Home Forums Hunting & Shooting Ask The Gunwriters J B, history behind SAAMI 62K PSI for 7 mag

John B, I know you mentioned it before but can't find it. Why did SAAMI go with a MAP of 62K psi for the 7 mag? Something about oddball pressure spikes?

Have you experienced any issues?

I know the 7 PRC is rated at 65 K.

There have been threads written on this subject here in the past:

Here is an interesting thread that goes deeply into the variables.

The late Bob in NH wrote what I think is a good explanation:


Re: 7mm Rem Mag at 64-65,000 PSI ?
bwinters#165561109/06/07
Offline
BobinNH
Caampfire 'Bwan
B
Joined: Jan 2007
Posts: 35,900
The real difference 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 custom barrels have been Douglas 9.5 twist, or Kreiger 9 twist. Loads were similar 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 Rem Mag 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 Rem Mag 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.

link this this thread:

https://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/1662047/1

Here's what I've heard from the folks at more than one pressure laboratory:

It's not pressure "spikes," but wider swings in high and LOW pressures when testing with some loads. This was discovered after SAAMI started to switch to electronic pressure testing several years after switching from copper-crusher testing, which wasn't sensitive enough to detect the swings. SAAMI tries to make ammo safe for use in all commercial rifles, so lowered the Maximum Average Pressure (MAP) for the 7MM RM so that the high swings would be safe--especially if some other "condition" (a worn chamber throat, high ambient temperature, etc.) might raise the pressure of the high swings. This tends to happen with specific bullets and powders.

Nobody knows exactly why it happens, but it also occurs in the .243 Winchester, which is why its MAP, and hence velocities, were also lowered by SAAMI after the change to electronic testing. It may be partly due to throat erosion due to the combination of the shoulder angle and short neck. (Also have been informed by more than one pressure-lab tech that contrary to popular belief, throat erosion does NOT necessarily lower pressures. During its early stages it tends to raise pressures, due to the roughening of the throat, often called "gator skin" due to its rectangular cracking of the steel's surface. As erosion continues pressures do tend to drop, do to the increasing diameter of the throat as pieces of the "gator skin" break off.

The symptoms a handloader would tend to see of the swings would be wider velocity variations when chronographing--though many would attribute that to other factors. (Plus, amazingly enough many handloaders still don't own chronographs.)

All of which is why I stick to pressure-tested data when handloading for the 7mm RM--or .243, for that matter. Of course, there are also many handloaders who believe they can accurately judge safe pressure from various "signs" from the cartridge case. I ran some tests on those signs, using three different cartridges,
over 20 years ago,thanks to the help of the piezo-pressure lab of a major powder distributor. I worked up a load for each round using pressure signs, and then each load was tested in the lab. One was just about matched SAAMI pressure, one was considerably under SAAMI pressure, and the other considerably over--which may reflect the odds of such pressure-guessing.

The 7mm PRC's shoulder and neck geometry is very different than the 7mm Remington Magnum's--with the 30-degree shoulder and longer neck that have proven to both enhance consistent power-burn, and reduce throat erosion.

My take:

I was wondering about this a few days ago and remembered that one of the more common aspects of this problem is a worn throat. I believe I’ve read that heavier bullets was also a factor.

I was thinking that as the bullet travels through the throat and engages the rifling, the enlarged throat allows the unsupported base of the bullet to bump up or rivet slightly. From what I’ve understood that’s right about the point where pressure is really ramping up and with any slight variation in bullet diameter comes pressure and velocity excursions.

The thoat geometries / angles and what not are different for different cartridges. This is why I am always suspicious of people who say things like load a 280 or 3006 up to 270 65k pressures and it is ok because I have been doing it for 40 years. I do know the angles and what not in leade influence shot start pressure which in turn impact max pressure and probably spikes etc…. There are few papers published by military that show this. The 270 win throat looks different than others so maybe that has something to do with higher pressure and why some roudns are slightly different. So I am not sure it is safe to assume things like I can just load to same pressure as another cartridge just because same caliber or similar case. Any way, few k psi (between 60,62k and 65k) is few 10 of fps not 100s so decided not to care

Lou

I have fiddled with at least three 7RM rifles and eventually sold all of them. They were built by very good gunmakers. I used the best available barrels.

Anyhow, each was very fussy. There was no consistency between rifles. In contrast I fiddled with many 270 and 300W rifles and generally the good loads in one would be as good as or nearly as good as the loads in other rifles. Each 7RM was just plain difficult. I was trying to get 175’s to 2950 and 160’s to 3050.

I used such loads on three hunts in Africa for plains game. They worked well but didn’t produce results any different than the 270W BUT the rifles were heavier, longer, and recoiled more than the 270 so I abandoned the 7RM.

