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Jim:I do know that Federal loads to about 58,000psi; at least that is what they told me in an email. What I mean is "why"; the technical reason that the cartridge allegedly produces these pressure excursions that are unique to this cartridge.

I can only note by analogy that the 7 Weatherby actually has a wee bit LESS capacity than the 7RM,yet never gets painted with this aspersion of extreme pressure spikes, even though it is loaded pretty hot by Weatherby and the load data is hotter than the Remington. I am wondering "why"?

I'm answering you before I read the whole thread so if this has been "asked and answered" I withdraw the comment.

JB: Thanks. That is the best explanation I've heard to date.Explaining this in the context of pressure "ranges" as opposed to "excursions" is helpful because it defines the parameters and avoids confusion. "Excursions" sounds pretty ominous; unusual, and out of the ordinary if you catch my meaning.

I have noticed large differences in velocity myself between two different 7 rem mags ; same load, same components.

I do not have the Asquare manual,so have never seen the data....

I am glad you made the point that the same thing goes on with other cartridges as well,even though maybe not to the same extent.

Bob, this is why one needs a 270 instead

When I bought my first one, in the late 60's, I set about working up an elk load for the thick, dark, dense stuff in Western Montana. I had a ton of 4831 at the time so it was first on my list. When I got to the maximum load in my Pacific Tool book I quit at the maximum and settled on a load 1 grain below max that shot 1/2 inch groups with the 175 Partition. The current max in Speer #13 is 5 grains below the max in that Pacific book and 4 grains under what I still shoot.

Anyone want a photo of that page in my Pacific book just let me know and I'll send it by e-mail.

I have been around the 7 Rem Mag for over 40 years and have had many friends load for it. I've never seen anything that would lead me to believe it's different than any other cartridge.

Wayne

Wayne,

Yeah, you will find all kinds of craziness in 7Mag loads. I charted the difference once and it was amazing. The probable reason is the unbelieveable variance in barrel throats that apparently acceptable in 7Mag rifle.

I once owned a Ruger M-77 rifle and a Ruger Number-One. The bolt rifle had the shortest throat I'e ever seen and the single-shot had a throat that was looooong. And, obviously, the load data for the two rifles was vastly different.

Interesting cartridge, the 7Mag, but I've learned to treat it as a wildcat. You just have to work up your own data ... but then, all cartridges are that way, to some degree.

Steve

Does anyone know if this trait is common with rifles from the same manufacturer, or does it only happen when comparing rifles in the same caliber, but from different manufacturers.

The reason I ask is, I know for sure that a difference in throat length can and will cause velocity and pressure variatiions, but not in the same chamber.

For example, compare two identical rifles, except one has a throat a few thousands, or maybe 32nds of an inch longer than the other.

The same load fired in the longer throated rifle will show less velocity and pressure than the same load fired in the short or no-throated chamber.

Put another way, can rifles from the same manufacturer vary in chamber dimensions from day to day, and the same thing happen with rifles from different manufacturers?

I suppose these things can happen, but it would seem strange that it would only happen with the 7RM.

123,etc: I think you're exactly right in the effect of the throating. But I think JB was refering, not to differences in pressures and velocities between different rifles, but differences in pressure encountered in the SAME rifle,load, etc, as demonstrated by lab tests.

This stuff can get confusing, so if I am not on the same page with you, please excuse....

Some comments on what has been posted:

One of the reasons for the vast variety of 7mm RM data is the incredible original popularity of the cartridge. The craziness over the .300 WSM is NOTHING compared to how quickly the 7mm RM took over the shooting world in the 1960's. Anybody and everybody was chambering for it, here and elsewhere in the world, often with little regard for consistent throat dimensions.

Short necks ALONE apparently have less to do with throat erosion than the angle of the shoulder right behind the neck. If the shoulder is such that it directs the hot gas to a point in front of the neck, and the cartridge is of fairly high powder capacity to bore diameter, then erosion begins sooner. But throat erosion alone is not the reason some rounds show wider variations in pressures. Sometimes it's just a mystery.

