Home Forums Hunting & Shooting Ask The Gunwriters I'd Like Some Proof....

There is this persistent rumor that floats around, here and there, now and then, about the 7 Rem Mag and these mysterious "pressure excursions". I read about them, hear about them,and some guys even repeat this stuff and use this as a reason not to use the cartridge.Many times these same guys will fall into a swoon over the 280 AI or something similar.

I've been handloading,shooting and hunting with the cartridge ( among others) since, I think, the late 70's, in more rifles than I can possibly remember,and while I've seen some that gave more or less velocity than some others, I have never had any difficulties with the cartridge at all.(The two times I had a real"problem" were with a 338 and a 280) For me,the 7 RM has NOT been difficult,has been easy to load, and has given very good field performance.

Having said that, I am always willing to be educated.

SO, does anyone have any REAL DATA,I mean, REAL ballistic lab proof from more than one credible source, of this kind of behaviour from the cartridge.NOT speculation, rumor, heresay,isolated "one-time" problem; but real PROOF, from ballisticians. I want to learn and am willing to listen

Have heard same and am just as curious.

I've owned and shot one since 1982 w/o incident.

Of course I've heard the same thing about the 243. I've owned one of those since 1975, again w/o incident..............

Bob,

Are the SAAMI pressure specs good enough for you? You can compare the 7mm RM to the other .532 belted brass cartridges and note the lower average pressure allowed (4,000 for most of them).

jim

I have wondered about what you write about the 7mm RM as well. Not so much that I have had no problems with the two I had as its only two rifles but the theory of it all. The 7mm RM is just another ordinary design as far as I can tell.

I suppose we have to accept that the pressures do indeed vary from factory loads in that cartridge. However what are the other variables as compared to other cartridges?

For instance is there one particular powder thats used extensively in the 7mm RM and not other rounds?

I don't know anything about pressure excursions but do know that the information given in the loading manuals does not pertain to my 7 RM. Powder charges given as maximum in the manuals translate more to beginning loads in my gun.
In order to obtain top velocity with 175 grain bullets my gun requires 4 grains or more of powder than the book maximums. Have been shooting these loads since the early '90's without a problem. It seems to be commonly accepted by many knowledgable writers that the 7 RM is somewhat unique in this regard.
If I went solely by book recommendations I would be shooting something comparable to a lightly loaded 7/08. I have not experienced this with any other caliber I have ever loaded for. Can't explain it but its a fact. YMMV.

Then why?

Is it the cartridge, chamber, throat design or some other aspect of the round or is it the components?

I used quite a bit of surplus 4831 in the 7mm RM and got top velocities with no problems but again it was only in two rifles.

What powder is common in factory loaded 7mm RM's?

Good question and good topic. This sort of thing always seems to lurk out there in the land of wink/nod. People will allude to such things, but I never see a really detailed article about them. I enjoyed the old articles in Handloader when S.S.E., pressure excursions, D.D.T. or whatever the preferred term happened to be were discussed. (That was in the days before such happenings were summarily dismissed as swamp gas of course.)

Bob,

No, the 7mm Remington Magnum does not have "pressure excursions." This implies that once in a while an individual shot somehow has pressures well above normal or safe levels. This is not the case.

What the 7mm Remington Magnum does typically show, in every pressure lab it is tested in, is a little wider range of pressures in a test string. Let's say, for instance, that the .30-06 normally shows a pressure range of plus or minus 3000 psi from average. This means that a load that averages 58,000 psi would have individual shots as low as 55,000 and as high as 61,000. (This is just an illustration, NOT what will be found in the .30-06 with all loads.)

The 7mm Remington Magnum, on the other hand, might show a pressure range of plus or minus 6000 psi. This means that a load averaging 58,000 psi would have individual shots as low as 52,000 and as high as 64,000.

This is NOT any sort of "pressure excursion." It is a just a little wider variation from average than many other cartidges show.

It is also not unique to the 7mm Remington Magnum. ALL cartridges exhibit certain characteristics when being pressure-tested. Some tend to exhibit lower pressure variations, others higher. This also does not show up with EVERY load, but is simply an overall tendency. In general the more "even" cartridges are those with less case capacity for their bore size, but this isn't a hard and fast rule.

This tendency of wider pressure variations on the part of some rounds wasn't really understood until piezo-electronic testing became common in the business. The other common method of copper-crusher testing is not nearly as sensitive to peak pressures above AND below average. This is why the standard SAAMI pressures (and hence muzzle velocities) of a few cartridges were lowered somewhat after piezo testing became more common.

Handloading the 7mm Remington Magnum is NOT dangerous. But assuming that because we can't "see" the wider variations that occur in some (not all) 7mm Remington loads doesn't mean that we should ignore published maximums because they "seem" safe in our rifle.

One other sidelight of piezo testing is proof that many of the "pressure signs" handloaders have relied on for decades are only seen when pressures are well above the 65,000 psi level. You normally, for instance, aren't going to see loose primer pockets or ejector-hole marks on the case head until pressures are up around 70,000 psi.

This also means you aren'tt going to see any "home evidence" of pressure variations in the 7mm Remington Magnum with published handloading data, because even the highest-pressure rounds in any group will still be well below 70,000 psi.

One thing I have noticed that also tends to vary, since starting to log onto the Campfire some years ago, is reading comprehension. (I had already noticed this, of course, from reading letters from readers for several decades, but it is more noticeable here.)

Evidently that is what's going on with this issue. One guy reads that the 7mm Remington Magnum shows wider pressure variations than other cartridges, and somehow that statement gets tranformed into "pressure excursions." Another guy reads the same thing and since, in his vast handloadiung experience, he has never seen any "pressure signs" from the 7mm Remington Magnum, he thinks the claims are all BS. The truth, as is often the case, lies between the two.

