Throat erosion

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jamcam1999

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I am wondering if any of you knowledgeable people have a picture or can tell me what throat erosion looks like in a rifle. My tired old eyes are obviously not good enough to see it and I would really like to know just what to look for. Obviously it can cause lower performance or accuracy but my shooting skills can also do that for me. Thanks in advance.
 
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You generally can't see it without a borescope, and if you can see it without a borescope, we're long past the functional life of the barrel. Essentially it's the forward progression of the leade. Another way of putting it would be the forward expansion of the freebore, the cylindrical section of the chamber between the mouth and the start of the leade. Some chamberings have lots of freebore, some have little to none. It's easiest to diagnose when you have a new barrel cut with the exact same chamber reamer to compare against.
 
OP, throat erosion that will effect accuracy is fairly easy to see.

As mentioned by Blast, if you can't see it, it likely isn't an issue, especially for a hunting rifle.

If you're a match shooter, you likely load your own cartridges and depending on the type of rifle you shoot, load the bullets out far enough to make contact with the leade. Match loads are usually HOT and accuracy will start to decline at anywhere from 1000 to 1500 rounds. I've had match rifles become unacceptable for match shooting with less than 1000 rounds through them. The barrels were still more than adequate for hunting rifles.

Most centerfire rifles give their best accuracy over the first 3500 rounds, depending on how hot the loads are. Then the accuracy slowly declines over the next 1500 to 3000 rounds. Some do better and some do worse, depending on a lot of different things.

You don't say why you're thinking the throat in your rifle is worn.

Most often it isn't a problem with erosion, caused by the flames of burning powder, but a problem with a build up of jacket fouling further down the barrel.
 
shotoutbarrel1.jpg


LdNFMvM.jpg


for reference
 
sfp said:
Great pic. Thanks
Click to expand...

Usually there is .050" from the leade to the rifling (Depending on SAAMI spec I know just trying to simplify). As the throat erodes, the lands are what erode in the barrel. So beyond a certain point, the bullet, when fired is unstable (wobbles) down the barrel until it can be engaged (and engraved ) by the rifling.

Think of rifling as a train on a track. The train will follow rifling precisely centered axis to the bore as dictated by the rifling lands. As the throat erodes, the bullet will oscillate in the barrel until it engages the rifling. At which point the lands will be the tracks that keep the bullet moving concentric to the bore.

BUT the issue with erosion is the bullet 'wobble' (actually a helical moment of movement) prevents the bullet from being engraved in perfect 0 degree alignment with the bore... put another way, the engraving is not centered along the bullet axis.

So when the bullet is finally free of the rifling and leaves the muzzle it will wobble as the spin is 'off center' of the axis of the bullet.

If that makes any sense.
 
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Hence why, when one reloads, you do not want to 'JAM' the bullet into the rifling, a dangerous overpressure situation can be the result. Seat too deep, and you induce wobble, seat too long and you have a jam. The careful reloader will figure out the correct jump from LEADE to full rifling, and set his COAL to that spec....

BUT after 5-800 rounds... the leade has moved forward, as well as the rifling is now a nice smooth ramp... but further down... so you seat out another few tenths of a thou to ''catch up'' to the rifling to reduce wobble... wash rinse repeat...

After enough time the leade is completely gone, and all you have left is rifling that starts from 0< to full rifling in a gentle slope, but if you can't seat the bullet out long enough to 'catch' the lands, you have 2 choices. Choice #1 is to have a gunsmith cut back the existing barrel 2-2.5" re thread, and rechamber AKA Set BACK the Barrel. Trouble with this is it is labor intensive, and the muzzle is now an issue as you will see...

Choice #2 is new barrel fitment.

With the current crop and cost of replacement barrels VS gunsmithing time, it is actually better to replace an old barrel with a new one.

Some might scoff at this but food for thought is that the last 4" of the barrel at the muzzle end can be ALMOST as eroded as the first 2" of chamber... if the gun has been shot with an eroded throat liong enough the wobble tends to wreck the rifling at the end of the muzzle and slightly oval the last 1" of the muzzle.

Keyholing, poor accuracy, inconsistent velocity, etc etc.

