Why 7MM Remington Magnum

[BigDMT]

I doubt the US Squad is worried about barrel life anyway, they are concerned about winning at all cost.

Agreed. Barrels are an expendable item on a firearm, especially expendable in a competition as important as that one.

 

[mysticplayer]

... Is a short mag backed off to 2800 a lot harder on a barrel than a straight 284 at 2800? If so, why? ...

The backed off Short mag will last longer then the 284 for that speed but when the bore goes, the short mag barrel will be damaged for more of the bore.

Here is my basic understanding of internal ballistics.

to move a bullet you need to kick it with so much energy down the pipe.

That energy is a combo of pressure, gas volume and heat. As long as the energy imparted is the same (bullet leaving with the same speed), you can vary these three parameters to suit.

Barrel steel can handle "low" temps for long periods of time with little to no damage (removing of alloy metals notwithstanding). However, get over a "melting" point (may not be the tech term but makes it easier for me to understand) and the steel degrade rapidly.

Fast powders deliver their energy in a shorter period of time vs slow powders. The FP pressure curve will be a sharper peak vs SP which will be a flatter wider graph. The area under the curve determines the energy available to push the bullet.

So you can burn "less" FP but you need to do so at HIGHER pressures to impact the same energy as SP (area under the curve to be the same). At higher pressures, we also know that powders have to burn hotter. Peak temp is higher but for a shorter period of time.

Since the actual flame contact happens right at the throat of the chamber, high pressure at hot temps will be more erosive then a slower cooler but longer "push".

The short hot high pressure blast can heat the steel right at the throat above the "melting" point so you get alot of localised damage. The throat erodes, lengthens and bullets don't engrave properly. You get flyers and wonky accuracy. The throats get worn fast but these barrels can be saved by setting back on a regular basis - say every 300 to 500rds.

conversely, with SP, the cooler longer pressure blast doesn't degrade the throat as quickly BUT it sends more energy down the bore. The throats survives much more shots but the rifling for the next many inches are also cooked. When the steel finally goes, it may leave in chunks and the rifling is cooked so far down the pipe that setting back makes no sense.

This is how I perceive my bore wear for better or worst.

For the longest bore life, you want to use as little powder with as little pressure as possible, ie rimfire, but that comes with very low ballistics.

The higher the ballistics desired, the shorter the bore life. There really isn't anyway around it - unless you are willing to put up with ALOT more recoil.

That is another story.

From my experience, F Open at max ballistics and tolerable recoil, offered today has peak accurate life 800 - 1000rds. For FTR, under 2000rds.

You can change the ballistics to increase or decrease bore life as a trade off for how much your bullet gets bounced around. Or how much your shoulder hurts..... There is no free lunch.

Best is to start the season with 3 to 6 barrels and all the above becomes moot.
Jerry

 

[LESLEY]

Please educate me. Why 7MM Remington Magnum for the US squad on the upcoming F-Class World Championship.

Because they were told they had to

 

[LESLEY]

Even if it was the 7mm Rem Mag it would still do the job w/ proper dies and loads in a good rifle. It is a very accurate long range caliber with bearable recoil. It is a good killer too out to about 1200 yards. There is plenty of good components and would be just as accurate as the other 7mm's. Efficiency doesn't necessarily mean accuracy as long as powders are available with the proper burning rate even if it is a little overbore.

I can tell you have never shot f/open have you. Come on tell the truth

 

[LESLEY]

I doubt the US Squad is worried about barrel life anyway, they are concerned about winning at all cost.

Yep

 

[DanConzo]

No I have never shot f-open but I have shot 3 or 4 barrels out at 1000 yards benchrest in 7mm Rem Mag and they are very accurate and not hard to take on the shoulder, and if you set them back after 800 to 900 rounds you'll get a competitive 1500-1600 rounds. I shot a 300 Wby in the 17 lb class (Light Gun) for 12 years also (with a brake) and got well over 1500 rounds out of them, that's 75 grs H4831 with the 210's. The early loads with the 7mm Rem Mag were 62 to 64 H4831 with the 168 sierra MK and a rebated BT Hornady 162 Match bullet. There was no Light Gun then but my rifles only weighed 18 lbs. I don't even know what f -open is but I know what accuracy is at long range since 1971. I'm too old to lay down on my belly. I never said I shot f-open class. Is there is a secret that only f-open class shooters know, I thought accuracy was the goal. The 7mm Rem Mag was the winner of the Wimbleton Cup at least twice that I know of and that is a hard match to fire. I just can't figure out why it would be any different.

 

[LESLEY]

when was the last time the 7mm mag won the Wimbleton cup. It was a while back. Also when was F/Open started in the W. Cup. Another ? what size terget do they shoot in the cup

 

[DanConzo]

Look it up for yourself. I'm not trying to convince you of anything, you didn't start the thread, I just stated my experiences and others with the 7mm Rem Mag. If good bullets are available for this caliber I believe that people would be very surprised how accurate this case is. I never shot in the Wimbleton but I do know that the competitor has no rest for his rifle. If you don't like the 7 Rem Mag that's fine. Just my opinion, since the person that started the thread brought it up.

 

[DanConzo]

The 1911 Colt 45 acp and 30/06 was invented a long time ago. Just a statement, if it doesn't pertain to you-ignore it.

