Bore Wear in Cast Boolit Rifles

[BruceB]

Gents;

Most of us have seen the reports of greatly-increased barrel life in rifles using only cast bullets, compared to those firing jacketed-bullet loads. Lyman published a finding some years back that one of their rifles using jacketed loads was pretty much worn out where a similar rifle having fired the same number of CAST loads showed almost no barrel wear at all.

Looking at the Lyman handbooks, I can see one glaring reason for this phenomenon, in my opinion.

That is: in the majority of loads in the rather dated Lyman CB Handbook, they use SMALL charges of fast powders, compared to the loads used in "normal" (jacketed) loads.

Heat and associated high pressure are the enemies of barrel steel. While firing my Garand on Thursday, I got it up to quite warm temps, using 39 grains of IMR 4831 powder. (My report is on the Military Rifle Board.) Compare the BTU heat content of such a load with the more-common CB .30-06 loads of perhaps 18-24 grains of various other powders. The 4831 loads were operating at much lower pressure than most Garand loads, so the actual exposure of its steel to HIGH heat was limited....but still the total heat inflicted on the rifle was higher than with, say, 18 grains of 2400.

When cast-boolit cartridges are loaded to near-standard-SAAMI pressures with charges similar to those of factory-type ammo, I am wondering if we aren't also inflicting jacketed-type wear on the barrel? Many CB-friendly cartridges can readily equal factory-jacketed levels with our beloved cast boolits...the .30-30, .32 Special, .303 and many others come to mind. My .416 Rigby happily goes far above factory-load levels. Bullet velocity is NOT what wears out barrels, witness the fact that the least wear appears near muzzles, where velocity is highest. Instead, barrels wear out in the throat area, where pressure and heat are highest, and bullet composition may have very little to do with this.

In addition, there is some evidence that the scouring abrasive action of larger charges increases the throat wear as unburned granules scrape across the microscopically-thin layer of near-liquefied steel on the surface of the throat. Our typical small fast-burning CB charges are consumed rather quickly, and certainly a bigger percentage will burn in the case rather than in the throat or barrel as slower powders do, which would surely tend to minimze such abrasive damage. Remember too, that those arch-culprits of heat and pressure are located BEHIND the bullet, whatever type we're using, and thus the bullet itself doesn't have a whole lot of involvement in the erosion process.

If I had unlimited funds and a trained monkey to crank out zillions of rounds, I'd like to test two identical new rifles in a side-by-side test, using jacketed bullets in one rifle and an EQUAL pressure/powder-charge load and same-weight cast bullet in the other. I have a sneaky feeling that the two barrels might be much closer in wear patterns and bore life than current "wisdom" may hold to be true.

This certainly won't cost me much sleep, because even if full-power cast loads DO wear barrels like the "other type", it's still WAAAYYYyy too much much fun to stop now...

[carpetman]

BruceB---That's an interesting observation that barrels don't wear out at the muzzle and that is the highest velocity. Despite that,I do think velocity is what wears out barrels. Because getting those higher velocities requires more pressure and heat. In Lyman manual there is glaring difference between jacketed velocities and those of cast bullets. Certainly pressure is reduced in a cast load vs a jacketed one. Perhaps beating on a railroad track with a rubber mallet vs a sledge hammer? Chicken or the egg? I'd guess the chicken as God wouldn't want to lay an egg. Who knows maybe the first one was a test tube deal? Where are those Navy SEALS when you need one?

[felix]

Bruce, it is heat, and only heat in the throat area, that causes the accuracy degrade. Full pressure (30K cup plus) of powder having 20-40 percent nitroglycerin will be especially damaging. My bench gun's throat has quadrupled since the 70's when retired shooting 4198s after about 1200 rounds. The gun has now over 12,000 rounds using an equivalent of 12.5 grains of 2400. Actual powder is N110 and N105 at equivalent pressures. The 4198 load was 19 grains. And, these are single base powders at that. ... felix

[MTWeatherman]

BruceB:

Are you old enough to remember Herter's? I say that because I have their original "Professional Loading" and loading data yellow book in which the issue was discussed.