Just my opinion though.
 
TangoKilo said:
Great explanation Gotjuice!!
Click to expand...

Thank you. There is more to it, but that is the simplest breakdown, and relatable that I could come up with. Pics are avail on the net where I got them. SAAMI and CIP specs are available for all commercial and some specialized calibers. There they fo in depth about leade length, dwell, and enough technical jargon to be as useful as not, but it IS relevant to reloaders, and enthusiasts. Gives you a better overall understanding about what goes on and why things are set up the way they are.


Here is some SAAMI stuff.

https:// saami.org/wp-content/uploads/2019/02/ANSI-SAAMI-Z299.4-CFR-Approved-2015-12-14-Posting-Copy.pdf
 
ChewyChewbacca said:
Right where the silver band is ahead of the fire checking, is that the throat and the lands start just ahead of that.
Thanks for posting the picture. I always wondered what it looked like.
Click to expand...

The Leade was where that band resides. Depending on the cartridge the barrel was chambered for it could be from .030-.076. Go No Go will be dependant on how tight your chamber is compared to SAAMI standards. I have 1 204 Ruger that is so damn tight, It gets it's own settings for dies and absolutely cannot be used in another firearm, and then reused/resized to the tight one.

One fine day, I will purchase a piloted finish reamer and bring the chamber to a tolerable spec. This one rifle is how I got my start in reading up on this and applying it in the field.
 
ChewyChewbacca said:
Right where the silver band is ahead of the fire checking, is that the throat and the rifling starts just ahead of that?
Thanks for posting the picture. I always wondered what it looked like.
Click to expand...

Look at the picture VERY carefully. See where the cut of the barrel is... the straight lines, look real close and you can see 'stairsteps'... those steps are where the cutting tool cut through the rifling along the bore axis. Allowing for a secant ogive bullet, take a good look.... there are NO stairsteps along the bore for 1.25" from the case mouth. This barrels is pretty much shot out. You can also see the coppering left from engraving... see how far that is from the neck? That is how far the erosion has gone. Almost 2 inches!!!.... nope, can't chase the lands any more......
 
Thanks Got Juice? appreciate that. Now I understand what guys have been talking about when they say that they have a barrel cut to certain dimensions and what a shot out barrel looks like.
So if I shoot a 100 grain Nosler partition in my 6.5x55 that I have to seat a 140 Accubond long to get the right jump for accuracy it would be like shooting it out of a burnt out barrel. It has to go so far before it is engraved in the rifling that it won’t be centered in the barrel anymore.
 
ChewyChewbacca said:
Thanks Got Juice? appreciate that. Now I understand what guys have been talking about when they say that they have a barrel cut to certain dimensions and what a shot out barrel looks like.
So if I shoot a 100 grain Nosler partition in my 6.5x55 that I have to seat a 140 Accubond long to get the right jump for accuracy it would be like shooting it out of a burnt out barrel. It has to go so far before it is engraved in the rifling that it won’t be centered in the barrel anymore.
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That is a fair assessment. Depending on where the ogive starts to engage the rifling and how far it travels to get to that point. Typically the lower projectile weight is a shorter bullet, and there is a limit to how long you can load whilst maintaining adequate neck tension for gripping the projectile. Conversely the heavier the projectile, you have more latitude in loading Cartridge Overall Length (COAL) In my 6.5 I have trouble getting close to the LEADE with adequate neck grip with 100gn projectiles. 120's are my minimum with the chamber as cut. 130=143's are easy, and 147's are too heavy for the Rate Of Twist. So once my 120 loads start to fall off in accuracy, I can chase lands up to heavier bullets to compensate, regain accuracy and keep shooting. To a point. Flame cutting, erosion, lands dulling will of course dictate how much life there will be left. Supposing I get 1500 rounds of stellar accuracy with 120gn loads, I doubt very much by moving to 130's I would get more than 4-600 rounds of stellar accuracy and then at 140's possibly another 3-400 rounds of stellar accuracy. After that point, the barrel will be completely shot out. Of course I am making a couple of educated guesses on the barrel life so I might be off a couple hundred rounds, but in the end, barrels are a consumable when shooting a fair volume.

Just my .02
 
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