A few questions about .357 Magnum and BBI test results

[Krator]

Hello!
This is my first post (and topic) in here, but I have lurked this board for a few years and thanks to you all, my knowledge about firearms, physics etc. has greatly expanded. Thank you for all of this.

However, there are a few questions that, I think, have never been asked or answered, or at least I can't find the answers myself.

1. .357 Magnum and long barrels.
Why is .357 Mag benefitting so much from really long barrels (up to around 12-14")? When compared to .357 Sig it seems clear that Magnum at equal velocity and bullet weight always benefits way more from longer barrels.

According to http://www.ballisticsbytheinch.com/357sig.html
http://www.ballisticsbytheinch.com/357mag.html

Cor-Bon 125gr JHP, 357 Sig: 4" barrel: 1468 fps, best velocity out of 14" barrel: 1747 fps. Difference: 279 fps. Increase in %: 19%

Cor-Bon 125gr JHP, 357 Magnum: 4" barrel: 1496 fps (so far it looks like an equally powerfull/slightly hotter load), best velocity out of 16" barrel: 2119 fps. Difference: 623 fps. Increase in %: 41,6%.


And if you take a look at any other .357 Sig or 9mm +P load, the same is always true - they benefit much much less from longer barrels than .357 Magnum does.

Now, 125gr is pretty much the most popular self defence bullet in .357 Magnum and I doubt anyone @Cor-Bon would be loading this kind of ammunition with lever-action rifles in mind, yet it seems it absolutely shines when fired from the rifle's barrel.

So what causes this HUGE difference in performance? Is it because .357 Magnum ammo manufacturers use v. slow burning powders? Why would they (theoretically it's just more flash, more report, for no real benefit out of most popular 2,5-4" barrels - or am I wrong)? Or does it have something to do with case length?


2. Revolvers - muzzle velocity - cylinder gap.

I have carefully read the whole section of BBI about cylinder gap and apparently its impact on muzzle velocity is rather small.
Now take a look at BBI testing and you'll notice that their 4" S&W 686, which is an excellent performer in muzzle velocity department, compared to their other wheelguns actually was doing a bit worse than their 4" test barrel setup, measured from the breech face.
This puzzles me, since 686 with 4" barrel has the advantage of a whole cylinder that effectively allows the bullet to be accelerated for a longer time.
I expected the revolver to achieve velocities slightly below those of 5 inch barrel test setup. Yet it seems that 4" test barrel already outperformed it.

Same isn't true for semi-auto pistols. The "real world" guns basically achieve v. similiar velocities to the test barrel. It's logical - similiar barrel length, similiar velocity.


So, what causes such a big loss of muzzle velocity in revolvers? Cylinder gap tests indicate that revolver vs revolver, 1 with 0,006 gap and another with no gap the difference is not so great, and I'm pretty sure their test 686 is WAY more tight than 0,006:
For Federal Premium, .357 magnum, 158 gr. Hydra-Shok JHP:
4" revolver barrel, 0,006 gap: 1211 fps
4" revolver barrel, no gap: 1282 fps
and we're talking about 2 revolvers here, so in both cases bullet has exactly the same conditions (no cylinder-length advantage.

Also it's very interesting that in this test all of the sudden .357 Magnum no loner performs so great out of long barrel:
Federal Premium, .357 magnum, 158 gr. Hydra-Shok JHP:
1)16" revolver, no gap: 1492fps
2) 4" revolver, no gap: 1282 fps

(as a comparison, test barrel setup: 4": 1332 fps - 50 fps faster than 4" revolver with no gap!, 16": 1741 fps - 250 fps more)

Cor-Bon, .357 magnum, 125 gr. DPX:
1) 1246 fps
2) 1526 fps

(test barrel 4": 1471 fps - 225 fps more!, 16": 1946 fps - 420 fps more, WOW!).


So:
What makes a revolver such an inferior performer compared to "test barrel setup" in almost all cases? Why isn't the additional space for the bullet to accelerate positively affecting the muzzle velocity?
What may have caused muzzle velocity of the revolver-with-no-gap to be that much lower than muzzle velocity out of test barrel?
Is it simply all because of flawed testing criteria/conditions? Or is the presence of the cylinder actually negatively influencing the muzzle velocity. If so - why? Or maybe the revolver testing setup was somehow "broken" and their cylinder gap test results actually lead to underestimation of gap impact on the muzzle velocity?


