Home Forums Reloading Big Game Rifles 7mm Rem Mag at 64-65,000 PSI ?

As I study the 7mm Rem Mag and 7mm Weatherby Mag cases, it strikes me that they are virtually identical except for the radiused shoulder angles on the Weatherby.

The big difference between the two cartridges is, as best I can tell, is the SAAMI pressure limits � the 7mm Rem Mag runs 52,000 CUP and 61,000 PSI while the Weatherby runs 65,000 PSI. (Hodgdon data shows Weatherby loads in excess of 55,000 CUP.) By contrast, SAAMI ratings are 65,000PSI for the .270 Win and 64,000PSI for the .300 Win Mag.

Given that, and given a modern firearm that is commonly chambered for the .270 Win or .300 Win Mag, I question why the 7mm Rem Mag can�t be loaded up to similar pressures?

Thoughts?

"Back in the day"...it was discovered the 7mmRM in particular had a quirk of wild pressure spikes. .243Win same same IIRC. Just an unexplainable quirk of the round. The 61Kpsi is just a safety measure. In theory 65K should be and would be safe, but that safe pressures are tallied at a max AVG safe pressure, and to maintain a 65K AVG, the top end spikes tend to be too high. Mule Deer (JB) explains this very well. But basically thats it.

I first read this theory on this very website some years back, and in the interim have spoken with several ballisticians who to a man disavow it. Rather, I have been told, the 7mm Rem Mag displays a wider pressure band, or in other words wider "swings" in pressure, low and high, not "spikes" upward...

So why is the sammi max for the 300 win mag at 64,000 instead of 65,000?

That may be right...cant say for sure. I'll take your word. But the bottom line is that the top end spikes were too high to be considered safe at an AVG of 65Kpsi, regardless of the low end.

BW,

The numbers put out by SAAMI are a "maximum average safe pressure." They have to look at the top end pressures some loads get...if they go over a certain number, the top AVG must be reduced so that the MAX pressures don't exceed XXpsi. I'm sure thats a simplistic explination...but basically thats its.

If you really want this in detail, Id suggest asking the GW's.

I�ve never bought into the idea that the 7mm Rem Mag (or the .243 Win) were unique in any way when it comes to pressures. It makes much more sense to me that other factors were influencing the results seen and that the results were subsequently misinterpreted.

At any rate, I�m not seeing any reason why the Weatherby can be safely loaded to 65,000 PSI and the 7mm Rem Mag cannot. (Or why the .264 Win Mag and .300 win Mag are safe at 64,000 PSI and the 7mm Rem Mag is not.) The 7mm Rem Mag and Weatherby cases are identical for all practical purposes, with no physical reasons I can see that account for the difference in powder charge levels. (I am open to correction in my thinking, however, if solid evidence can be presented that shows my thoughts are wrong.)

At any rate, the reason I�m asking is that my 7mm Rem Mag 160g North Fork loads at 3048fps are redundant with my .300 Win Mag 180g loads at 3032 and 3048fps (TSX/MRX and North Fork, respectively). As a result I�m working up loads for 140g TSX/MRX and would like to hit 3250fps.

Based on the work I�ve done so far with H4350, H4831SC and H1000, I think I can get there with a couple powders, maybe all three. To do so looks like it will require exceeding 7mm Rem Mag max loads but stopping short of or at 7mm Weatherby starting loads.

The twist rate is differant. Even Weatherby twists the 7mm Rem 9.5" And the 7mm WEA 10". Reminton twists the 7mm Rem 9.25"


The faster the twist the higher the pressure. And I would think the more likly to creat spikes???

Have pondered the same question myself. I've measured case capacites of both cases and they are almost identical. I attribute a slightly higher velocity and associated powder charge to the Weatherby freebore. Most 7mm Wea data states that the data was developed with standard weatherby chambers which have freebore. The freebore allows a couple extree grains of powder vs the 7mm RM. Even acounting for the freebore, there seems to be an abnormally large gap between Wea and RM data...........

Might also add that most of the 7mm RM data stops at less than 60K psi; presumably to account for the wide pressure band.

To me though, there is still a big difference in safe velocities between the 280 and 7mm RM. Both have ~ 60K psi SAAMI limits and the RM is 20+% larger. That's about 5.25-5.5% more velocity for any given bullet weight or ~ 150-200 ft/sec.

Or stated another way, the 280 gets 94.5% of the velocity of the 7mm RM - 160's ~ 2800, 140's ~ 3000.

Boys, I'm merely repeating what I have heard as the reason. I think JB has explained it here too. What you choose to believe and the pressures you choose to load to are a matter of personal preference. The published velocites, which I assume are acheived at SAAMI pressures, are plenty for me. With that said, the .280 is a different animal as to why its not up to the 65Kpsi mark...and I load it to 65Kpsi standards. So, I get what you're saying. In fact, thinking on it, it could be the difference is the 7mmRM is loaded in so many configurations...Autos, pumps, Singles, Bolts...so...who knows.

BW,
I am sure that you are right regarding the freebore in the "Roy". Very similar to backing up a bullet in anything. The closer you get to the lands it seems the higher the pressure given everything else equal. As I am sure many know some barrels and chamber combinations are more and some are less sensitive to added powder (out of the book loads). I had a 7MM RM on a Mark X with a Douglas SS barrel that would go over with a 145 Speer and anything over 65.5g of RE22. Another .2gr of powder and it was burnishing the bases. The Speer manual says 68g is max. No way with this rifle. Later I found that the lot of RE22 I was using was a very hot lot which is what probably caused the low capacity compared to published capacities. Given all of that and after replacing a couple of barrels on that 7MM I decided that it wouldn't hurt to slow them down a little. Running a 7 or a 300 at max or out of book powder charges looks good on the chrony but it sure is hard on rifling. If a man want to run a RM at "Roy" velocities it might be a good idea to just buy a "Roy."