In general, you couldn't believe a thing Herter's said as long as they were referring to their products. However, they did tend to have good information at times if they simply stuck to the facts.

Pressure and temperature are directly related...raise one and you raise the other...lower one and you lower the other...all other things being equal. That's a fact. You are therefore quite correct in maintaining that Lyman's information was meaningless given the low pressure loads...they also had low temperature and negligible throat erosion. I am in the camp that says its pressure (and therefore temperature) that causes premature wear in a barrel...and that wear(actually erosion) is primarily in the throat. You can wear out a barrel due to friction with a bullet (and that increases with velocity) but that takes many, many bullets.

What Herter did was to correlate the chamber pressure with chamber temperature or "flame temperature" and came up with the following relationship.

PRESSURE------TEMPERATURE(in degrees F)
25,000 ------ 2,000
40,000 ------ 3,300
50,000 ------ 4,000
55,000 ------ 4,350
60,000 ------ 5,050

Herter claimed that 3,300 deg was a critical temperature in that it was the temperature at which gun barrel metal began to melt. As the temperature increased above this value, the melting (and erosion) of the metal increased. Erosion is simply melting of a tiny layer of the surface which is then removing by the high velocity gas.

Since the 30-30, .32 Special, etc. class of cartidges operate near or below that 40,000 pressure, it would mean that even full power loads would have little erosion. Get up to the overbore magnum class of cartidges firing jacketed loads at over 60,000 psi and erosion would be pretty severe. That's pretty much in line with what we observe.

Can't verify the actual pressure and temperature values Herter came up with, but the logic appears sound. No reason to doubt it though since he had nothing to gain by doing so.

[9.3X62AL]

Interesting thoughts, Bruce. I'd like to have the time to attempt burning the throat out of a few rifles--although my usual suspect (12.0-18.0 x 2400) might keep such effects on the slow side as you say.

[44man]

MT, your mention of Herters sure brings back memories. That was a fun catalog and I bought a lot of stuff from them. I am still using some of their knives. Too bad the new Herters is just involved in duck hunting stuff. Much more expensive too!

[MTWeatherman]

MT, your mention of Herters sure brings back memories. That was a fun catalog and I bought a lot of stuff from them. I am still using some of their knives. Too bad the new Herters is just involved in duck hunting stuff. Much more expensive too!

44Man...I went into mourning when Herter's went under. Understand that the banning of mail order firearms started their slide though suspect management had to have some role in their demise.

Like you, I bought a lot from Herter's. Unlike some of the younger people, it was Herter's not Lee that got me started in reloading and casting. I've still got the original "Super Model 3" reloading press on my bench for when I really need a heavy duty press but mainly use it for push through Lee bullet sizers. I still have a number of Herter's dies...still use one set to turn out .32 Special ammo on it, but other than that, they're pretty much in retirement. I purchased one of their revolvers in .44 Mag but sold it years ago and replaced it with a Super Blackhawk. It actually was a fine revolver. Great grip...more like a Bisley than the Blackhawk.

It was a kick reading their advertising...everything was "world famous", "guide quality", etc. Although not usually the "world's best" as advertised, they did actually have some pretty good equipment.

Herter's was the Cabela's of its day...actually more like combining Cabela's with Midway and adding mail order firearms...all a big discount. Yep, Herter's got a lot of us old timers started in reloading...I miss them and their catalog. They're not the same company today.

[bfoster]

Just a note: Once upon a time I was a tool maker and tooling designer.

Obviously, there are considerable differences in the thermal conditions that steels encounter when used in die casting dies and in a rifle bore, but the similarity of "wear" in the leade area of a rifle barrel used with "standard" smokeless loads, be it 416R stainless or 4XX0 chrome-moly or chrome-vanadium-moly alloy, and the steel used in die cast dies (H-13, usually) used in making aluminum parts is striking.

This form of wear is called heat checking- this forum isn't the place to go into the details of how it happens: that's covered thoroughly in engineering reference textbooks that detail the various modes of failure in steels.

