Barrel & Chamber Erosion using Solids

[93x62]

Speaking for myself, my owner is currently using 235 GR Rhino Solid Shank Core Bonded Bullets and is considering the use of 180 GR LWHV Impala Lightweight Solids thinking that he will consistently get a flatter more accurate shot onto the target. This forum has already told a story about the improved accuracy and other positive comments about Impala units. Rhino shanks are solid copper units and may therefore be deemed to be similar to the Impala shanks. Rhino units are however heat treated for better expansion, meaning that they are softer than normal cold worked copper. As with most hunting rifles I am terribly one-eyed and looking at the lathe cut Impalas I cannot convince myself that they are heat treated, meaning that they will be quite hard wearing on my chamber and my rifling. First Question: -Do Impala use heat treated copper for their units. If Yes then I would like to see it on the packaging. If No, then I would love them to use heat treated or annealed copper. Second Question: - Are there any references available regarding research done on pressure - and associated increased temperature regimes on chamber and rifling erosion. I think we all think we know the answer but I would love to read a scientific / metallurgic report on the impact on the life span of the barrel.

Looking forward to your replies,

Signed SIG 93x62 Swiss Hunting Rifle

[A-R]

Dear 93 x 62. Glad to hear from a real old medium bore. Tell your owner the following:

This is only my (educated) opinion, but I will be the first to admit that I can't see inside the barrel, while the bullet is passing down the bore...

Bullet friction is of little to no consequence w.r.t. barrel life. Most (all?) bullets are much softer than the barrel steel. It is the powder that erodes the steel inside the barrel. Most erosion happens in the throat area, so this is where we should focus. There are exeptions to this "rule".

There are basically 2 processes that cause barrel throat erosion:
1. On ignition, the bullet "jumps" forward, and momentarily stops where it contacts the rifling. Pressure increases to the point where it is high enough to push the bullet to "engrave" into the rifling. During this pressure build-up, some hot gas (flame) passes around the bullet and through the gaps where the bullet has not yet sealed into the rifling. This hot gas (flame) causes some gas cutting (erosion) in the throat area. The duration is very short, but the damage accumulates with high shot counts. Slow powders cause more erosion than fast ones, all else being equal.
2. After the bullet has left the case, the rest of the hot gas/burning powder gets blasted down the bore. The case/chamber neck causes this hot "jet" to be "focussed" in the throat area, causing flame cutting/erosion to the throat. This gets more serious in "overbore" cartriges.

There is not much you can do about no 2. above, except changing the shoulder angle and neck length of the case. Even then, the erosion is not negated totally.

No 1. above, has been adressed in many ways. GS Custom, for example, have invented and patented their "driving band" technology. This causes dramatically reduced engraving pressure, better and quicker sealing, and less throat erosion. Every one tries to copy it... Other manufacturers have simply reduced the diameters of their mono bullets. This dramatically reduces the engraving pressure, but has the side effect of allowing gas leakage along the whole length of the barrel. Gas leakage will lead to gas cutting/erosion, with extended use. Also, "soft" bullets will engrave and seal easier (faster) than "hard" bullets.

So, factors to consider are, amongst others, bullet diameter, bullet hardness, bullet construction, cartrige expansion ratio, powder burn rate and likely a few more. What I'm trying to explain, is that this is quite a complex matter, with no short and clear answers. Personally, I would mike the various bullets, and avoid the undersized ones.

All factors considered, a 9,3 x 62 is not known for high barrel wear. And, while your type usually comes with real long throats (free bore) already, a little troat wear is not likely to cause much more trouble. Use a fast powder and preferably a Magnum primer. Crimp cases tightly into the bullets' cannelure, to bump up shot start pressure. When the bullet starts moving, it will cross the leade in the shortest possible time and slam into the rifling at max possible speed. This will reduce any bullet "wobble" in the freebore and help to engrave the rifling much quicker, reducing gas leakage and flame cutting/throat erosion. The result should be max barrel life and excellent accuracy.

Happy hunting!

[treeman]

This dramatically reduces the engraving pressure, but has the side effect of allowing gas leakage along the whole length of the barrel. Gas leakage will lead to gas cutting/erosion, with extended use.
Dude - ! you sure about this ?.

[93x62]

Thanks for your comprehensive reply A-R. It makes a lot of sense. I think you have hit the target with fast burning powder and proper seating / sealing in the rifling. It also means that the bullet manufacturer should use annealed copper or a somewhat softer alloy for the construction of monolithic solid bullets.

This hot gas (flame) causes some gas cutting (erosion) in the throat area. The duration is very short, but the damage accumulates with high shot counts. Slow powders cause more erosion than fast ones, all else being equal.
The case/chamber neck causes this hot "jet" to be "focussed" in the throat area, causing flame cutting/erosion to the throat. This gets more serious in "overbore" cartriges.
Also, "soft" bullets will engrave and seal easier (faster) than "hard" bullets.
Personally, I would mike the various bullets, and avoid the undersized ones.

[treeman]

The Gs C bullets are a alloy and engrave very well, hence my question about full barrel length blow by. Although softer than pure copper and cold worked, they are still harder than cup and core. My .270 uses a .20 thou bigger GSC because of over size barrel. I realized the barrel was oversize when shooting thin skinned bullets better than premiums.
For reasons we are still trying to fathom - my .458 x 2" only shoots Impala bullets after about 2200fps, it shoots them very well -1' at 100m easily. I have also noted that the engraving is very shallow and even hard to see. The gases in barrel are traveling much faster than the bullet and therefore there would be blow by. Now funny thing is after shooting Impala bullets, my barrel is at its cleanest. I have theorized, but got distracted and never followed up, that the Impala are acting like a scraper and cleaning the previous shot out each time you shoot. Try me on this - shoot 6/7 conventional bullets run 3 patch's through. Keep in cleaning order. Thoroughly clean barrel - do same with Impala's.
AR's statement about slower and faster powders also rings true and is more the reason for not wanting un burnt excess powder in barrel than wastage. Read somewhere that that is a real barrel eater, the powder which was not burned in the bullet barrel time. In colder climates they shoot over snow to see if unburt powder is ejected. Hope above is relevant.

[A-R]

This dramatically reduces the engraving pressure, but has the side effect of allowing gas leakage along the whole length of the barrel. Gas leakage will lead to gas cutting/erosion, with extended use.
Dude - ! you sure about this ?.

Yes, I'm very sure. But, as I said, that's a logical(?) deduction, not something I can measure. Apparently thin-jacketed undersized bullets can (and sometimes do) "upset", sealing better. This largely depends on pressure curves and a few other variables. But I think we can safely conclude that flat based and boat tailed mono copper and brass bullets wil not "upset" at any safe pressures. Have you ever inspected the grooves in a well used 303, with oversize barrel? The grooves usually are rough (and black), the lands shiny. MV is usually down quite a bit, up to 200fps and more. Hence my statement above.

But I don't want to turn this into a hijack...

[A-R]

On a related topic: I recently received 20 turned brass bullets, from a not to be named manufacturer, for T&E. First, I dug out the micrometer and checked diameters. Interestingly, the bullets all miked out at 0.308" (converted from metric) in the center of the bearing surface. The bases, however, miked at 0.307" and the front of the bearing surface miked at 0.309"! There is no way I'm going to force them trough my (very accurate) 308W rifle. Where does the manufacturer intend the extra 0.001" of brass to go? This is not lead that can be "squeezed" down...