Do you get additional speed from WSM or WSSM cases?

[budlight]

does any body have proof that the short fat cas can drive a bullet faster on the same or even less powder?

6 ppc vs the same or larger 6mm case

like 243 wssm vs 243

25 wsm vs 25-06

270 wsm vs 270 win

300 wsm vs 300 win mag


I was told that the case shape is more efficient, but I need some real life test results. I found this statement.

6 mm BenchrestAnother influence was a series of 6 mm cartridges developed for bench-rest target shooting competitions in the 1970s. The idea behind these cartridges was that a short, fat cartridge would be more "efficient" than the traditional long, narrow cartridge, as more of the powder column would be in the immediate vicinity of the primer as it detonated. In turn, this would mean that a cartridge of this type would be able to propel a bullet at speeds comparable to those of "magnum" cartridges of the same caliber using significantly less powder.

 

[Greyfox]

I have read that the biggest benefit of the short fat case is the consistency of the ignition process, and pressure ramp, thus narrowing the velocity spread, and providing better precision. Also, the amount of powder that comes in contact with the wall of the case seems to effect the efficiency of the powder burn. There is much less of a wall effect with the fatter case.. As to higher velocities given the same powder charge I'd good test would be the comparison of the 6.5-06 and the 6.5x284. Case capacities are very close between the two. Judging from the many discussions on this site comparing the two, you can't draw any definitive conclusions as to a velocity difference. There does seem to be a general consensus here and elsewhere that the 6.5x284 is more accurate at extreme distances where a tighter velocity spread would make a difference.

I do have a 270 Winchester and a 270WSM. I do get an extra 250-300 fps out of the short mag using the same bullet/powder. I can get an extra 7.5 grains of power in the short mag. That's about 40fps for each additional grain of IMR 4350. I get an additional 50 fps for each additional grain when working up loads in my 270 or 308...about the same or a little less with my 270wsm. Doesn't seem to be velocity advantage. I would say the two 270 WSMs I have owned were .25-.5MOA shooters out to 500 yards.

 

[INTJ1]

Just like I posted on the nest, and just as we've known sine the 1940s, case shape doesn't have much influence on velocity. The modern day proof is the data in loading manuals. Where our individual rifles will vary a bit from published data, there is no more apples-to-apples comparison than a bullet company testing the same bullets in different cases. This is done under very controlled laboratory conditions.

I recommend you just look all this up in a few different loading manuals comparing whatever rounds you are interested in. About the only thing you'l have to do is correct for barrel length. Manuals that show pressure would be very useful, but I think those are all powder company manuals and the are sometimes inconsistent as to the bullets they use.

When you are all done, it would be great if you put all the into in a spreadsheet and then posted it. You might find an exception or two, but in general, the larger the case the more the velocity regardless of shape. Be careful when comparing the 30-06 and 280 as the data for both of those chamberings is often derived at lower pressures due to the rifles these calibers were originally chambered in.

 

[gunwalker]

The reason the wsm and wssm cartridges are pushed to higher velocities is their ability to withstand higher pressures. I shoot a 7mm WSM and just compared it to a 7mm Rem Mag. Using the same powder H1000, the Rem max load is 70 gr and produces 2905 at a pressure of 49600 psi, Where as the WSM max is 69 gr and produces a velocity of 2969 at 63400 psi. These are pulled from Hodgdon.com.

 

[INTJ1]

65,000 PSI is pretty much the standard SAAMI max pressure for most modern cases, and that limit would apply to both the 7mm RM and the 7mm WSM. I think what you are seeing is the 7 Mag data is shown in C.U.P where the WSM data is in PSI. Most of the older data is in CUP. While there is not an exact correlation between PSI and CUP; a rough guide is 65,000 PSI equals 52,000 CUP.

 

[gunwalker]

afp1 you are right, when I was looking at hodgdon's data they are in both psi and cup and I wrote the wrong one. They still show the 7mm Rem at a lower pressure, with a max of about 59000 psi.

 

[INTJ1]

I too have noticed some loads Hodgdon shows are well below the max acceptable pressure. Perhaps when the hit the pressure limit that particular load was very inconsistent.

Regardless, when loaded to the same pressure, the larger case will shoot faster, all else being equal.

I think what is confusing guys on this are two things. First, a smaller case almost always gets more velocity per grain of powder than a larger case. That is mostly because the smaller case takes less grains to generate an equal amount of pressure. In that same vein, we only gain about 25% of the case capacity in velocity.

For example, a case that has 21% more capacity--like a 300 Win vs a 300 RUM--will only generate 5.25% more velocity. At a nominal velocity of 3000 fps, the RUM theoretically gains 158 fps over the Win, and that seems about right, at least in my experience. If we take a 200 grain bullet at 2900 fps from a 300 Win and a 200 grain bullet at 3050 fps from a 300 RUM, we see the Win gets 32.2 fps per grain of case capacity while the RUM gets only 26.75 fps per grain of case capacity. However, the RUM still gets more velocity in the same length barrel at the same level of pressure.

The second thing is the actual case capacities of the WSMs. A 7mm RM hold 83 grains of water while a WSM holds about 80 grains. Throating long vs short could easily eat up that capacity difference. Also, the WSMs are about 1/2" shorter so in equal length barrels the WSMs actually have 1/2" more barrel, which is worth 15-20 fps. So a 7mm WSM may well indeed shoot faster than a 7mm RM. However, neither will keep up with a 7 STW even though the STW is longer, slopes more, and has a shallower shoulder than the WSM. The STW holds about 98 grains of water.

 

[WildRose]

does any body have proof that the short fat cas can drive a bullet faster on the same or even less powder?