FYI, in the aluminum die cast process, the alloy is heated until it is molten, 1300+° F, and forced into the die cast die by a plunger at pressures ranging from a few hundred to ~5000 lb/in².

Bob

[StarMetal]

Felix said it was heat and heat alone that wears out the throat. I thought it was that when the barrel metal was in a more fragile state due to the heat from the powder combustion and bullet friction that powder kernels that weren't burned yet had more of a sandblasting effect on the heat metal then cold metal, also the tight fitting jacketed bullets has to cause more damage on this heated metal too when it slams into it, then if it were cold. Thing about a clutch on an automobile. The flywheel certainly gets heat checked and it definately wears from the clutch disk which softern then the flywheel.

Joe

[felix]

Joe, you are NOT wrong, so don't even think about it. Friction as you say plays a role, but the heat caused by the friction (on a micro-level) plays the 90 percent role. Notice that car springs wear down 30 percent of their springing power after about 6 months, right? How come? ... felix

[StarMetal]

Metal fatigue from flexing, which is probably the heat created from friction of the molecules moving and rubbing one another.

Joe

[JBMauser]

Well as I see it, barrel wear and throat erosion are two different things. A barrel can loose metal and erode due to high pressue with in turn runs temps way up as the Herter's data stated would support. Hard cast bullets are not going to run those kinds of pressures. barrel wear is caused by abraision. mike Venterino wrote recently about paper patched bullets wearing off and rounding the leading edge of the rifling in the old buffalo hunters barrels through abrasion. I do not think even hard CBs would do the same. IMHO JB

[45 2.1]

Mike Venterino wrote recently about paper patched bullets wearing off and rounding the leading edge of the rifling in the old buffalo hunters barrels through abrasion. I do not think even hard CBs would do the same. IMHO JB

Those buffalo rifles used barrels with VERY SOFT STEEL in comparison with todays steels. They would wear quicker with any form of bullet.

[Leftoverdj]

I would be willing to bet that you could shoot out a throat with blanks, and probably quicker than with bulleted cartridges because of the higher gas velocity. We KNOW that heat is a major factor from the speed with which machine gun barrels burn out in sustained fire. Even with bolt actions, there have been a good many reports of shooters getting overly happy in a prairie dog colony and ruining a barrel in a few hundred rounds.

My belief is that pressure is chiefly important as it relates to gas velocity. The gas itself can cut if velocity is high enough and one component of most powders is the abrasive graphite.

[Bass Ackward]

When cast-boolit cartridges are loaded to near-standard-SAAMI pressures with charges similar to those of factory-type ammo, I am wondering if we aren't also inflicting jacketed-type wear on the barrel?

Instead, barrels wear out in the throat area, where pressure and heat are highest, and bullet composition may have very little to do with this. In addition, there is some evidence that the scouring abrasive action of larger charges increases the throat wear as unburned granules scrape across the microscopically-thin layer of near-liquefied steel on the surface of the throat. Our typical small fast-burning CB charges are consumed rather quickly, and certainly a bigger percentage will burn in the case rather than in the throat or barrel as slower powders do, which would surely tend to minimze such abrasive damage.


Bruce,

Answer to your first question is no.

Here are some more things to think about.

There are many factors to throat erosion. Very little to barrel wear itself. Just run a lead slug through your bore. After it is sized down, it travels easily. The key phrase there was after it sized down. Do you choke?

Quality and type of barrel steel makes the most difference. The second is case size to bore ratio. (volume) Diameter and the length of the throat. A tight throat down close to bore diameter sees more heat. Throats see more wear from copper because there is gas leakage before the bullet seals. So it is not just pressure. Microscopic copper molecules shoot ahead of the bullet and are impressed by the bullet causing .... "coppering". A cast bullet seals almost immediately.

Old timers threatened to quit buying commercial bullets from vendors if their makers used antimony. The higher the antimony content the more abrasive the bullet. Oooops. Too fast. Try that one again. The higher the antimony content the more abrasive the bullet. In addition to that .... the harder the bullet, the more abrasive the bullet. Shooting soft bullets increases barrel life and decreases throat erosion.

