Is pressure the only factor?

[Tallinar]

When determining if your gun is "strong" enough to handle a given load, is chamber pressure ultimately the only thing to be concerned with?

I'm trying to wrap my head around why this is. Here's an example. Hodgdon's online reloading data for .45-70 Trapdoor loads shows the following:

405 GR. CAST LFP -- IMR 4198 -- max load: 32.0 gr, 1462 fps, 19,000 CUP
405 GR. CAST LFP -- Trail Boss -- max load: 13.0 gr, 1007 fps, 25,600 CUP

If pressure is ultimately what we're looking at, wouldn't it seem logical that if the gun can handle 25,600 CUP under Trail Boss, that the gun must also be able to handle more than 32.0 gr of IMR 4198? Wouldn't this seem to (theoretically) suggest that I have at least 6,600 CUP of pressure that I can play with?

What other factors am I neglecting?

 

[RandyP]

The 'trick' is determining that very fine line between a tiny increase in charge and a kaboom. Grains vs pressure is not necessarily linear, often it is exponential.

I would tend to trust the folks that did the original testing who had the means to conduct pressure tests? Unless of course you have a pressure test chamber?

 

[Tallinar]

Yeah, I understand that pressure is not necessarily linear. I know that once you reach a certain point, pressure can begin to spike violently even with a single grain of powder.

However, I'm just kind of interested in understanding how to interpret these variations from various reloading manuals. Since we're talking about 4198 in a .45-70, 35.0 grains is nowhere near filling the volume of the case, so there shouldn't be a dramatic increase in pressure with those extra 3 grains. But still, I'm sure they have their reasons for listing 32 grains as the max. The Lyman 49, however, lists 35.0 gr as the max, but obviously lists it with a slightly higher CUP.

It just seems that some published data has a very dramatic difference in CUP on their "max" loads for the same bullet/different powder. I guess I just don't understand where the max comes from all the time. If it becomes a simple case volume issue, then it makes perfect sense. But these don't always appear to be case volume related.

Another example from Hodgdon's online data for Trapdoors:
385 GR. CAST LFP -- H4198, max load: 32.0 1483 14,700 CUP
385 GR. CAST LFP -- IMR 3031, max load: 52.0 1819 25,900 CUP

Over 10,000 CUP difference, and I know for sure that they can fit more than 32.0 gr of 4198 in that massive case.

 

[Clark]

I have read on the internet that trapdoors blow up at ~ 55 kpsi.

My brother had his trap door at my house and we shot it with black powder.
Then he cleaned the minty bore with some SimpleGreen liquid detergent from my kitchen, flushed the bore with water, and dried it off.
That took the linseed oil finish off the stock in places.
He rubbed some motor oil on the stock, and it looked like new.

 

[rbernie]

The load data that you present in post #3 is NOT the load data in the printed Lyman #49 manual and is either incorrect in presentation or was transposed incorrectly. Lyman #49 shows 44gr of IMR3031 @ 15800CUP and 34gr of H4198 @ 16000CUP, with both loads acceptable for Trapdoor actions and with both loads yielding approximately 1450fps. The printed manual does not contain any data for Trailboss.

In the printed manual, the starting pressures for all listed powders are analagous and the max loads all show relatively similar pressures. Where are you reading the data that you presented?

 

[Tallinar]

the load data that you present in post #3 is NOT the load data in the printed Lyman #49 manual and is either incorrect in presentation or was transposed incorrectly

Sorry, I failed to specify where that data came from. That was from Hodgdon's reloading data website for Trapdoor .45-70's. Edited post to erase any confusion.

 

[rbernie]

Different test fixtures and measurement techniques will yield differing data. You also need to be very careful to not inadvertently compare data that interchanges CUP and PSI.

It is also common to see load data adjusted over time as test procedures changes. Since not all load data is retested for every iteration/version of a given vendor's manual, you will often see some minor 'drift' between loads for a given chambering (especially as new powders come on the market). I suspect that addresses your first post, in that the online manual is showing both the 'traditional' Trapdoor loads that were worked up in years past and a new load (suitable for modern cowboy action leverguns) using the new powder.

A 26000CUP load is not normally considered acceptable for a Trapdoor action.

 

[cougar1717]

I think one factor that needs to be brought up is that the pressure is not a single point like we see in the loading data. Strain gauges are used to determine the pressure of a cartridge over a period of time (although very small). I'm sure some powders cause pressure spikes worse than others. Without a strain gauge and other testing equipment, we rely on several different sources of loading data.