The 7mm Weatherby Magnum doesn't exhibit the same pressure variations because of the "freebore" common to Weatherby chambers. This has always cut down on pressure problems with Weatherby cartridges, one of the reasons it was developed. Also, The Weatherby has a longer neck than the Remington which helps delay throat erosion.

Cartridges which have a very high powder-capacity to bore ratio often have problems with pressure variations. These can be mitigated in a number of ways--one of which is testing to find the powders that work most reliably.

This was indeed a problem with the .220 Swift when it was introduced in the 1930's. There weren't as many powders available back then, and many were touchier--especially at really high pressures. Plus, everybody who had a Swift wanted to push it to the limit--or beyond--and no home handloaders had chronographs. Data was also scarce, and the Swift case is prone to stretching and neck thickening, both of which can cause real pressure problems. So some primers got blown out of cases and a few rifles were wrecked.

Consequently, loading the Swift today is a lot different than it was back then. Just because we can load the Swift today with no problems doesn't mean that there weren't any in 1936.

Similar problems with finding the powders that act civilized can occur with any new high-intensity cartridge. This was indeed so tough with the 7mm RUM that more than one loading manual didn't list it for a while. They simply hadn't found reliable, safe loads. Now they've had time to develop them--but that makes it all the more important to follow TESTED DATA.

Finally, one of the reasons the publishers of reloading data don't reveal ALL their test results is that too many handloaders will interpret the data in very weird ways. In fact, believe it or not, many handloaders assume that because they've been loading for a while and haven't blown any fingers off yet, that they know a lot more about pressures and powders and ammunition in general than the professional ballisticians who test hudnrfeds of thousands of rounds a year.

The more information this type of handloader gets, the more he assumes, and the more likely he is to bloew some fingers off. This is exactly why the people who publish handloading manuals strongly suggest following their data, UNLESS obvious signs of excessive pressure occur.

It is also why they provide the absolute minimum amount of information. If they provide more, a certain type of handloader thinks he knows everything and starts experimenting.

That made way too much sense as even I understand that one. Great post Steve.

Steve's advise about loading the 7 RM as a wildcat is very good.Been my experience as well.

I agree with all of the above. I've had 7mm Rem. Mags. that would barely make 3000 fps. with 150 gr. bullets without exhibiting high pressure signs, and I've worked with others that would make 3000 fps. with 175 gr. bullets. Throat and lead dimensions, actual bore diameter, chamber dimensions, etc., etc., can vary a great deal between individual rifles, even from the same manufacturer. Steve's right - just about ever 7mm Rem. Mag. you run into seems to be sort of a law unto itself, so it pays to be cautious as you get to know each one as the individual rifle that it truly is.

Powder selection has, at least in the past, been an issue as well. In the old days, for example, H870 was very popular with those who preferred 160 and especially 175 gr. bullets in the 7mm Rem. Mag., and pressure spikes weren't at all uncommon with that powder.......

AD

I can't seem to find it - we can only search back 2 years, but I remember Charlie doing some experimenting here on a 300 WM with his pressure equipment. THAT was an interesting read. Secondary pressure spikes in the barrel, going WAY over the line on reloads with no normal indicators of a hot load only to end up blowing off the last 5 inches of the barrel. He was going to do more work but he never posted it...

MD maybe you remember that one?

Hi Denton,
this pressure graph you showed here is drawn from a childrens hand and never from industrial pressure recording equipment. I would return transducer and electronics back to the manufacturer for a free replacement when I would watch those curves. I have measured some thousand rounds loaded with very slow powders and never seen such pressure traces.
When the plot looks like this, it has nothing to do with combustion anomalities, this is electrical noise. Especially after pressure maximum you'll find no large spikes. Some spikes may occur in the early ignition phase at beginning of pressure rise. If they are significant, the powder-primer-cartridge composition is bad and the user should try a different powder. A good device for watching those signs is the Oehler Ballistic Lab, which contains appropriate filters. In standard piezo-based equipment a 20 KHz filter is used to damp electric noise. For those who use strain-gauges without filters serious barrel ringing often occurs. This ringing even increases when the bullet is leaving the barrel, because the barrel's tension drops abruptly.
We are talking about rifles. For artillery equipment spike's behaviour is something different.

