Discuss why light 357 loads cut top straps & erode forcing cones?

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GJgo

GJgo

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I've read all over how 110 & 125 gr bullet weights in 357 loads cut top straps & erode forcing cones, while heavier loadings do not. The only reasons I've read are that the slow powders aren't fully burned by the time the bullet leaves the case, and the burning powder sand-blasts the steel as it blows through the cylinder gap.

So, if you're using a longer heavier bullet with the same slow powder, does it not cause the same damage because:
A) The longer bullets engage the rifling before leaving the case, so they slow down a bit and all powder is combusted in the case or..
B) The loads are enough slower that the powder all burns before it gets to the cylinder gap?
C) ??

Well, in both my 686 & my Blackhawk, (A) is busted. The 158 grain still has a significant jump out of the case before it hits the lands. Also, both the 125gr & the 158gr do engage the lands before they clear the cylinder gap.

So that leaves (B). Is there really that much of a difference? Considering that there is no resistance to bullet speed till it engages the rifling, wouldn't the speed that it clears the brass till it hits the lands be the same?

What are the odds that this really has to do more with powder choice?

Hope to understand rather than just accept. :)
 
Shoot your revolver in low light, if you see a fireball out the muzzle, then all the powder is not burnt before the bullet left the muzzle.;)
 
Lighter bullets burn more powder and that increases flash duration and heat. It has nothing to do with pressure but with heat.
 
So then if all the powder is not burning regardless, and it has to do with heat, wouldn't this still point more towards powder choice than bullet weight? Combination?

The impetus for me thinking about this is that I have a big run of 125s w/ 296 loaded up before I heard about any of this. I was planning on stashing it, and working on a new run of heavier bullets. Can't decide between 158s or 180s, 296 or AA9.
 
The majority of the powder burns inside the case in the chamber.
The fireballs and muzzle blast are mostly created by the high pressure gas plasma coming in contact with free oxygen in the air, not still burning powder.

My personal theory is, the lighter bullets not only burn more powder, but the light bullet starts out of the cylinder quicker before the powder burn is 100% complete.

The remaining burning and unburned powder is blown out of the B/C gap at very high velocity.
And acts as "sand blast" media when it hits the hard stuff.

With heavier bullets accelerating slower, more of the powder is consumed before the base of the longer bullet reaches the B/C gap so the sand blast effect is reduced considerably.

I base this theory on the fact the chamber throats right in front of the brass cases do not get "flame cut", and neither do the soft brass cases. And it just as hot in there, with more pressure, as it is at the B/C gap.

But the burning powder granuals are being pushed through them, not hitting them at a 90 degree angle like the top strap and lesser angle at the forcing cone.

I also base this theory on the fact even an oxi-acetlene cutting torch is not able to cut a steel dog food can in the brief mili-second the top-strap of a revolver is exposed to the flame each shot.
It simply can't get the thin steel can that hot that fast.

It also seems to me that "flame cutting" has become more prevalent after the use of ball or spherical powder became more widespread.
That happens to coincide with the beginning use of lighter 110 & 125 jacketed bullets in the early 1970's..

Prior to the 1970's, 158 lead bullets loaded with 2400 flake powder was about all there was in .357 magnum factory loads.

rc
 
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GJgo said:
I've read all over how 110 & 125 gr bullet weights in 357 loads cut top straps & erode forcing cones, while heavier loadings do not. The only reasons I've read are that the slow powders aren't fully burned by the time the bullet leaves the case, and the burning powder sand-blasts the steel as it blows through the cylinder gap.

So, if you're using a longer heavier bullet with the same slow powder, does it not cause the same damage because:
A) The longer bullets engage the rifling before leaving the case, so they slow down a bit and all powder is combusted in the case or..
B) The loads are enough slower that the powder all burns before it gets to the cylinder gap?
C) ??

Well, in both my 686 & my Blackhawk, (A) is busted. The 158 grain still has a significant jump out of the case before it hits the lands. Also, both the 125gr & the 158gr do engage the lands before they clear the cylinder gap.

So that leaves (B). Is there really that much of a difference? Considering that there is no resistance to bullet speed till it engages the rifling, wouldn't the speed that it clears the brass till it hits the lands be the same?