 

[Tallinar]

That explanation helps, rbernie. But I still have trouble understanding how the same gun with the same bullet could somehow be "safe" with a 25,000 CUP load with one powder, but be "unsafe" with another load using a different powder for significantly less CUP pressure.

Is there something besides CUP that would render a load dangerous with a given powder, while another powder with a higher CUP can be considered "safe"?

Maybe I'm having trouble wording the question. Hopefully I'm making some sense, hehe.

Thanks for the replies so far.

 

[rcmodel]

One possible reason is the velocity, not the pressure was used as the stopping point for trapdoor loads.

If you look at all the various trapdoor loads with the 405 grain bullet, they all top out in the 1500 - 1700 FPS range for which the older rifles sights would be regulated.
All at very low max load pressure.

Except for the Trail Boss load, which can't quite get there on velocity, even running well more pressure then any of the other powders listed.

rc

 

[KeithET]

Without actually asking Hodgdons about this we will never really know the why. As a guess I would point out that as a reloader we are taught to look for signs that indicate potential issues with a new load. One of those is signs of excessive pressure. Another is excessive velocity variations (if using a chronograph). The words "excessive variation" can also be added to any discussion of pressure as well. When excessive variations in either of these parameters is encountered it is a sign that something is not right. This could be a sign that the pressure is right on the edge of a dangerous increase. At this point the wisdom is to back off on the charge a bit to avoid those variations. My guess is that the Hodgdon technicians doing this testing did just that. Upon encountering excessive variations (in either velocity or pressure) they backed off on the charge to avoid any surprises for themselves as well as those using the data.

KeithET

 

[Jim Watson]

SAAMI standard for .45-70 is 28,000 CUP.
I think it a reasonable precaution to load to 18,000 or so for the Trapdoor.
I don't know why the same vendor would show it both ways.
Keith has a point, though. I read that the industry standard is to load one standard deviation below the peak. So if a given powder is rather variable, the printed load is going to be well on the low side.

I think a lot of people let Trail Boss' low density/high bulk trick them into thinking it is a lot slower burning powder than it really is.

 

[fecmech]

I'm no expert but here is my take. #1 Trail Boss is a pretty fast powder right up there with Green Dot, AA2, imr PB etc so maybe it is pressure limited in the 45-70 trap door. OTOH powders such as 4198 and others may be somewhat velocity limited due to the weak locking system on the trap door and bolt thrust issues. Just a guess.

 

[GaryL]

As fecmech alluded to, the pressure curve makes a big difference. A fast rising pressure spike is like a hammer to brittle metal. The metal may have the strength, but can't handle the shock. A slower burning powder will build pressure at a more gradual rate.

CUP is an integration method of pressure measurement. There is a force acting on the copper slug the whole time, which deforms until some point a little after the pressure peak. PSI, on the other hand, is an average peak detection method. There are several ways to measure it, and a fast acting transducer with a high speed Data Acquisition system will provide an accurate profile of the pressure curve, but anything less than that will tend to average the data points and reduce the peak value.

 

[Jesse Heywood]

Adding to what GaryL said.

Part of the issue is the time elapsed when the pressure occurs. The older system of CUP involved measuring a strip of copper place in the chamber. The thickness of the copper is measured before and after firing. The pressure is calculated from the amount the copper is compressed. There is no time curve involved. Therefor a slower burning powder can have a higher pressure shown with less shock to the gun.

The newer method of measuring PSI uses electronic sensors that will also show when the peak pressure occurs.

Without knowing when the pressure occurs, the CUP method relied on someone raising the pressure until something went wrong, then backing off for the top load. With either method the published data does not show the time factor, which leads to confusion when there are major differences with a fast and slow powder.

 

[Jim Watson]

A strip of copper in the chamber?
All the US style crusher test setups I have seen illustrated had a HOLE into the chamber. Not something you can do with your favorite rifle. A steel piston is set into the hole and a heavy yoke clamps a small cylindrical copper gage (or lead for shotguns) against the piston. Firing the gun compresses the copper. Each production lot of gages is calibrated with a dead load of weight or hydraulic pressure and the pressure versus deformation listed in a tarage table. The time and energy required to deform the gage plus the dead load calibration mean that the actual peak pressure is not recorded like an electronic transducer.