Thank you in advance for your responses. Sorry for all the mistakes I've made - unfortunately my English is still rather poor, but I'm working on it even right now.




Now this is just mind-boggling:
http://smith-wessonforum.com/s-w-re...sults-38spl-9mm-38-super-9x23-winchester.html

Taurus 905 has only 2 inch barrel and it pretty much matches Beretta 8000. Which may indicate that the problem with revolver muzzle velocity being so low is actually caused by something in cylinder. 9mm headspaces on the case mouth. Is it possible that differently cut cylinder causes all the difference, allowing the revolver to all of the sudden match the "normal pistol" setups?

 

[BobWright]

I've been a handgunner for over sixty years, and a handloader for nearly that long. And I've puzzled over ballistics tables just as you have, and conducted my own experiments with revolver barrels of varying length and bullet/powder combinations. My own observation over the years has been to take ballistic tables with a grain of salt.

Two revolvers, or pistols, as near identical as possible, will give velocity differences using the same ammunition. It is generally accepted that longer barrels allow more complete combustion of the powder and therefore generate higher velocities. However, in my experience, some powders, even slow burning powders such as Hodgdon's H110, will give higher velocities in the 4" barrel over the 6" barrel. Very fast burning powders, such as Winchester 231 or Hodgdon's HP-38 tend to give uniform velocity increases as barrel length increases, but chamber pressures peak before true magnum velocities are achieved.

What I'm saying I suppose, is to use ballistics tables only as a guide and use actual firing observations as to the effectiveness of you gun/load combination. Bullets on the target are a far better gauge than tables.

Bob Wright

 

[DT Guy]

The .357 Magnum has greater case capacity, and can take larger loads of slow-burning powders, where the .357 Sig was specifically created to duplicate that 125 grain load with the case capacity of a necked-down .40. The slow-burning powder can often benefit from more barrel length.

The smaller case capacity of the Sig makes it quite a bit less versatile, but it does what it was intended.

Larry

 

[Krator]

But why would anyone load 357 Magnum self defence ammo for long-barrel performance like that? Isn't it all noise and flame when fired out of shorter barrel?

I think majority of .357 Mag revolvers are 4" or less and for those ammo behaving similiarily to .357 Sig would most likely be the best ( I don't mean same bullet weight, but simply - good performance out of 4-6" barrels, above that little gain from barrel length) - yet ammo manufacturers think otherwise apparently. What's the reason?

 

[R.W.Dale]

It comes down to two things.

Case capacity

And Pressure.


In short barrels such as in handguns pressure is by in large the determining factor in performance. Case capacity matters relatively little.


But in longer barrels it's a different story. Case capacity has a chance to go to work. In simple terms case capacity = a larger volume if powder, a larger volume of powder = more expanding gas to drive the bullet . This results in a much longer pressure curve on the base of the bullet providing larger gains in longer barrels.

You can load a 357 like you do 9mm with faster propellants at much lower volumes. When you do you see similar unimpressive long gun gains as you do 9mm or 357sig

It's all in the powders magnum revolvers can use and how much of it

 

[jmr40]

A couple of things.

No two guns will shoot the same ammo to the same speed even with equal length barrels. This explains a lot of the odd numbers you see on sites like BBI or when people post their results online. Differences in manufacturing tolerances will make some faster. Sometimes significantly. Take a look at the BBI page where they test real world guns. The 6" Colt python is significantly slower than the 4" S&W 686 with every bullet tested. This is an extreme example, but it illustrates the concept.

Magnum revolver rounds were conceived and designed around 7"-8" barrels. Semi-auto rounds were conceived and designed around 4-5" barrels. All of the published test data is from barrels of this length. Years ago longer barreled revolvers were more common, but today 3"-4" are by far the most common. With semi's though 4"-5" is still the most common length with few shorter than 3.5". Since all semi's are made pretty close to the barrel lengths the ammo is designed for actual speeds are very close to published numbers.

Some people seem surprised that a barrel 50%-75% shorter than the ammo is designed for does not perform nearly as well as expected. It would be like shooting a 300 magnum rifle with a 9" or 12" barrel and expecting it to match published numbers.