Today I think a 140 at 3000 is about the optimal level of energy, recoil, rifle weight for BG hunting until a 375 is needed. If you really think you need 180’s at 3000 then get a medium big 300, easy no effort.

I tend to agree--or even less. This is partly due to today's bullets.

One of the big "problems" here is that so many handloaders believe another 100 fps (or whatever) will make a vast difference in "killing power," or trajectory. It doesn't.

MD
Yup. Today less can be very effective. When I mentioned “140’s” I should have said in a bore diameter between .264 and .284.

This all seems a bit like a tidal wave in a tin cup sort of a thing to me. My dad has been hand loading the 7MM RM since the early 70’s and I have been for 30 years anyway. I’ve not seen these issues.

I’m 47 now and bought my first chronograph when I was in High School and have had some flavour of one ever since. I’ve never seen pressure spikes of any kind in the 7MM. Ever. I’ve not noticed it in sudden velocity spikes, sudden traditional pressure sign presentation, or anything else for that matter. And I’ve run 7 RM’s through entire barrel life cycles. I have had this happen with both the 30-06 and the 280 Rem but never the big 7.

I can beat the 270 with similar weight bullets by 200-300 fps. That does make a trajectory difference. Am I saying this can’t happen? Not at all. But I think it’s been stretched a bit thin.

You will never see this with traditional pressure signs, because meaningful pressure signs generally don't occur until around 70,000 PSI.

They are NOT "spikes," just wider variations in pressure and hence velocity. The variations don't occur with all powders and bullets, but can occur with certain combinations.

The OP asked why SAAMI established the MAP of the 7mm Remington Magnum at 62,000 PSI. I explained why, and the reason has been documented for decades in various pressure labs.

Would appreciate a description about the "pressure spikes" you've encountered in the .280 and .30-06.

Wider variation? Is there a way to quantify that?

This is why I bring up the 06 and 280. I’ve had a couple of rifles so chambered that showed a lot more random velocity variation than any 7MM I’ve owned. To me this seems like an urban legend that has grown wings. Or there is much more to it than just the cartridge itself.

This is interesting. Did the same thing with a 7mm Rem Mag in the early 1970's. Loaded up a dummy round with the 160gr Speer seated to the base of the neck and had a smith relieve the throat so the bullet was just off the lands. It became a totally different rifle. Super accurate and load went up two grs. It was on an L61R Sako and I suspect had a longer magazine than other rifles. had a 6mm Rem in a 700 Rem that I couldn't seat bullets out far enough in and magazine wouldn't take 100gr bullet's seated that long. Didn't care much for that rifle! Didn't shoot very well at all.

Ballistics labs see the variation every time they test. If you compare the variance of the pressure distributions of the 7 Rem mag and something like the 308 Winchester there will be a difference.

I’d love to see that data somewhere.

Sometimes a chrony yields some interesting numbers.

There is far more to it than just chronograph error. Especially in the last 10 years.

Yesterday, in addition to the 7mm, I also shot a proven 223. The ES in the 7MM was less than 10 fps while the 223 was almost 100. This even though group size was under 1/2”. Both rifles were clean. Bare metal bore scoped clean. Both had the initial shot land within the group.

I have a good idea what’s going on with the 223. I’m almost certain it’s a combination of neck tension and barrel wear. The neck tension on the virgin WW cases is all over the map. In fact some of the cases, when seating, would allow bullets to be pulled by hand. I suspect fired cases that are annealed, sized, and run over a mandrel will change that. 10 rounds through the clean bore will help as well in this particular rifle.

I’m not saying the 7MM Remington Magnum or the 243 (a cartridge I have also loaded for a lot) don’t exhibit pressure anomalies (apparently pressure spike’s isn’t correct). But I am saying it should be revisited with fresh eyes (with the current crop of new 7mm’s this is likely never to happen). Taking a look at Noslers current reloading manual tells me they aren’t worried about it. So why should I be?

Well said PF76. Mind your P’s and Q’s, take care of your gear and you’ll likely never see so much of an issue.

I’ve only had 6-7 different 7 Rems and loaded for a handful of others. Never had any problems or scares. Could happen I guess, but I’m not holding my breath.

In statistics, that's what the SD quantifies. The visual representation of this is a flatter Bell Curve.

The variables inside the chamber and bore are so insignificant compared to one’s ability to hold the crosshairs on the spot and trip the trigger. Easier to become a scientist from the couch vs a better shooter in the field. I am guilty.

Shoot your chrony.

The military is using bimetal cases and 80+kpsi So other than beating the crap out of the brass is there really that much to worry about if the load conforms with regular signs? No excessive primer flattening. No sticky bolt lift. And lastly primer pockets that last a reasonable amount of firings.

If the brass lasts 7 or 8 firings and the bolt ain’t sticky. It’s all good you guys worry too much.