten up" or "obturate" and somewhat engrave itself into those lands. Even bullets that we "full/form" or actually engrave the rifling prior to loading, are not being "force" into the groves so pressure readings via quick loads can be sketchy at best. Then, we have no direct knowledge of how much pressure our barrels can withstand, prior to bursting? Obviously, a .40 cal. barrel that is 1.25" in the chamber area will have more thickness than a .45 with the same dimension. If my barrel (which it is) is 1.300" and a .45, how much more pressure will it contain than the 1.250" barrel? Currently, we have no one on the forum, that I know of, who is doing Pressure Traces? And even so, that does not tell you how much pressure your barrel can contain?
Sometime back, Jeff was going to experiment and attempt to blow up a barrel using increments of powder to see the point where it gives out? Like everything else, running a business can cause restraints on time and that test never happened. Yes, I did send Jeff a old .45 PN barrel that I was not using. So where does that leave us? Personally, I think our #17 contour barrels from Brux will handle all the pressure we are willing to endure. Safely! Where our biggest problem lies is with marginally thin barrels (on break action rifles) that are double charged. The problem here is the rapid rise in pressure that cannot move the load in front of it.............so...............the barrel will rupture. The thinner the barrel, the greater the risk. I was surprised however when FoxGrape called me to advise his #17 .45 barrel and rifle I put together, had burst from an admitted double charge. I have bulged three barrels since 2007 and probably some 20,000 muzzle loader shots. These barrels were salvageable by cutting off 3" and rechambering. Why his burst and mine did not??? Could have been just his charge? or he might have had another full load of powder and bullet where I only had two bullets...........that could be an extra 70 gr. of powder weight and just enough to cause the damage.
It would be nice if we had a mechanical engineer in the crowd that was capable of giving us an actual "burst" pressure on a given size barrel.
Just as an example, when Jerry was doing pressure traces and did one with my 6/65...110/4198 and a 300 Aeromax. (he was using his own direct ignition modules which were 3/4" long and had a .125" hole). He bulged his 1.350" .45 barrel and blew out the extractor. The pressure trace went up off the chart and the digital reading stopped at 115.000 psi. So that gives you some idea of how much pressure these barrels are capable of.
This is why Steve, its hard to put a number on Safe Pressure? I have always said that it is not the fact you load an extra two or three or four more grains of powder, its the TWO 300 grain bullets that the powder charge cannot move fast enough that will cause the sharp pressure rise and burst the barrel.

If you are shooting HIS out of a bolt gun, should you worry if the bolt is a little sticky if the modules show no sign of pressure? Primers don’t look flat.

That's a pretty stout load with 325 gr bullets. If your getting heavy bolt lift that is always a indication of pressure and you should back down from that load. I do shoot 105grs with a 300gr Aeromax in my 1.250 straight no taper barrel with a 209 ignition. I see no signs of pressure on the primer. With the 209 primer I measure the diameter of the new primer and then the fired primer and check the diameter of the unsupported portion just ahead of the bolt nose and see what it measures if its starting to expand more than .002 I don't go any higher its the best way I have to gauge pressure on that ignition system.
If you really get into high pressure you will see leakage between the cup and the housing that is not where I want to be.

With primer modules if you have primer pockets that are loose after only a few firings your to hot from the start and if there sticking when you open up the bolt after firing that might tell you that your working with to much pressure but if your using a Rem. 700 factory action your bolt timing might be off far enough that you won't have enough primary extraction so that could give you a false reading on pressure.

Bottom line I would say 105grs of H4895 with a 300 & 325gr bullet is maxed out. If you want more speed you could go to a slower powder but you will need to add a lot more to gain only a little more speed and probably less accuracy.

OK, so taking this discussion a little further, we have somewhat established that when you get up in the 55 to 60,000 psi area, you might notice primer pockets loosening or even completely opening up so that the spent primer falls out of the module. This is good information and definitely where you should be backing down on your load or the weight bullet your are shooting with that charge. As Bill mentioned, 105 gr. of H-4895 with a 325 grain bullet is about maxed out for degrading your modules. Now drop to a 300 grain bullet and you may find no damage to that module.................weight vs. charge. That being said, it does not mean that if you bumped that charge up to 115 grains with the 325 bullet that you will blow up the barrel? It may kick like a mule and destroy your module but in all probability, not destroy the barrel.
I was right next to Jerry when he shot/tested that duplex load and I saw the "blip" on his PT unit go off the top of the screen at 115,000 psi., it could have been 120,000 for all we know? But, we did not see barrel failure. We saw it blow the top portion of his steel flange head direct ignition module completely off.......gone! We did see it blow out his Sako extractor (but not destroy the bolt. The breech plug was able to be removed unscathed. So, we still do not know what the strength is for any given thickness barrel before complete failure will occur?
It seems that somewhere way back when we first started playing with changing out Savage barrels that a number like 110,000 psi was being tossed around, whether by Savage or whoever, that was supposed to be the rupture point on their barrels? Don't know it it was hearsay or what? So a question I have is directed to our barrel manufacturers...........Can you tell us what the "burst" point (in psi) of say a .45 cal. barrel that is 1.250" or 1.275" or 1.3" or whatever dimension?.....the same for our .40 barrels? I am sure there are qualified engineers at these companies that specify barrel strength for liability purposes?

