Home Forums Hunting & Shooting Long Range Hunting Help me choose a 7mm Chambering......

Dober,

Merry Christmas to you, too! I hope you're enjoying this time with family and friends

If you end up heading this way in March (or any other time), please let me know, and I will definitely make myself available to grab some lunch! It'd be nice to finally meet the Dober dawg *grin*

PM inbound with phone number.

They aren't more "efficient". ........people load them hotter,to higher pressure, and get more velocity......(ask Johnny B;they have run the tests with the 300WSM and 300H&H)

Even Federal will tell you that,if you ask them.

Since most all of us have no way of accurately reading pressures,we don't "know" what level we are really loading to.....we can only "guess". If we take two cartridges of similar capacity (82 gr vs 82.2),keep all things equal,and one goes faster than the other, it is only because we are loading it to higher pressure....there are no other reasons.There is no "magic" in a short fat case.(Although in barrels of the same length,a bullet from a short/fat does have a bit more barrel to travel through.)

I have (many times)loaded a 7 Rem Mag to the same velocities shown here for the 7mm WSM.....why? Because I loaded it hotter.....one load with a 160 gr bullet showed 3080 from one rifle, and a bit over 3200 fps from another of the same length(!).

Clearly, the load was fine in one rifle, and too hot in another,although I had no outward "signs" of pressures.

There is no "magic" in a case of equal capacity vs another of the same capacity but a different shape....if one is going faster than the other you are loading hotter,or something else is not "equal".

Merry Christmas

Yup, good post

Mr. B will take issue with the notion of fast & slow barrels, too. Velocity = pressure. Higher velocity (than published data) = higher pressure.

That's regardless of a lack of the classic pressure signs.

Or so I'm told.

140gr factory NBT's run 3300 out of a 7wsm. The holes generated by close range impact are impressive.. I've got a few pics around here somewhere..

I bet! The 150 NBT is supposed to be a tougher bullet, although the one time I used one on a deer from my 7-08, it was pretty explosive.

You miss your 7 WSM Calvin?

7 Mashburn Super AI best of the best

No. I have a 7saum that shot just as well, and just about as fast as the 7wsm. And I like the trigger and stock better on the saum.

Ah, a SAUM guy.....

I don't think so. Different barrels have different chamber dimensions, bore dimensions and variances, etc. Some barrels are faster than others because they build more pressure with the same load.



You're told correctly, but a powder charge that generates X velocity in a loose, long-throated, sloppy-bored rifle, will generate Y velocity in a tight-chambered, short-throated, tight-bored rifle.

I had a long conversation with him about this. My intuition is as you say

However- if I understand him correctly- the influence of those variables is trivially small in modern factory rifles, "pressure signs" notwithstanding*, and in fact pressure and velocity do correlate very closely, across rifles.

*most pressure signs are signs of sloppy tolerances stacking up funny in various ways

That's the condensed version, and filtered through ME big time, so let me be clear, this is me saying what I think he said, grin.

(because my intuition says, as I believe yours does Jordan, that one can derive a minimum pressure needed to make a velocity. However, one cannot derive a maximum pressure to make a velocity. Therefore, the two do NOT correlate both ways. And yet, pressure testing equipment tells a different tale, apparently!)

???????

Yeah... lol... crappy post.

Basically as I understand it the notion of fast and slow barrels is, practically speaking, false.

Take two modern factory 22" 30-06's running 180's at 2875 fps. One has a sticky bolt lift and short brass life; the other does not.

The common wisdom is to say the rifle running that speed "cleanly" has a fast barrel; that it is making that speed at safe pressures, while the other rifle is running at a higher pressure to make that speed.

In fact, they are at very similar pressures, because pressure can be accurately derived from velocity. However, in our hypothetical example, the rifle showing us pressure signs has lugs machined crooked (sticky bolt lift) and a sloppy chamber (shorter brass life).

This, I am told, is what modern pressure testing equipment shows. Velocity = pressure = velocity. The notion of fast barrels that make high velocities, beyond what the "books" get for instance, without higher pressure than the labs measured, is false. Barrels getting higher velocities, are doing so at higher pressures.

Hey Jeff,

I think you've got two separate ideas crossed- the existence of fast and slow barrels, and the idea that pressure=velocity.

I agree that total pressure equals velocity. We have to remember that pressure is what moves a bullet down the barrel. Period. Expanding gases from deflagrating powder create pressure which pushes the bullet down the bore. That pressure builds as powder burns, peaks, and then gradually declines. The velocity that the bullet carries out the muzzle is generated by the total pressure that has pushed that bullet, from start to finish. When we talk about traditional pressure signs, we're talking about signs of peak pressure only.