Case design affects where the powder burns but volume is still the key. Case volume is a measure of how long the heat lasts per shot. Then there is rate of fire for the gun. Benchresters heat the barrels up and then fire for score on a hot barrel to maintain temperature. Hot throats erode faster than cold throats regardless of all other factors. No matter how over or under bore the case. And they usually alloy for more antimony so they can shoot harder bullets. They shoot shorter barrels so they have higher exit pressures. In essence, benchresters race to see how fast they can ruin their gun. Like building a car to get up to 90 MHR between stop lights.

But as far as the barrel itself goes, the military did copper tests after WWII with barrel steels of the time and found accuracy to improve up to 8000 rounds. These were high pressure, jacketed loads. Even though throats eroded, the most damaging wear affecting accuracy actually occurred at the muzzle from escaping gases around the base of the bullet upon exit. So the amount of pressure upon bullet exit from the bore is the worst culpret of all. And what's the best way to guarentee low pressures at bullet exit? Either shoot faster powders that peak quickly and then start to drop sooner. Shoot longer barrels. Or ..... that's right, increase .... bore diameter. Smaller bore diamters erode faster because of longer pressure curves and thus slower pressure decreases. (higher muzzle presure.) Same for shorter barrels.

So if you get a good stainless barrel, the longer the better, in a large bore diameter, and don't go over bore on case volume, shoot soft ACWW bullets or softer, with slow burning (ooops, try that one again) slow buring powders that start out with low pressures (and heat) in the throat area and burn at lower temperatures out in the bore, firing at a slow rate monitoring barrel temperature, your questions will be, "Throat erosion? What's throat erosion?"

[buck1]

I think it was handloader a few years back. Anyway they ran several tests with diferent burning rate powders. They concluded that the BBL would wear out in relationship to the speed of the powder burn. Slower powders burning longer in the BBL would erode the fastest.
It makes sence to me , large volumes of slow burning powder should subject the bbl to a heck of blast. After/wile it ALL blasts by the throat in a semi soild state that is. So I agree with the thought its not the speed of the boolit but, the means used to get that speed that kills bbls. My $.02 worth ..Buck

[floodgate]

Joe:


"...and it definitely wears from the clutch disk which softern then the flywheel."

Right! In a grit-loaded environment, the harder metal will wear MORE than the softer one: the grit embeds in the softer material and stays put - and cuts away the harder one. This is why copper is used with diamond or emery powder in honing steel - or lead for lapping a barrel. Of course the softer material is rendered useless afterward (except for more honing), because of the embedded grit, but the harder steel is made smoother and more uniform.

floodgate

[Ballistics in Scotland]

It might be more a matter of saying it IS closely connected to velocity, but the means used to get it make quite a difference. For example, obtaining a high velocity with a fast powder, although well suited to a light bullet such as the 87gr. .25, can produce greatly accelerated erosion. What Lyman did was to use very light loads, with the result that bullet material became significant. But usually it isn't. Of course bore erosion is such an unpredictable business, with so many variables involved, that side-by-side testing of two barrels proves only what happens in those particular circumstances, if that.

I think the commonest opinion among experts these days is that it is a matter of heat change. A very thin layer of barrel surface is indeed heated close to melting-point, but that isn't of itself the major part of the problem. Next that heat is led away, either into the cooler outside of the barrel (and eventually the air) or into the gases and air inside the bore. The period is so brief that if you could stick your eye to the muzzle a split second after firing (you'd wanbt good coordination for this), you probably wouldn't see a trace of the red heat the bore surface must surely have.

In the process, the metal is hardened amd embrittled by absorption of nitrogen from the powder, a process much like nitride hardening of dies etc. It is this extremely fast heating and cooling which results in expansion and contraction, and crazing of the surface, just like you could do with an enamelled steel mug. At this stage a hard-jacketed bullet ("especially if oversized" would be my guess) might be more inclined than lead to break away the pieces between this microscopic pattern of cracks.