Steve...how does powder column height figure on here?

The load above is a very accurate load, in some rifles the optimal cahrge weight might vary a bit but usually from 102-105 is the accuracy node.

This load is also pretty stout, both on the shoulder and on the animal, when kyle first introduced the 325 Aero and sent me some test bullets I was in awe at the exit holes in the deer I tested them on, I also shot a .167 at the Challenge @100 with that same load, it is also VERY temp stable load, as stable as anything I have tested to date, it sure lets you know when you pull the trigger too, even with a really good brake.

It is definately not a load for the masses!

Just for comparison and to show the limitations of H4198 with higher weight bullets. Note: this is with a 28” barrel. Likely 50’/sec less  , for a 26”, but same peak pressure.

I'm a mechanical engineer :-).  I don't work in the gun industry, but spent a decade designing mobile industrial equipment.  For things like hydraulic cylinders or steel pipe/tubing carrying pressurized fluids, typically we'll use what's called Barlow's Formula.  It's a pretty standard basic formula for pressure vessels.  There are more complicated formulas that take more variables into account (like is required by some ASME codes), but this one is generally a good starting point.  The formula is:
P = (2*s*t)/D, where:
    P = pressure (psi).  Typically a target working pressure for an application
    s = material stress (psi)
    t = wall thickness (inches)
    D = cylinder outside diameter

Typically for design purposes, for "s" we use some agreed-to value for a safe working stress.  Normally this is the material's yield point, divided by a factor of safety which will be chosen based on how confident we are in the data, the criticality of the application, how much shock-loading is expected above the normal working pressure, etc.

The above formulation outputs an allowable working pressure.  Rejiggering it to predict the what stress the material would exhibit depending on the internal pressure it is exposed to, the formula is: s = (P*D)/(2*t)

Times when you had a bulged barrel, the pressure exceeded the yield stress of the barrel steel but did not exceed its ultimate tensile stress.  When a barrel bursts, that means the pressure exceeded the ultimate tensile stress causing fracture.  There's all sorts of things that can happen in between and ways that behaviour can vary, like the ductility % of the material and how much it strain-hardens, its impact-fracture energy, and depending on the ambient conditions a degree of cold-embrittlement could come into play.  Dynamic factors (I'm guessing including the speed of sound in the steel...) would also come into play.  As everyone who's looked at a QuickLoad calc or a pressure trace knows, this **** is complicated haha.

Taking a really quick, back-of-the-envelope crack at it, if we assume using a 4140 steel with Yield of 105,000psi and an Ultimate of 120,000psi (numbers just off the top of my head, they will vary from material to material, manufacturer to manufacturer, lot to lot), your barrel OD of 1.30" and taking the ID as being the outer diameter of 5/8" breech plug threads, Barlow's would give the below chamber pressures for yield and burst:
P(y) = 54,519psi
P(u) = 62,308psi

If instead taking the inner diameter to be 0.458", we'd get:
P(y) = 68,008psi
P(u) = 77,723psi

NOTE: these values are for DICSUSSION PURPOSES ONLY!!  These are NOT values to design to, I don't have any data for the actual material properties of typical barrel steels let alone any individual barrel, these are not safe predicted pressures, dear sweet lord no one try to do anything practical / physical with anything I've said above without doing a heck of a lot more work, reading, and taking your own responsibility for what happens next.

I'm hoping to get into pressure-tracing in the soon(ish) future, and later in life when I can build the garage and machine shop capabilities I want in it I'd love to get into firearm testing and data acquisition.  But that's some years off still at this point :-(

Cheers from Canada,
Mitch

Ok now I'm curious what the actual material properties are. I've reached out to the barrel maker I use, to see if he has any material property information about the barrel steel he buys. Will update if I learn more.