One of the problems here is that most handloading manuals do not publish pressure variations in a given load. Instead the most they'll normally publish is average pressure of a given load. Most also do not publish loads that DIDN'T prove safe in the lab, for obvious reasons.

One exception is the A-Square manual. If you want evidence of the wider pressure variations of SOME 7mm Remington Magnum loads, look at that manual.

I have visited a number of pressure labs over the years, and even done a little work in a couple. This is where I learned of this phenomenon (which I again emphasize is NOT unique to the 7mm Remington Magnum). None of the ballisticians I've yet met have any hard evidence of why it happens, but do have some guesses. As noted above, in general wider variations show up in cartridges with relatively large powder capacity for the bore.

Also, many of them have relatively short necks. It HAS been proven in labs that even a slight amount of throat erosion tends to cause wider pressure variations. It's also been noted that cases with shorter necks tend to accelerate throat erosion UNLESS the shoulder is very steep, say 30 degrees or more.

My first reloading experience was with the 220 Swift in the mid-1950's. I read in several loading manuals at the time that the Swift was a difficult cartridge to load and that it was dangerous to load maximum pressure loads due to the wide variance in pressure. I never had any problems (or maybe I was just not smart enough to recognize them if they occurred).

This post lists specifics.

1. Short neck may cause more erosion and therefore pressure.

2. Large capacity for the bore.

3. Shoulder less than 30 degrees accelerates erosion in short necks.

I have also wondered for some decades about the conservative loads published for the 7mm RemMag. After having collected several hundred cases from different manufacturers I figured out, that there is no other cartridge I know with such extreme variations in case capacity. I believe those guys developing loads with pressure barrels found the same situation and therefore keep the charges low.
For larger cases like the 7mm STW, the handloader might run into problems with very slow powders (RL25-MRPII), even with Mag. primers, producing non-consistent pressures at evenly velocities. So a chronograph is no help to detect those pressure inconsistencies.

I also have had very good results loading the 7MM RM for over 30 years but I have noticed that the loading data for it really seems to vary as to max loads and when I first had mine the loads of IMR 4350 that were published in some sources were far too hot in my rifle. One of the best loads was 79 grains of H-870 with any 160 bullet as pressures seemed much milder(filthy powder but dirt cheap at the time) and there was not a huge selection of slow burning powders like there is today. I have read a number of times that the reason for this varied load data was because of the Number of 7RM being built around the world at the time with slightly different chamber and throat dimensions. Looking back at my first loading manuals a lot of the 7 MM RM loads were smoking hot by todays standards. It was news to me as a young inexperianced reloader that chambers varied at all. When I had my Belgium browning rebarrelled with a shilen barrell and chambered with a American speced reamer I was shocked that my brass fired would not fit in the new chamber even when fully resized.

Mule Deer

You wrote
"One other sidelight of piezo testing is proof that many of the "pressure signs" handloaders have relied on for decades are only seen when pressures are well above the 65,000 psi level. You normally, for instance, aren't going to see loose primer pockets or ejector-hole marks on the case head until pressures are up around 70,000 psi.

This also means you aren'tt going to see any "home evidence" of pressure variations in the 7mm Remington Magnum with published handloading data, because even the highest-pressure rounds in any group will still be well below 70,000 psi"


Why haven't you ever mentioned this before in any of your articles or posts here????

(TIC) < means I am kidding...
Royce

This is what real pressure excursions look like.

These were produced in a 30-06, and seem to be the result of less than full loading of H4831.

I've always found the 7Mag quite easy to handload. The pressure and muzzle velocity always seems to follow the powder charge. I'd say that there is no problem here.

The rumor probably started because Remingon DID HAVE some probs with the 7 Ultra. I really don't want to get into that, but it all worked out well in the end.

ANY cartridge, when loaded with a low charge of relatively show burning powder can experience pressure excursions. Slow burning powders should always be loaded to standard pressures and never under-loaded more than ten percent. There was a big SEE (secondary explosive effect) scare back in the 1960s and we learned to never load H-4831 in half-charges and other such stupid things. Always use the right powder for the job and everything is cool.

Steve

John, Denton, and Steve - excellent stuff - thanks. I imagine that it gets tedious at times to have to repeat information, but I think that doing so really provides a valuable service to remind those that already "knew" that and to catch those who didn't. Once again, thanks. John

John,

Repeating never gets old ... if you're helping someone. Helping folks is totally what writing is about.

When readers ask a question and either don't listen or start arguing with you, that's another story.

Steve

Excellent commentary!

So what you're saying is that cartridges like the 300 Win Mag are actually inferior afterall...?




And unintended inferences being made?

Okay, I curl my posterior extension firmly into the crack of my smartazz and go lie down in the corner.



Actually, I have wondered some about the same thing. And why isn't an even "worse" cartridge, the 264, named? I wonder how much of the 7 Mag flap has had to do with extensive published material that was later retracted? I know Speer made a (to their credit) incredibly open and honest retraction of previously published 7 Mag data after discovering that their test barrel has some of the same issues you referred to regarding barrel throat wear. Besides reading comprehension, I think many folks fail to understand basic scientific (logical) principles or fail to apply them.

(BTW, wasn't IMR 7828 or some non-canister similar powder the, or a, classic propellant for the 7 Mag?)

I suspect the reason you hear less about this issue in cartridges like the 264 mag etc is there are far fewer of them in circulation. The 7 mag is by far the most common everyday magnum cartridge.