 

[Desert Fox]

I doubt the US Squad is worried about barrel life anyway, they are concerned about winning at all cost.

Dan, This has nothing to do with barrel life since I'm pretty sure the US squad will have ample of barrel at their disposal. The object of the thread is the US squad choice of cartridge in this competition.

 

[Desert Fox]

So you can burn "less" FP but you need to do so at HIGHER pressures to impact the same energy as SP (area under the curve to be the same). At higher pressures, we also know that powders have to burn hotter. Peak temp is higher but for a shorter period of time.

Since the actual flame contact happens right at the throat of the chamber, high pressure at hot temps will be more erosive then a slower cooler but longer "push".

A lot of internal ballistic has to be taken on faith since we can't see what's going on inside a close breech. 7RSUM is a short fat cartridge with shorter powder column. I doubt if the peak pressure gap between the 284 and the 7RSUM is that much significant. With the proliferation of a modern powder, the gap is probably even smaller. I'll say don't use magnum primer on that 7RSUM.

 

[DanConzo]

Desert Fox--I fully agree with you about not having to do anything with barrel life, I never stated that it did, others did if you would review the whole thread. Barrel life for this world match would not even be a consideration. Also if the US squad plans on using the 7mm Rem Mag, as far as I am concerned it is an outstanding choice.

 

[BigDMT]

So you can burn "less" FP but you need to do so at HIGHER pressures to impact the same energy as SP (area under the curve to be the same). At higher pressures, we also know that powders have to burn hotter. Peak temp is higher but for a shorter period of time.

Since the actual flame contact happens right at the throat of the chamber, high pressure at hot temps will be more erosive then a slower cooler but longer "push".

A lot of internal ballistic has to be taken on faith since we can't see what's going on inside a close breech. 7RSUM is a short fat cartridge with shorter powder column. I doubt if the peak pressure gap between the 284 and the 7RSUM is that much significant. With the proliferation of a modern powder, the gap is probably even smaller. I'll say don't use magnum primer on that 7RSUM.

It's "RSAUM". Stop assuming everything. If you are assuming, you don't know.

Its really splitting hairs here. All these rounds will work, they chose the RSAUM, there's no changing it.

You say the thread is not about barrel life, yet here you are again talking about pressure for some reason...

 

[Desert Fox]

It's "RSAUM". Stop assuming everything. If you are assuming, you don't know.

Its really splitting hairs here. All these rounds will work, they chose the RSAUM, there's no changing it.

You say the thread is not about barrel life, yet here you are again talking abut pressure for some reason...

Hey! calm down. No need to get bent out of shape here. So I'm missing an A in RSAUM but you're missing an O in abut, that means we're even

Again this is not about barrel life. The thread is about the choice of cartridge. You might call it splitting hair and 7RSAUM might be a good choice after all. But who can say that a 284 Winchester is not.

 

[mysticplayer]

The whole point of using a larger case is to get the speed without the tuning issues that come with excessive pressures.

The 284 is a relatively small case and pressures will get pretty toasty when trying to push 180gr bullets into the 2800fps range. That increase pressure can cause some serious tuning headaches depending on the rifle and barrel.

And ambient conditions.

The larger case of the 280 AI/ Mystic and RSAUM just gets rid of that headache at the same speed. At 2800fps, moderate pressures. At 2900fps, toasty but not breaking anything - RSAUM is not super tough brass.

Larger cases like the 7RM can break 3000fps without fuss so boils down to what ballistic level you want.

The US team obviously sees 2900'ish fps the speed they want to run BUT do not want any tuning risk in the heat of the NM desert.

Smart...

Jerry

PS DF, try this little test. From a cold barrel to the touch, fire 4 or 5 rds fast using a fast powder that shoots well. Then do the same test with a slow powder. Feel where the heat generates in the barrel steel.

You might just feel the heat further down the pipe with the slower powder.

 

[Desert Fox]

As a matter of fact I'm contemplating doing that. I talked a friend into chambering his rifle to 284 Winchester. He told me that he can drive the 180 Berger at 2900 with no fuzz using 52 grains of Reloader 17. I'm going to do a side by side test with my 7 WSM using the same bullet but with a reduce load of Reloader 25 to match his velocity. Both barrel has identical contour. I will use an infra red thermometer to record temperature. Who knows this might convince me to forgo a 284 build and keep my 7 WSM after all.

Thank you Jerry.

 

[mysticplayer]

AFAIK, most in Canada have given up on the Re17 with the 284. Our temp swings can be massive during a match from morning to afternoon and tuning became a pain.

I found Re17 to be a very peaky powder so when it was on, awesome. When it was off, trainwreck. Loading on a razors edge.

I tried in my my 6.5 Mystic and 260improved. Yes, there was more speed BUT the accuracy was spotty. When loaded to where the accuracy returned, so did the same speed that I usually get.

It didn't become a wonder powder for my application. It is temp sensitive in my application.

YMMV

Interesting to see how your tests go.

Jerry

 

[DanConzo]

Jerry--Did you ever come to the conclusion from barrel cleaning and/or borescoping that thr Reloader series of powders was more corrosive than Hodgdons from about medium slow to slow burning rates? Just wondering.