John and Steve - more good information. Thanks again. I am still in my normal state of wondering why there is such variation among throats. Once again, it seems to me that anything with the potential for significant problems that are involved with creating the pressures with which we deal would call for closer quality control. As always, just my wonderings. Best, John

My Rem 700 BDL LH in 7mm mag was also one of those that wouldn't come close to the speed it was supposed to. Would come about 200 FPS under what it should and showed spuratic pressure signs,
it now wears a new Shilen barrel chambered for the 7mm WSM. Problem solved.

Not so, my friend.

The illustration is from Dr. Brownell, a physics professor at University of Michigan. It was done with a strain gauge transducer, using a Tektronix oscilloscope, in the late 1960s. At that time, there was no better equipment.

Dr. Brownell and his graduate students produced a large volume of very high quality work in ballistics, over a period of several years.

The earliest work I know of on this issue was from J.B. Goode and D.E. Wealde of the Royal Armament Research and Development Establishment of Woolrich, London. Their work was done on behalf of NATO, and published in their journal in 1966. Dr. Brownell cites their work as the probable theoretical basis for the phenomonon he observed.

Some pressure test equipment manufacturers deliberately "roll off" the response of their amplifiers above about 3 KHz, in order to reduce random noise. In such a system, faster events are smoothed away. You could use a commercial 3 KHz system forever, and never see the rapid spikes, even if they happened with every shot.

Although I can't tell you exactly what amplifier Dr. Brownell used, I can tell you that the bandwidth of the 564 oscilloscope that he used was 10 MHz (over 3,000 times better bandwidth), which is abundantly fast for such events. One of my first projects at my first job (Tektronix) was to work on the electron gun in that particular oscilloscope.

Maybe, with such equipment you'll get readings you wonder about. In that time electronics were in a early phase of development, and research uses every device available on the market. When I was student in the sixties, I was involved in such test equipment and we all including our Prof. were not able to interpret every observation correctly.
The 20 KHz low-pass filter is standardized (mandatory) by CIP (and also by Mil-Specs and SAAMI). With this you will see every important spike on the plot. And those artifacts shown in the sketch may be possible with artillery cannons where the barrel time is longer than the speed of propagation of waves traveling back and forth, but not with rifles and handguns.
With rifles one may observe erratic pressures, because of poor ignition, having SD about up to 50% in pressure readings, but showing low SD in velocity. We see dependencies, but cannot predict this phenomene.
Please note: do not trust every publication, in former times it was scientifically proofed that Earth was a disk !

Allen: RL25 seems to be working well for me lately with 160 gr bullets.

Thus far I agree with you. I still see no specific reason why the 7mm RM should stand out unless its the one Mule Deer just mentioned that a lot of 7mm RM's were made in the 60's and perhaps some of those were off spec.

Otherwise, in theory, the 7mm RM should not stand out as a problem.

Bob-R25 has been good to me in my 7 Mashburn Super, I also liked H1000 with the heavies but day in and day out 7828 has been my go to powder for the big 7's.

This is my experience only, but I've been running some kind of big 7 or another for a long time. I've no doubt theres others in the world that have run more rounds than me , but most likely not by much.

I've never (to date perhaps I should say) run into any challenges running them, working up good loads and or had any pressure spikes.

I've only worked with one round that I felt had some weirdo pressure curves and that was the 6.5/06.

Big time chilly willy hear today, was going to the range but it's too cold for that, perhaps some ice fish'n is in order...<g>

Dober