What are the odds that this really has to do more with powder choice?

Hope to understand rather than just accept. :)
Click to expand...
This explanation from a gunsmith might help give you a better understanding.
Now for forcing cone and top strap flame cutting (metal erosion).... which is a totally different issue. Cracks in forcing cones are caused by repeated high pressure loads ... erosion is caused by the powder burn itself. Here's what happens ... pressure and temperature peak at the same time. The longer it takes for powder to burn, the more it will flame cut. It's really a matter of the metal being exposed to the very high temperature (5000 deg C) and high pressure for a longer time and the "timing" of the bullet location. With lighter bullets and slow burning powders in a 357 Mag, peak pressure and temperature happen about the same time as when the base of the bullet is flush with the forcing cone. The metal in the top strap and the forcing cone is exposed to very hot temperatures under high pressure for a considerable period of time, which increases erosion a lot. Heavier bullets take longer to start moving so peak pressure and temperature happens while the bullet is still in the cylinder. This reduces high temperature exposure time considerably and also reduces the effects of flame cutting, although it is not totally eliminated. Even fast burning powder will flame cut a little. The bullets that cause the worst flame cutting due to powder burn timing with slow burning powders are 110 grain. 125 gr bullets are not as bad but still do considerable flame cutting. 140 gr and heavier bullets are delayed enough where flame cutting is minimal.

Howard
 
Thanks roaddog, that explanation helps. Makes me wonder though if moving from something like 296 to a slightly faster magnum powder like AA9 might help alleviate this some?
 
it's all about pressure and time. high pressure for a long enough time.

i get flame cutting with a 180gn bullet out of my model 29. i hear the 454 casull is a flame cutter. the 357 maximum, too.

murf
 
Whenever I have seen this subject discussed, it was in reference to non-stainless S&W K-frames. Does this problem show up in other 357's?
 
yes, flame cutting shows up on all my 357s. i don't see this as a problem, though. just part of shooting.

murf
 
roaddog28 said:
This explanation from a gunsmith might help give you a better understanding.
Now for forcing cone and top strap flame cutting (metal erosion).... which is a totally different issue. Cracks in forcing cones are caused by repeated high pressure loads ... erosion is caused by the powder burn itself. Here's what happens ... pressure and temperature peak at the same time. The longer it takes for powder to burn, the more it will flame cut. It's really a matter of the metal being exposed to the very high temperature (5000 deg C) and high pressure for a longer time and the "timing" of the bullet location. With lighter bullets and slow burning powders in a 357 Mag, peak pressure and temperature happen about the same time as when the base of the bullet is flush with the forcing cone. The metal in the top strap and the forcing cone is exposed to very hot temperatures under high pressure for a considerable period of time, which increases erosion a lot. Heavier bullets take longer to start moving so peak pressure and temperature happens while the bullet is still in the cylinder. This reduces high temperature exposure time considerably and also reduces the effects of flame cutting, although it is not totally eliminated. Even fast burning powder will flame cut a little. The bullets that cause the worst flame cutting due to powder burn timing with slow burning powders are 110 grain. 125 gr bullets are not as bad but still do considerable flame cutting. 140 gr and heavier bullets are delayed enough where flame cutting is minimal.

Howard
Click to expand...
This the way it's been explained to me too. :)

http://www.1911auto.org/forum/showthread.php?3815-Help-with-10MM-relaoding

Pic #3, post 3 has an AA #9 gap flash I took several years ago, not much of muzzle flash, but you can see a little flash at the rim before the mouth of the case expanded enough to seal off gas escaping rearward.

My scandium alloy N-frame M327 TRR8 has a metal insert in the top frame to prevent top strap flame cutting.
 
I still say it is "bead blasting" from the powder granules blown out of the B/C gap at very high velocity.

You can't cut a top strap with an oxi-acetylene cutting torch in the millisecond the gap flash has time to heat the steel and melt it.

And if it was hot enough to cut the top strap in a millisecond of exposure?
Why doesn't it melt or erode the cylinder throats and brass case too??