 

[41 Mag]

I think majority of .357 Mag revolvers are 4" or less and for those ammo behaving similiarily to .357 Sig would most likely be the best ( I don't mean same bullet weight, but simply - good performance out of 4-6" barrels, above that little gain from barrel length) - yet ammo manufacturers think otherwise apparently. What's the reason?/QUOTE]

While I will agree with you on most factory loads being loaded to a certain level, it wasn't that long ago that the average barrel length was around 6" as a standard or even short length for a popular brand 357 mag.

Personally all of my revolvers have 6" or longer barrels. Not so much to ascertain the max velocities but more so for the extended sight radius. The ability to get added velocity is just a bonus.

 

[Krator]

6" barrel is a standard, but...

While many .357 Mag revolvers indeed have 6" barrels, it doesn't explain why SD ammo is loaded in a way that it benefits so much from long barrels.
Once again - such ammo in barrels any shorter than 6" will just produce more flash and report and for no real benefit....


...unless I'm wrong about flash and report and there's no downside to using big powder charges that work the best in really long barrels.



And a few questions remain unanswered - even thogh every gun shoots differently, the 250 and 420fps difference between revolver + test barrel with no gap setup and the "standard" test barrel setup, in favor of standard test barrel, is way too big to just be caused by a slightly tighter barrel. This difference is simply HUGE. And actually the revolver should be achieving higher velocities, thanks to cylinder acting like an additional 1,5 inch of unrifled barrel, while exact opposite happens....


...and it seems to be also true for "real world guns" - 686, which is the best out of all "real world revolvers" is still far behind the 4" test barrel, even though the revolver, again, has the advantage of its cylinder acting like an additional 1,5 inch of unrifled barrel.

So far it seems that those lower muzzle velocities have something to do with the cylinder, and not with the cylinder gap itself - that would explain why revolver with no gap test setup is simply inferior to the standard test setup.

Can ANYONE explain this to me? Why all revolvers - both normal guns and the revolver test setup from cylinder gap tests - achieve much lower velocities?
Same isn't true for autoloaders, so it seems it really has something to do with how the revolver is built. If it's not the cylinder gap, then what?

 

[Kman314]

I have not followed the links you provided and so don't really know exactly what the test setup is, in other words, I could be totally wrong here. My guess as to why the difference in bullet speeds with "normal" gap and "no gap" is that even though the forcing cone or barrel is supposedly touching the cylinder, there is still a small gap. It may not be visible, but it's enough to relieve a little pressure. Hence the difference between a test barrel and "no gap."

As far as the cylinder goes, the cylinder bores are slightly larger than the barrel bore. There is probably some leakage of pressure both around the bullet and around the case.

Again, I could be totally wrong, but that's my story and I'm sticking to it -- until I get a better one.

 

[antiquus]

Random thoughts.

Long barreled .357's with hot loads approach .30-30 levels of energy.

Revolver loads are hugely more flexible than auto loads, which depend on some interval of energy to make the gun work correctly. Typically revolvers, and especially magnum's can be loaded with powders faster than Bullseye to something as slow as a small caliber rifle powder. Loads from manufacturers are done with a large range of powders, but usually in the .38 family the target load is for a 4" barrel. But is you want more oomph, hot load companies like Buffalo Bore will sell you something that will perform awesomely in a 16" barrel.

 

[Krator]

The cylinder explanation makes sense, but then... - why don't they just elliminate this weakness somehow? Apparently "test setup" has no such issues, so the problem exists only in revolver cylinders. Is headspacing the cartridge on the case mouth AND the rim simultanously possible? Theoretically a tighter seal between the cylinder walls and the projectile would be created.


On the gap - I think BBI crew actually managed to elliminate the gap altogether.
http://www.ballisticsbytheinch.com/gaptests.html



Now this is just mind-boggling:
http://smith-wessonforum.com/s-w-re...sults-38spl-9mm-38-super-9x23-winchester.html

Taurus 905 has only 2 inch barrel and it pretty much matches Beretta 8000. Which may indicate that the problem with revolver muzzle velocity being so low is actually caused by something in cylinder. 9mm headspaces on the case mouth. Is it possible that differently cut cylinder causes all the difference, allowing the revolver to all of the sudden match the "normal pistol" setups?