1

The real differenc between the 7 rem and Weatherby is, I think, the freebore. I've built 3-4 7 Rem Mags with "long throats, that is, long enough that the 160 Nosler Partition was seated with its base even with the base of the neck. It takes a H&H-length magazine box to do this.By the way, ALL my costom barrels have been Douglas 9.5 twist, or Kreiger 9 twist.Loads weresimilar to the 7 Weatherby.

Set up this way, it was very easy to get 3130-3150 with a 160. Friends did the same with their rifles, so we shot thousands of rounds set up this way without a problem.I hunted with a rifle set up this way for about 10 years all across the west, no problem.

Federal emailed me in response to a similar question to say they load the 7 RemMag to 58,000; the WSM to 63,500 psi.

I think this pressure spike issue has nothing to do with the cartridge itself, but to the wide variation of chamber, throat, and barrel styles in the various rifles chambered to it after it came out.I say this because I have recorded chronograph data showing 2 different 7 Rem Mags giving velocities varying over 200 fps from the same ammo, same day, same everything except the barrel.I have found, for example that my Douglas barrels have given higher velocities with the same charges than the Kreigers do; said another way,it takes heavier charges in the Kreiger to hit the same velocity levels.

I have even built a couple of 7 RemMag barrels with "oversize" groove diameter,ie. 285 instead of 284 groove diameter.(I have done the same things with a number of 300 WM's and 270's)This actually worked very well, the barrel not being prone to touchy pressure increases as near as I could tell, and accuracy still excellent. The charges will. however, be a good deal heavier than what the manuals show.

I have generally used the cartridge with 140's at 3250-3300, and 160's at 3050-3100 without any incident.Don't know if this helps, but there it is.

I don't think that's it. If a ballistician from lab XYZ said he measured a certain variation in pressure from shot to shot in a test string I'm inclined to believe he used the same test set-up for the entire string.

Also I don't believe the words "pressure spike" convey the right meaning. My take is that sample probability density functions of pressure test strings for the 7mm RM have more weight in their tails than do sample pdfs for other cartridges.

mathman

How about a loose conversational thought experiment, with differences exaggerated to illustrate the point? Note that I'm not trying to put a physical cause behind the observables. I'm taking a shot at why loading recommendations are different given the observables.

Suppose that cartrige A, when loaded to an average peak pressure of 63,000 psi, never has any individual shots in a test string measure outside of a narrow window. Let's say [60,000 , 65,000] psi.

Suppose that much testing has produced data showing that cartridge B, when loaded to the same average peak pressure of 63,000 psi, typically needs an interval like [56,000 , 70,000] to catch all the measurements in a test string.

Certainly cartridge A could be loaded with it's average peak pressure set nearer the individual maximum allowable pressure without having particular shots going over the individual MAP.


My take is that modern faster acting pressure measurement methods that report pressure-time curves have shown test ballisticians that the 7mm RM is more like B than A.



What other factors cropped up for these cartridges that could not have just as easily affected the tests of others?

mathman

I like your example mathman.
I have heard for years that the 7mm Rem. has had issues with pressure variance.
If I owned and loaded for a 7mm Rem(I don't) I would simply keep my loads on the mild side.

Mathman: It could be as you say; I really do not know. What is curious is the same "problems" have never been noted for the 7 Weatherby, a cartridge with about the same capacity, using the same bullets at about the same velocity. Why one should be subject to erratic pressure variations, and the other not, makes me suspect of the research.Could be Gremlins.....

Along the same vein, I remember an article by, I think, John Haviland on the 30/06 in Handloader, where he discusses Charlie Sisk running into strange pressure variations of up to 12,000 psi in a customers' rifle that did not manifest themselves in velocity variations, but showed up in erratic grouping. Sisk traced it back to primers, curing the problem by switching to fed 210's. I do not remember the cartridge which caused the problem, but don't think it was a 7 rem mag. Some here may remember the article. Point is,many other cartridges may be prone to the same problem, if it exists.

Anyway, it is very clear to me that,at least, variations in barrels and throating can make a significant difference in velocities and pressures with the same components.It is very hard to keep things exactly the same all the time.

Bob,

I think significantly lengthening the throat on the Rem. basically turns it into a Wby. and so the initial pressure rise is smoothed off a bit. It may be that this capacity/powder/bullet combination gets along with available propellants better with a little freebore, regardless of whose name is stamped on the case head. Sound reasonable? I'm sure not claiming I can explain it.

mathman

Bob,
Your right on the money about diferent combinations producing different velocities. Here is some data from this morning that is pretty interesting.
8:30AM 65 degrees No wind (all flags still), clear sky.
7MM RM Sako 75 w/25" Spencer barrel

66gr RE22 140g Nosler CT 5 Rounds on a clean barrel
2927, 2895, 2924, 2905, 2955

Gonna keep working up with this load and will have more results tomorrow. Accuracy was OK but not great.

64.5gr IMR4831 150 Nosler BT 4 Rounds on a clean barrel
3028, 3051, 2973, 2979
Group size .249" Fouler was 1/4" under the last three shots in the group that were all in a cluster touching.
This barrel might be one of those that isn't going to shoot big velocities but it looks like if a guy can hold it still it will shoot where it is aimed.