Because the powder granules are not hitting the brass case and chamber throats at 90 degrees.
They are flowing through it in the same direction as the holes.

rc
 
I think it may also be a matter of material and manufacturing. I see this happen on my Dad's model 60. A 3" barrel but in every other way essentially a j-frame. On his k frames, this doesn't happen. Bigger and stronger forcing cones. On my SP101, even though it is a "j-frame" size revolver, its forcing cone is the size of a "k frame". I don't have any problems with my ruger. But this is just my observations.
 
If the top strap was bead blasted there would be sufficient mass of particles against heat treated forged frames such as those made by S&W and Colt. Also, one would expect the bottom of the frame to show the same bead blast effect as the top strap given the same particle mass/velocities.

While powder 'streaks' can be seen in the the Power Pistol pic, none can be seen in the AA #9 picture, either at the muzzle or cylinder gap.

One of the basic safety rules of shooting revolvers is to always keep your fingers behind the cylinder gap because of potential flame cutting injuries. Not long ago there were pictures posted on a forum showing what remained of a shooter's finger that was placed too close to a top strap.
 
Also, one would expect the bottom of the frame to show the same bead blast effect as the top strap given the same particle mass/velocities.
Click to expand...
Why would anyone familiar with a revolver expect to see that??

Not only is the bottom of the frame much further away from the B/C gap, but:

The gas ring or cylinder bushing on the cylinder is shielding the ejector rod and the bottom of the frame from any blast that comes out of the B/C gap in that direction.

I think that maybe might be why it is called a gas ring??

Maybe thats why gas rings get gas cutting too sometimes?

rc
 
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Forcing cone erosion and flame cutting are caused by the heat generated in firearms by powder burning at high temperatures. The high heat leeches out carbon from the steel, causing it to become brittle and to erode with subsequent firing. Obviously the erosion depends on the 1) type of steel and 2) exposure and length of exposure.

Stainless steel stands up to flame cutting erosion significantly more than standard steel. This is because the chromium content in stainless steel resists carbon leeching much more than regular carbon blued steel.

Another interesting aspect of stainless steel was discovered years ago when the Navy conducted salt spray tests on a Model 60 stainless revolver and a Model 36 carbon steel model. The blued steel corroded much faster than the stainless steel up to a point, then the stainless began corroding faster than the blued steel.

Go figure.

.
 
Roaddog28 - I gotta agree with your explanation except for one thing. I think the 125 grain Magnums were the worst offenders. 110 grain loads (except for Super Vel) were not loaded nearly as hot as the 125s.

Sent from my Ally
 
rcmodel said:
Why would anyone familiar with a revolver expect to see that??

Not only is the bottom of the frame much further away from the B/C gap, but:

The gas ring or cylinder bushing on the cylinder is shielding the ejector rod and the bottom of the frame from any blast that comes out of the B/C gap in that direction.

I think that maybe might be why it is called a gas ring??

Maybe thats why gas rings get gas cutting too sometimes?

rc
Click to expand...
Thank you for explaining that the gas (not bead blast) ring/sleeve is positioned around the ejector rod. :)
 
I'm not sure of the whys, but a couple of months ago I called S&W customer support 1-800-331-0852, & asked about the flame cutting in BBLs with the 125gr .357 load. I spoke with a fellow named Jeff, he told me the life expectancy using 125gr loads in their S&Ws was 2,5000 to 3,5000 rounds. He said using 158grs loads the life expectancy was 5,000. Now 2,500 rounds is a lot of shooting with full house loads.

Jeff also told me to check the 6 OClock position in the forcing cone as that's where the wear will ocorr. He said what actually happens is the forcing cone wears till it gets thin & will then crack.

We had a very informative conversation & I am really pleased with the Customer Support at S&W at least answering questions.
Hope this is of interest to some.
Frank

PS: Jeff told me the 110s wear a bbl quicker still, but I don't remember if he told me just how much quicker.
FV
 
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High pressure, heavy powder charges, blast effect from the burning powder grains and higher sustained temperatures.

There are photos on the web of an S&W 329PD cutting clean through the little stainless blast guard in 1,000 rounds, next up would be the alloy frame.

It really is not much different than a rifle in this respect and throat erosion, Speer reports a volocity loss of about 150fps after 6,000 rounds throgh a model 29.
 
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