"Throat Erosion" and "Burning out a Barrel" -- What, When and Why?

[Broncazonk]

This is what I know:

1) Throat erosion is bad,
2) Too much powder, or velocity (or something) burns out a barrel and it can happen in a hurry,
3) I limit my .284 and .30 cal. loads to 3025 fps to prevent throat erosion.

Questions:

1) Is "throat erosion" and "burning out a barrel" the same phenomenon?
2) Powder can causes a "plasma torch effect" that erodes the leading edge of the rifling. Is this a "too much powder" issue or a "rate of burn" issue? (Are slower powders easier on barrels?)
3) Does the sheer velocity of a bullet going down the barrel erode the throat or rifling?
4) Is 3025 - 3050 fps in a .284 and .30 cal. load conducive to long barrel life?
(Or is 2800 fps the limit?)

Bronc

 

[Rust]

Barrel wear, erosion, burning out the barrel, all amounts to the same thing.

Wear on a barrel is determined by:

1. The amount of powder burned (how many BTUs of heat are produced)
2. The diameter of the bore (what area in square inches is exposed to the heat)
3. The pressure of the load (higher pressure increases heat transfer)
4. The heat build up, at a match firing twenty rounds in 20 minutes gets a barrel good and hot.

So obviously a 7-08 will not wear a barrel as fast as a 7mm RUM. Along the same lines a .338 RUM will not wear a barrel out as fast as a 7mm RUM even though they burn similar amounts of powder due to the larger bore.

That said, in a match rifle maximum accuracy at a useful velocity is the primary consideration, barrel life is secondary.

 

[Broncazonk]

Muzzle Erosion

How is muzzle erosion related to throat erosion, if at all?

Bronc

 

[fdshuster]

Bronc: I've never seen any evidence of throat erosion at the muzzle, and I've done a lot of close-up inspecting with my borescope. Even the barrels that have severe heat related erosion in the throat, rifling "gone", fire-cracking to the point that there are chunks of steel ripped out of the bore surface, lands that are worn away, etc. will show no damage if you move forward a few inches, maybe 6" for the worst examples, Other than "banging" the muzzle against a hard surface, most muzzle/crown damage is the result of poor cleaning practices, cleaning rod wear/drag, brush damage, etc. One of my barrels had been fired to the point of the erosion being serious enough that the barrel was set-back, now has a total of 3800 rds. fired, and the muzzle/crown is still the original as cut when new, and remains perfect.

 

[Rust]

There is a theory floating around that hot gases are sucked back into the muzzle once the bullet exits. I've had a couple of rifle for forty years and haven't seen it.

Most muzzle damage relates to poor cleaning equipment and practices.

 

[Bob3700]

Burning out a bbl is largely related to throat erosion.

Once a bbl chambered for the likes of a 243, 6XC, or 6.5x284 gets 500 rds thu it, you can borescope the bbl and find:

Cracking in the bbl steel in the first two inches of bbl. Some call this alligator cracking because it begins to look like a dry lake bed or the irregular pattern of skin on an alligator.

The throat (tapered part of the lands) begins to wear/recede from the mouth of the case. That is why you "Chase" the lands with bullet seating as the round count down the bbl increases.

This rough area begins to build up with copper fouling that is torn from the bullet as it passes. This affects accuracy.

The actual land diameter also enlarges/tapers towards the muzzle. Chamber end of the bore will measure larger than the muzzle. If you rechamber a used bbl, you will see that the pilot that fits the muzzle is way too small in diameter for the breach end of the bbl.

Muzzle deterioration/wear comes from cleaning rods and from rounds fired. The high temp/pressure gases do begin to take the sharp edge off the crown after 1000 rds or so. That is why benchresters recrown their bbls to keep that area sharp.

So if you want the high performance cartridges like 22/250, 243, 6.5x284, then be prepared to pay the price of short bbl life. If you are a hunter, this may not be an issue. For us competition shooters (where a lot of rounds are put down bbls each year) it is an expense we have to accept.

That is one of the reasons that the 6BR line of cartridges are so popular. Good ballistic performance with long bbl life.

Keep the throat of your bbl smooth/protect the crown and you will see a greater number of accurate rounds out of it.

Bob

 

[Dgd6mm]

This is one of the rasons I shoot a 6CM, the other is it is so accurate.

 

[Broncazonk]

Advancing the Chamber - Recrowning the Muzzle

Thanks Rust, fdshuster and Bob3700 for the great info!

Questions:

1) When advancing the chamber forward in the barrel (setting back) is it standard practice to recrown the muzzle as well? (Even if the crown usually doesn't need it?)

2) Setting back requires (eats up) 1/2", 3/4", 1" of the barrel on average?

Bronc

 

[fdshuster]

Bronc: No set rules: amount/severity of erosion will depend on the cartridge, powder, # of rounds fired, barrel temperature's reached (ex: prairie dogg'in vs single shot benchrest), etc. My 6ppc with a documented 2410 rds. fired (all single shot), has visable damage/firecracking to approx. 3" in front of the chamber/ mouth of the case, so to get to the "good" undamaged bore surface, will be set-back 4", for a "new" barrel length of 20". This is as seen with my borescope, so is a true measurement, not a guess. I've also checked that crown also, and it too is damage free, so will be left alone. Gotta think of a barrel as a replaceable item, same as primers, powder & bullets, and if fact, can add the cost of replacement to the true cost of each shot that is fired. Example here: Barrel $315 + cost to install $175 = $490 divided by # of shots fired = 2410 = approx. .020 cents per shot.

 

[Bob3700]

In order to determine the amount of setback you need to bore scope the bbl. That will let you see where the firecracking is diminishing.

One other thing that the borescope won't tell you is how the lands have enlarged at that point. You need to put a gauge pin or reamer bushing into the bore to gauge the enlargement. You may be throwing good money away if the bore has gotten too big.

These factors will determine whether it is feasible to set the bbl back. Another factor is whether the gunsmith wants to risk his reamer in this task. The metal in the throat/breech end of the bbl has now been flame hardened and often will not cut like virgin steel. In rechambering some used bbls, you will see the reamer cease to create a evenly cut chip and begin to produce granulated chips. Almost like the reamer is chipping small particles of the hardened bbl away.

I can't believe that hitting this hardened steel is doing your reamer any good.

Rechambering is always a difficult decision. If you have the equipment and can do it yourself, give it a try. If you are paying someone else to do it, your money might be better spent chambering a new bbl.

Bob

 

[fdshuster]

Bob3700: Good points. As mentioned, the throat area has been "measured" with my borescope, the slightest trace being actually 2 1/2" in front of the chamber mouth, so the barrel will be set-back 4", for "good measure", and yes, the land dia. will also be checked with the pilot bushings from my spec'd. Pacific chamber reamer. All my chambers are cut with my own reamers. Had not considered the hardness of the metal in the throat area (another good point), and what if any effect it may have on the quality of the cut, so will run that by the 'smith for his thoughts. For that reason, if he sees it as a possible problem, will have the chamber cut for the new Krieger 24" first, and the 20" Shilen set-back second. I've had several other barrels set-back & rechambered, one in particular had been a 22-250 with some serious firecracking, set-back & rechambered to 22BR. 'Scoped it out with the "Hawkeye" and it is the most perfect chamber, throat & leade you will ever see--- looks like it was cut from a virgin barrel blank, and is one of my most accurate. This 6ppc re-barrel is being set-up as a switch-barrel with the 20" Shilen and a 24" Krieger, (new).