Whether antimony or unburnt nitrocellulose grains make much of a difference, I'm doubtful. It wasn't unknown for the old-timers to get the wrong idea. But we can see from the above theory why the mere fact that they are softer than barrel steel, doesn't rule out the possibility. Stainless steel offers its advantages by not absorbing nitrogen as much, and not being as much hardened. Chromium plating is much better still, and has an extremely valid application in military barrels, but can't be deposited with the consistency a top-quality civilian rifle barrel requires.

We all know that rapid fire is much more destructive to the bore than slower shooting. Of course machine-guns erode faster still, but not as much faster as you would think. The above theory is a pretty fair explanation of this. There isn't time for so much of the cooling part of the cycle to take place.

The quality of the seal produced by bullet size and upsetting may also have a great influence on throat erosion. Gas passing through the grooves acquires a much greater velocity than it has behind the bullet. A fair analogy here might be when your hot-water tap will deliver bathwater just as hot as you can bear to lower yourself into when it is still. But running over your hands, it can cause severe pain and scalding. The reason is that there is no chance to build up a boundary layer which is cooled by heat loss to your body. Similarly a revolver can cut into its topstrap at the cylinder gap while the bore erodes very little, and a rifle's extreme muzzle area can erode considerably, especially with boat-tail bullets, since that is where even the lower-pressure gases at that point get a chance to accelerate greatly. Note that the seal from a given bullet can vary very much with pressure, a hard alloy being unable to upset at pistol or low black powder rifle velocities, but all right at high pressure. A flat-based jacketed bullet is much more likely to seal well than a boat-tailed one.

The much greater erosion in the throat area is due partly to temperature and pressure, sometimes partly to failure bullet sealing, and also partly a matter of time. The throat area is heated for all of the bullet's bore time, and the muzzle area for a small part of it.

[StarMetal]

I believe the initial use of chromed bores by the military was with the intention to find the corrosion from corrosive ammunition. I don't know who was the first to use it, but know the Japs used it pretty early on. Anyone know the history of what country chromed their bores first and when? As far as chrome not being put on in a consistant coat I believe is a thing of the past. There are some custom match target AR15 builders using chromed bores and they shoot every bit as accurate as noncoated. I have an early pre-ban model of the Colt AR15 HBAR that I will put up against anyones similar rifle. For what it is, it's an amazingly accurate rifle.

Joe

[BOOM BOOM]

HI,
Think of it this way.
Friction is the great enemy of all machines!
It is friction (which can cause heat just buy itself) & heat that casues wear & tear on all machines (car or rifle).
Yes pressure has some effect but take a look at railroad tracks, they have alot of pressure but last a long time.
If I remember right the army did a study a long time ago on 30/06 machine gun barrels. To sum it up 1) a machine gun barrel would last about 10,000 rounds on the average. 2)the rate of fire had a clear effect, at cyclic rate of fire for the M-60 they acually shot out much faster (less than 5,000- rounds) & the barrels began to bend downward due to gravity as the metal was so hot.
Also I believe it is accepted by all of us that higher vel. rounds like the 220 swift will shoot out a barrel faster than a slower vel. round like the 30/30.
Pressure in how hard 2 surfaces are pushed together increases friction which increases heat.
So all 3 play there part & it may be almost imposible to totally isolate the effects of friction, heat, pressure, & velocity as they are all interelated and effect each other. Also rate of fire also can play a major part in barrel wear.

[giz189]

Maybe, just maybe, when Ole Elmer said if you ignite the powder from the front end, instead of the rear end, you get less wear in the throat, a little more velocity, and cooler barrels. They just get a little hotter at the muzzle end instead of the chamber. Duplex Loads is what he and his compadres called it back then. I also noticed that Knight Muzzle Loadinf devised a variant of this in their .52 calibre muzzleloader. The breech plug has an extended tube that extends about half way of the powder stack. This effectively ignites closer to the front than the other breech plugs, starting the ball on the way and purportedly reduces pressures and somehow increases velocity. Elmer was supposed to have done this with the 50 calibre BMG during WWII. I don't know, cause I wasn"t around then. However, it could make sense if you thought about it.