6 ppc vs the same or larger 6mm case

like 243 wssm vs 243

25 wsm vs 25-06

270 wsm vs 270 win

300 wsm vs 300 win mag


I was told that the case shape is more efficient, but I need some real life test results. I found this statement.

6 mm BenchrestAnother influence was a series of 6 mm cartridges developed for bench-rest target shooting competitions in the 1970s. The idea behind these cartridges was that a short, fat cartridge would be more "efficient" than the traditional long, narrow cartridge, as more of the powder column would be in the immediate vicinity of the primer as it detonated. In turn, this would mean that a cartridge of this type would be able to propel a bullet at speeds comparable to those of "magnum" cartridges of the same caliber using significantly less powder.

Some simple physics... .

The larger diameter of the base means more powder ignites faster, and thus the total volume burns more quickly evenly.


Even if you have the same volume or less of powder, burning faster and more evenly in a larger diameter pressure vessel exiting through the same diameter opening, it allows for greater pressure too build before the vessel is opened (bullet separating from case) and then greater pressures exerted as the bullet travels down the barrel resulting in higher velocities.

The greater pressures also mean higher temperatures and more rapid erosion of the throat area thus burning barrels out faster with max loads.

 

[SpencerSS]

does any body have proof that the short fat cas can drive a bullet faster on the same or even less powder?

6 ppc vs the same or larger 6mm case

like 243 wssm vs 243

25 wsm vs 25-06

270 wsm vs 270 win

300 wsm vs 300 win mag


I was told that the case shape is more efficient, but I need some real life test results. I found this statement.

6 mm BenchrestAnother influence was a series of 6 mm cartridges developed for bench-rest target shooting competitions in the 1970s. The idea behind these cartridges was that a short, fat cartridge would be more "efficient" than the traditional long, narrow cartridge, as more of the powder column would be in the immediate vicinity of the primer as it detonated. In turn, this would mean that a cartridge of this type would be able to propel a bullet at speeds comparable to those of "magnum" cartridges of the same caliber using significantly less powder.

There's a problem or two with comparisons in this group. There are too many variables being changed. For an accurate comparison you need two cart.s with the same volume, with the same components (powder, primer, bullet...), loaded to the same pressure (and see which uses less powder, or load with the same powder charge and see which produces the higher velocity). The only thing that should be different is the case shape, since that is the variable being tested.

300WSM vs 300WM is closest. still not a great comparison.

The WSSMs are different volume, higher pressure than their usual comparative partners. The 270WSM is closer to the 270 Wby mag, etc...

 

[INTJ1]

How about this test? This is from Robert Rinker's book "Understanding Ballistics," Second Revised Edition, 1998, page 177.


"A series of tests were described in 'The American Rifleman' during the summer of 1946 and mentioned again in 1981. Three cases were tested, all of the same capacity and all necked to take identical .22 caliber bullets. One case had a conventional body taper and a long 14 degree shoulder. Another had a very long body taper and a 35 degree shoulder. The third had a little body taper and a concave radius shoulder.

Velocities and pressures were measured by both electronic-transducer and copper-crusher methods. It was reported that the performance was almost identical, within the expected error. [Bold is in the original text] This very carefully controlled labratory experiment found no difference in ballistic performance.

 

[WildRose]

How about this test? This is from Robert Rinker's book "Understanding Ballistics," Second Revised Edition, 1998, page 177.


"A series of tests were described in 'The American Rifleman' during the summer of 1946 and mentioned again in 1981. Three cases were tested, all of the same capacity and all necked to take identical .22 caliber bullets. One case had a conventional body taper and a long 14 degree shoulder. Another had a very long body taper and a 35 degree shoulder. The third had a little body taper and a concave radius shoulder.

Velocities and pressures were measured by both electronic-transducer and copper-crusher methods. It was reported that the performance was almost identical, within the expected error. [Bold is in the original text] This very carefully controlled labratory experiment found no difference in ballistic performance.

Propellants have changed dramatically since 1946.

From what you quote here there was no comparison of larger vs smaller diameter cases (OAD) such as are the difference in the short magnums vs those mentioned in the OP.

 

[budlight]

There's a problem or two with comparisons in this group. There are too many variables being changed. For an accurate comparison you need two cart.s with the same volume, with the same components (powder, primer, bullet...), loaded to the same pressure (and see which uses less powder, or load with the same powder charge and see which produces the higher velocity). The only thing that should be different is the case shape, since that is the variable being tested.

300WSM vs 300WM is closest. still not a great comparison.

The WSSMs are different volume, higher pressure than their usual comparative partners. The 270WSM is closer to the 270 Wby mag, etc...

The best test would involve the WSM which is nearly the same or even slightly smaller than it's magnum counter part. load the exact same primer and amount of powder. Then see if the case design actually does create higher speeds \

I don't care about pressure and all the hoopla. I would take the cartridge with the most velocity and the least amount of powder. I have owned my share of way over bore rifles. 30-378, 7stw, 338 Lapua

The 7mm mag and WSM are close in volume

 

[INTJ1]

Propellants have changed dramatically since 1946.

From what you quote here there was no comparison of larger vs smaller diameter cases (OAD) such as are the difference in the short magnums vs those mentioned in the OP.

The results wouldn't be any different with modern propellants, as the last decade or so of loading manuals show. Take a look for yourself.

There just isn't any ballistic free lunch.

 

[budlight]

There just isn't any ballistic free lunch.

I've been pouring over reloading manuals. I might have come across a free lunch, if I'm reading things right. The 25 WSSm is a smaller case than the 25-06 and with considerably less of the same powder can attain the exact same speeds.

But i have always had a distrust of reloading manual specs and only use them as a rough guide. Even for the same bullet they have discrepancies of velocities with different types of powders. It's like the editor didn't even proof read the work before publishing.