So while total pressure does equal velocity, we have to use consistent variables when comparing peak pressure. This is because peak pressure is largely dependent on the burn and pressure profile of the individual powder at hand, in the cartridge it's being burned in, and may not adequately reflect total pressure unless the pressure profiles that we are comparing are the same.

Pressure is affected by chamber dimensions, throat dimensions (and consequently OAL), bore dimensions, etc. This is why one rifle may make 3100fps with a certain load, and another using identical components, will make 3300fps. I have also heard JB say that if your rifle doesn't make the velocity that the books say it should, it may be a safe idea to add powder until you reach that velocity, assuming no traditional pressure signs are present. This is simply because the dimensional variables in one rifle are different from another rifle, which affects the pressure and velocity that a given load generates in both rifles.

So yes, my rifle and load are generating more pressure than what is reported by IMR in their load data. But if I'm using a book load, and I'm not seeing traditional pressure signs, I'm not too worried.

Well, no, that's not what I'm told is correct.

The variables you mention- chamber dimensions, etc- have an effect, but it's a very minimal effect on velocity. In your example, the rifle making 3300 fps is running at considerably higher pressure than the one running 3100 fps.

Hey- my intuition is in line with what you are saying, and, what you are saying is certainly the conventional wisdom. However, pressure testing equipment says otherwise.

I'm sort of in the same boat looking for a 7mm hunter, but I don't think any of those you listed are on my short list. The 7mm Dakota would be except for the brass issue that has already been covered. The 7mm RUM has had some problems with pressure in a case that size according to some reputable sources that have had to deal with returns of those guns. The 7mm WSM is so close to the 7mm Rem Mag that I don't see an advantage where ammo selection is concerned. You can get 7mm Rem Mag ammo about anywhere. The MV is pretty much a wash where the 2 are concerned.

Win M70 vs Rem 700 is just a preference thing. Either action will do just fine, but it sounds like he's concerned about the rebated rim. All of my rebated rim rifles are push feeds (Rem 700 or Sako on a 300 RUMs & a 338 RUM )and I haven't had any issues there at all.

Dober,

I like your choice of music...

Marty

Does that account for such things that (allegedly) reduce pressures and thus allow you to put more coal on the fire, more velocity at the same pressures? Bullet coatings or bore coatings?

Not an expert but I always thought that was the original draw of moly. It made the bullet very slippery which lowered pressures and thus allowed the reloader add more fuel to the fire and get higher velocities at the same/original uncoated pressures.

Thx Marty you're welcome to join us in Calgary.

Dober

Sorry for the hijack, but I am going to have to agree with Jordan here. My understanding of "fast and slow" barrels is...

Let's say .30-06 gun "A", 22" bbl of military manufacture, 6 - groove, "short" throat, and minimal cross sectional bore area.

V.S. gun "B" of modern manufacture, 22" bbl, 4 - groove, of generous throat (think Wby) and "large" cross sectional bore area.

So when I stuff factory ammo in either gun (granted its loaded on the mild side) I get nearly the same velocities in either barrel. Are the pressures close, probably.

Now I handload...starting at 47 gr of 4350 - both guns OK and duplicating factory velocity, about 2750 fps. Now try 47.5 gr, gun A shows all sorts of pressure signs and case head expansion, and still only gets to 2775 fps, time to stop. Gun B no pressure signs and still gets 2775 fps. In fact gun B gets all the way to 2900 fps and almost 50 gr of powder!

What I think is happening, is gun A slams its bullet into a relatively small bore, in a short throat very fast, creating a pressure peak very early, and regardless of trying a powder with a flatter pressure curve, you still get early pressure problems, limiting velocity. Now gun B "eases" that same bullet into the barrel, giving time for a powder with a flatter curve to add its energy over a longer section of barrel, forcing the peak pressure to be later than gun A. The result is the ability of gun B to generate a higher velocity by sustaining a higher pressure over a longer section barrel. Make sense?

Higher pressure only equals higher velocity, all else being equal. Barrel, chamber and throat dimensions vary greatly from gun to gun and have a huge effect on how much and when peak pressure is realized. Again think Weatherby. For all practical purposes a 7 Wby and a 7 RM "should" perform nearly identical, as they have nearly the same case capacity. However, every reloading manual will show between 100-200 fps advantage to the Wby for the same load at the same pressures. Why - its in the throat. Does that make some barrels "fast" and some "slow", I think so, but that's only one reason, perhaps the easiest to visualize, but there are many other factors that affect when and where a particular barrel generates peak pressure.