Maximum loads...

[tstone]

Just wondering....
Some manuals (Lee) list pressures in the load data for max loads. For example, powder "X" has a max charge of 60.0 gr. and 61,000 cup. Powder "Y" may have a max load of 55.3 gr. and 53,000 cup. If the rifle can handle 61,000 cup's, why not load powder "Y" to 61,000 cup's, whatever weight that may be? Is it because the pressure/velocity curve of powder "Y" starts to become non-linear soon after 55.3 gr? Just curious. Hope the question makes sense!

[Unclenick]

It is an interesting question with several possible answers. One thing to keep in mind is that their pressure tests are in a pressure barrel. If well made, that barrel will have a SAMMI minimum chamber and throat to produce worst case maximum likely pressure, and so, in many commercial chambers the pressure will be lower. In some tight match chambers it may be higher. Not to mention that the powder lot variation, brass brand and sometimes brass lot variation (because capacity variation between cases changes the peak pressure determining volume the powder works in), and primer brand and lot, can all change the peak pressure, too. So you have to take all those listings with a grain of salt. They give you a ballpark idea but you still have to work up the loads in your own gun.

With all that in mind, here are some things that may cause them to choose different maximum peak pressures using different powders:

The case ran out of powder room (look for "compressed" listings)

Observed shot-to-shot peak pressure variation was high for that powder, and they felt they needed to obtain a margin of safety by lowering the average, which is what they publish.

The powder formulation is known to exhibit higher lot-to-lot variation than some others (e.g., 296/H110 will vary peak pressure by 10%, depending on lot) and they felt they needed more margin to avoid a lot related increase.

Other pressure signs, like casehead expansion, loosening primer pockets, splitting necks, head separations, or sticky extraction, began to appear at higher charges, despite the safe pressure reading.

Another pressure sign that would cause them to stop increasing the load is that the next incremental increase in charge achieved no increase (or perhaps even incurred a slight drop) in muzzle velocity.

Another sign, while not exactly a pressure sign, but which usually occurs near peak pressure is when a further increase in charge is accompanied by groups opening up. They would see no value in publishing a load beyond that point.

Given that powder measures are hard to keep within +/- 0.2 grains, they may cut powders off when that much additional charge causes them to exceed the peak pressure limit?

Any of the the above might be reasons they cut a load off, and there may be others.

Nick

[Jim Watson]

There could be a number of reasons.
Lee does not develop the loads shown, they just print powder company data in their own format, which opens up a lot of possibilities.

If you would cite a specific example I could look up, I would try to figure out the difference.

[mrawesome22]

Maybe going higher with given powder charge yielded little to no increase in bullet velocity?

I've wondered about this myself.


Edit. Woops, UncleNick already said it.

[mactex]

You should also consider the length of the barrel they use vs. the length of your barrel. The change in length will raise or lower the max pressure for a given load. Most of the reload manuals will be conservative to take this into account.

[buckster]

I went the range today and I noticed the min loads were more accurate than the max loads and the gun still cycled and my .45 felt like a 9 mm + you use less powder. Oh, 4.6 -4.8 gr of TiteGroup behind 185 Berry's. My groups really tightened up with my own loads.

[crowbeaner]

The newest IMR guide lists some pressures in PSI and some in CUP. Could you be mistaking the difference between the two? 61,000 cup is WAAAY too high for my standards, but 61,000 PSI is about 50,000 CUP I think. [I don't know the conversion from CUP to PSI] so maybe some kind soul will list the conversion factor for me and I can learn something new.

[tstone]

Crowbeaner,
The numbers I used were just arbitrary but you are right about the psi/cup. I did find a specific example in my Lee's 2nd Ed.

.270 win
130 gr jacketed bullet

H414 Never exceed load is 53.5 gr @ 63500 psi
Win Mag Rifle Never exceed load is 58.9 gr @ 53500 psi

I guess this could be a typo (should be 53500 cup)? If not, my question again is if my rifle can handle 63500 psi (with the H414), why not load Win Mag Rifle to 63500 psi? (All variables being equal, eg barrel length, temp, etc.)
Thanks for all the input thus far.

[mrawesome22]

Win Mag Rifle Never exceed load is 58.9 gr @ 53500 psi

I didn't know they ever created a cartridge without a number in the name.

Which Winchester Magnum are you talking about?

I didn't even know you were talking about 2 different cartridges. I thought you were talking about different pressures in one particular cartridge.

(All variables being equal, eg barrel length, temp, etc.)

All things can never be equal with two totally different cartridges.

[tstone]

mrawesome22,

Win Mag Rifle is a double-based ball POWDER. My reference is to only one cartridge, the .270 win. Sorry for any confusion.

[Jim Watson]

All I can say, like the King of Siam, it is a puzzlement to me.

I had to dig to find Winchester literature that showed WMR, they don't make it any more. Looking through the tables, it seemed that almost ALL Winchester maximum loads were to lower pressures than other brands.

Now that Hodgdon is distributing Winchester powders and putting out load data for them, the reported pressures are more in line. Unfortunately, they don't make WMR any more to compare. But let's see what they say about 760, the slowest remaining Ball powder:

270 Win, 130 gr bullet,

14th edition: 52.0 gr 2990 fps 49500 CUP
2006 edition: 53.5 gr 3003 fps 50,800 CUP
website now 53.5 gr 3003 fps 50,800 CUP
www with
Barnes bullet 52.5 gr 3059 fps 62,100 psi

[Unclenick]

Crowbeaner,

There is no exact conversion factor between CUP and PSI. There isn't even a single PSI standard (there are three), so comparing apples to apples is pretty difficult. There is a correlation between the SAMMI piezo pressures arrived at by a ported chamber and piston acting on the transducer (much the same way a copper crusher is done, but substituting the reusable transducer for the copper slugs). The two match pretty closely at just below 30,000 of either unit (e.g., .45-70 SAMMI peak pressure spec is 28,000 in either CUP or PSI). It appears the copper slug crushing loses linearity in the higher pressures, and under-represents them. There also seems to be a shift in the relationship that depends on the case geometry, such that the same exact peak pressure in a .223 case and in a .308 case will not give the same exact CUP or PSI by the SAMMI measuring methods. This Denton Bramwell article arrives at a correlation between CUP and PSI as measured by SAMMI standards, but when you look at the data scatter, you won't want to rely on the number in any particular case.

