Home Forums Hunting & Shooting Ask The Gunwriters Mule Deer -- 7mm Rem Mag Preassure Spikes

I was looking at an old magazine last night that I really enjoyed. First your article on the 8mm-06, and then your article on the various Ackley Improves. All very sound thought.

Anyhow, as I went to bed I thought: why does the 7mm RemMag do this? The 264 WinMag and the 338 WinMag have never been noted for any such problem.

So, can you or someone else tell why it is?

By the way: Col Charles Askins loved the 8mm-06, and tried the 8mm-06AI, and could see no difference to make it worth the effort. Book sold by the NRA. I still have it somewhere. Charlie should have red your AI article on bigger bores.

Swarf,
I'd very much like to read that article on the 8mm-06. Can you point me in the right direction?

swarf,

Nobody knows why the 7mm Remington Magnum does it, but it's been documented in every pressure lab I've ever visited.

They aren't really pressure spikes, just wider variations (both up and down) in pressure than most other cartridges show.

It only happens when Santa is shooting at the Easter Bunny with a 7 Mag.....

I wonder if having a gain twist barrel would help smooth out the variations? Something along the line of a Bartlein T twist with 5r style rifleling or is it in the cartridge itself?

Dogshooter,
wow, someone who has been around longer than me AND has fewer posts! That's impressive.

Crimson Tide:

The name of the magazine is: "The Hunter's Rifle" by Col. Charles Askins. It was published by the NRA. Copyright 1984.
It is a magazine sized boob of 176 pp. ISBN 0-935998-51-9.

It is a combination of articles reprinted from Peterson's hunting, Guns & Ammo, and the Rifleman. The article list loads for the 8mm-06, and 8mm-06AI. He also has some pictures of his other 8mm cats. He became an 8mm nut. The article in question is about 4 or 5 pages.

Hope this helps guys. Scratch that itch! Research and contemplation is cheap.

I've always preferred the larger book sized boobs myself

Can a boob of any size be a problem? LOL!

And its a dang shame, the 7 RM, generally, is extremely accurate easy to shoot round. But the pressure labs show these variations, and they show up on target on long range shoots all the time. Enough so that the only faster 7mm that hasn't dealt with these issues on the firing line that I"m aware of is the 7 wsm. And I"m not sure if its been shot enough yet to prove anything but it does have, at this time, a better record than teh RM re variations that cause larger groups, IE fliers.

It just doesn't make sense to me why this phenomenon doesn't happen with other bore dia. Either it does, or these "pressure spikes" are being blown way out of proportion. If these spikes are causing problems downrange I've never seen it. And why would the big 7 be so widely used for long range blasting?...

Any comments on the hypothesis that pressure spikes are associated with short necked overbore cartridges (.243 and 7mm Remington Magnum among others) because the rough and getting rougher (but inconsistently) throats associated with short necks then change internal ballistics just as long or short throats affect internal ballistics - that is the throat impacts internal ballistics in more and different ways than is commonly considered.

Clark that's a loooong sentence. I also haven't seen any inconsistencies at the chrono with 7RM's. SD's aren't any different than other cals.

Exactly--or at least that's one of the reasons.

Appartently some people didn't read my response to the OP, something that isn't uncommon on the Campfire. I explained (probably for the 20th time) that it's not "pressure spikes" but wider pressure variations, both low and high, than with most other centerfire cartridges.

And even those variations aren't as wide with some powder/bullet combinations. So if you develop a 7mm Remington Magnum (or .243 Winchester) handload that shows minimal velocity variations the pressure obviously isn't varying all that much.

But yes, the wider pressure variations have been documented many times in professional pressure labs in both cartridges, and tend to get worse as the throat erodes.

John,

It would be interesting to see data on specific loads, powder/bullet combos, that caused the least pressure variations in the .243 and 7RM.

From your research and time spent at these labs, are there data comparing such loads? Another thought would be to identify loads causing the most pressure variations, i.e. heavy bullets, slower powder, etc., looking for trends associated with this phenomenon.

If it's a scattered occurrence without trends, there may be no conclusions other than to lower top SAAMI pressures, which seems to be the prevailing answer.

DF

It seems to occur more with heavier bullets, especially in the .243. One of the production guys at a major ammo manufacturer told me it was entirely limited to bullets in the 100-grain class in the .243, perhaps because they created greater friction in eroded throats.

I also seem to recall that the real problem in early 7mm Remington Magnum factory loads was with the original 175-grain factory load, listed as over 3000 fps. The 175's factory load was reduced by 150 fps after piezo-electronic became common.

Both the .243 and 7mm Magnum also have shoulders that tend to funnel hot powder gas toward a point in the throat in front of their short necks. This is why the .243 is known as more of a barrel burner than the 6mm Remington, which has a sharper shoulder and longer neck.

I can easily see how a longer neck would help preserve throat by simply being there to get chewed instead of the barrel, but I can't completely get my head around the shoulder angle explanation.

As soon as you draw a line down one side of the shoulder pointing at the throat, you can draw a line down the opposite side of the shoulder where the gas flow would cause a cancellation. Maybe this results in erosive turbulence instead of a direct jet cutting effect.

Turbulence is definitely part of it.

Recently I've done some reading of military research into barrel erosion, and the increased throat erosion from cases with shoulders that funnel the gas in front of the neck are well documented. But gas that whirls around in a certain area rather than passing more directly down the bore also tends to increase erosion. In fact, once the surface of a throat starts cracking, erosion speeds up because of increased turbulence due to deflection off cracks perpendicular to the bore.

FWIW, Mic McPherson's line of cartridges is designed to keep that turbulence inside the case. The pic on their website shows it in cartoon...
http://superiorballistics.com/

FWIW Brownell's pressure factors book (originally excerpted in Handloader) has an extensive discussion with some references about shoulder angles and internal ballistics.

Neck length is a common topic with examples of one or two see frex: The 257 WBY Mag Improved OR My Attempts (meaning a wildcat with improved results not a blown out .257 Roy) in The Accurate Rifle for April 2003 written by Bob Greenleaf with a reference to Mic McPherson



Some say a shorter neck is easier to keep straight and so minimize run out or otherwise is likely to shoot straighter.

My own purely anecdotal experience with the 7mm Remington Mag is that it works just fine as a hunting rifle but lacks the magical advantage I once thought it had over the .270 once I bought a chronograph (moral don't buy a chronograph). My wife and I once had indistingishable from the outside rifles hers in 6mm and mine in .243. I found over the whole range of loads I had to throttle back the .243. Again no idea of actual pressures (I did measure pressure ring to tenths in my ignorance) or speeds but taking the 6mm as an improved .243 with minimal improvement I could at least push book limits and put more powder in the 6mm than the % difference in case size - of course the larger case needed more powder to achieve the same result and still more to go faster so that doesn't really prove anything except that I found pressure signs in a .243 I did not find in a 6mm from the same maker.