Just to add to the confusion, the European CIP measures PSI with a piezo transducer like the SAMMI apparatus, but is configured with its pressure port nearer the caseneck, so the European PSI numbers often don't match the SAMMI PSI numbers. The third piezo transducer method, considered by most to be the most accurate, is the conformal transducer in which a thin piezoelectric film is wrapped around the case. The transducer is destroyed in this test, so it is pricey and not as commonly used.

[Jim Watson]

I allus liked the British axial gauge.
It read what the gun's action saw, and did not require drilling holes in a perfectly good barrel.
Somebody in the USA built one with a large selection of thread adapters so he could offer the service of testing your loads in your barrels - unscrewed from your rifle, of course, but unaltered. Probably obsoleted by the glue on strain gauge devices.

There are several different terms that apparently have real meaning to SAAMI. Maximum, maximum average, maximum product pressures come out with different numbers.

[tstone]

Thanks Jim Watson, et al, for the input. I agree, info on WMR powder is scarce. Apparently is wasn't made for long. A puzzlement indeed........

[crowbeaner]

Thanks UncleNick, I got most of the info per your post. It'll take me a while to digest it all but I understand that there is no set conversion table. Sure would be nice if the manufacturers would decide which set of pressure readings they want to use to clear up any confusion that they're causing. I'll have to rely on what I can glean from the latest guides and the several older manuals I have and work up with any new powders I try. Now that Hodgdon, IMR, and Olin powders are all distributed from Kansas, I wish that Hodgdon would bring out a 4 lb. caddy of all the powders they carry to make ordering a little easier to save on the hazmat and shipping fees. I think Midway states that any powder is shipped from the mfg., and it sure would be nice to be able to order 4 lbs. of four or six different types at the same time. I appreciate your input on the conversion. Many thanks!

[sanson]

with rifles it seems the hotter charges are more accurate. so I work-up slowly inspecting cases as I go. pistols don't seem to make much difference as far as accuracy goes

[WESHOOT2]

IME powders can act linear, or not.

Up to a point.......

Good info already posted here.

Suggest if loading for MAXIMUM using exact component combination published, and chrono-working your way up (until you meet your goal, find that "un-linear" location, or flat chicken out. I suggest the chicken route, especially if it's a favorite gun).

And, frankly, there needs to be some understanding of exactly what more velocity can accomplish.

[mousegun]

Goal Based Reloading

Reloaders often take the wrong approach to reloading manual data. "Maximum load" is not a goal. It is a limit. Note that some maximum loads show a lower pressure than others in the same manual. Maximum pressure has only a restricted application for reloading as mentioned below.

Acceptable and even stellar performance can be had with lower pressure loads. Just because a rifle can use 61,000 psi loads doesn't mean it should. A reloader should be looking for

1. a load that satifies penetration, expansion and velocity requirements at the target,
2. an acceptable maximum range for these requirements and
3. sufficient accuracy at that range for the target size.

Maximum velocity may or may not achieve all three, but is more dangerous to develop and harder on the gun.

Some points to ponder:

• Load Range. Loading data usually consists of a starting load and an absolute maximum load. The data best describes a reasonably safe range.
  • Starting Load. This is very mild and low enough pressure to keep out of bad trouble if the firearm has problems. Joe Reloader can stop before a disastrous mistake, and it's high enough to stay out of detonation trouble. The starting load, if chronographed, will show what effects, if any, are the result of component substitution or weather conditions. Higher than manual velocity is an indication of caution. If this load is OK, then progress slowly (1-2% steps) toward the
  • Maximum Load. This is the load most manuals instruct never to exceed. There is good reason for this. High pressures reduce the safety margin and make variable conditions like temperature, manufacturing tolerances, component aging or substitution, mistakes, etc. dangerous. Work toward the maximum, but let other factors choose the best load. It's best to have a good idea ahead of time what velocity level will satisfy the three requirements above.
• Ballistic Performance. There is no practical difference in performance between a maximum load and a load with 5% lower velocity. The paper ballistics look better for the maximum, but field experience shows otherwise. Sight and wounding differences are indistinguishable at hunting ranges.
• Accuracy and Load Density. Best accuracy is with powders that operate in the 90-100% loading density range. Look for loads that (nearly) fill the case at the desired performance.
• Appropriate Caliber. If desired performance can only be had with maximum loads, get a bigger gun. Moving from a 30-06 to a 300 magnum would be a safer approach. Hotrodding a small car won't improve the life of the machine, and certainly degrades its safety.
• Standards Firing some factory loads over a chronograph will show what a reasonable load will do in a particular rifle. Compare the data with factory ballistics to show what can be expected from reasonable loads from that gun. Otherwise, reloading is flying blind. Mr. Chrony is your friend.

PSI vs. CUP? Using either is problematical since PSI/CUP data is measured by unknown methods with proprietary equipment. It can be ignored if the reloader selects appropriate components, has a good idea how a practical load performs in a particular rifle, can recognize common pressure signs, and uses a chronograph to monitor load development. The only reason pressure data might be important is if the reloading is done for a special case such as an M1 Garand or a trap door Springfield where the firearm has a weakness or pressure curve requirement of some kind.