Throat Erosion - Test with Go Gauge?

[BillB204]

I have a .284 Win on a Savage short target action. I've put about 1000 rnds through it using mostly 175 SMKs with 4831sc or Reloader 22 at about 2850 fps seated on the lands. Recently I've been noticing a drop off in accuracy from about .40 MOA to about .70 MOA. I tested my seating depth and saw that I had to seat the bullets quite a bit further out to get to the lands. When I did this, my groups tightened back up to about .50 MOA. But I'm concerned that this means I've had some throat erosion. I don't have a bore light. But I tried my go gauges. The go gauge was fine and the no go gauge was a no go. Does this confirm that I do have throat erosion? And on a Savage, can I just reset (screw in) the barrel a little further to get some additional life and accuracy out of the barrel? Thanks.

 

[rcmodel]

No.

Headspace gages have nothing to do with measuring what is out in front of the chamber inside the bore.

All they can check is headspace.

rc

 

[Jim K]

"... can I just reset (screw in) the barrel a little further to get some additional life and accuracy out of the barrel?"

No. The chamber has not changed, only, as rcmodel says, what is in front of the chamber. So you would need a chambering reamer at a minimum. And setting the barrel back might be feasible but you might have to set it back a full turn and that could mess up the bedding and/or cause a gap at the foreend. If there are no iron sights you could set the barrel back less than a turn, but it might look funny to have sight screws on the bottom.

Jim

 

[Jim Watson]

Look at the gauge. Does it have a bullet diameter protrusion out into the chamber throat that could detect the erosion you have found with bullets? No.

There is nothing wrong with seating the bullet out farther to chase the erosion, as long as you can get a reasonable seat of bullet in case neck.

After that it is often done to set the barrel back and rechamber for a clean throat. But if you have let the throat advance very far you must set it back a good bit, an inch or more, not a turn. I read of one guy who was very particular about it and as soon as he could detect any erosion, would set the barrel back a quarter of an inch - still four turns - and rechamber. Most shooters wait longer and only set back a barrel once or maybe twice in its accuracy life.

 

[BillB204]

Thanks a lot for the quick response guys. I've got the receiver sitting on a Whidden V-block. So setting the barrel back shouldn't interfere with the bedding or foreend. I am able to seat the bullets out to the lands and still have enough bullet in the case to get a good seat for now. So I think I'll keep periodically measuring the seating depth, and when I get much more erosion I'll set the barrel back and rechamber. Thanks for the advise.

 

[fguffey]

“But I'm concerned that this means I've had some throat erosion” If you have concerns about throat erosions, have concerns you have muzzle erosion also, for muzzle erosion, counter boar or shorten the barrel. Throat erosion and you said “I had to seat the bullets quite a bit further out to get to the lands”, throat erosion is measured with a tapered gage, the taper measures the diameter of the bore at the beginning of the rifling, erosion means the bullet is seated further into the rifling before it makes contact.

I have no problem with making a modified case as when drilling the flash hole/primer pocket our of a case head and using it to determine the length of the chamber from the bolt face to the rifling, I have no problem using a gun friendly hammer to drive the bullet forward into the lands, then drive the bullet (back out) out of the lands to examine. Throat erosion is measured in thousandths as in 1/1000 or .001” etc.. I do not have a bore scope, with enough light I can see throat erosion, with an grinder I can make tappers, with datums I can check a taper, as I said, I can make a modified case that allows me to determine the amount of throat erosion.

If your barrel is tapered, setting it back will open the barrel channel between the channel and barrel, moving the barrel back will shorten the length of the chamber from the bolt face to the rifling. Not a suggestion, there are rifles with 3/4” ( .750” free bore), not a problem but most reloaders do not how much free bore their rifles has, some are accused with having too much time on their hands, ‘in the past’, not the same as ‘once upon a time’ before the Internet is was common to increase the length of the threat (increase free bore) to take advantage of large case capacity as with the 300 Win Mag, the 300 Win Mag has a short neck, by increasing free bore in the 300 Win Mag the shooter, reloader could move the bullet forward and increase case capacity and use longer and heavier bullets. Let us not forget, increasing the length of the case from the tip of the bullet to the case head, let us not forget, by increasing the OAL the reloader could cause problems with the length of the magazine and the OAL of the case.

Again, I am not a fan of ‘to the lands’, I am the fan of the running start, I want my bullets to have ‘the’ jump start.

F. Guffey

 

[MtnCreek]

If that’s what the rifle likes, chase the lands till you run out of good bullet/case neck contact. Considering the cost of having an inch cut from the camber and muzzle, chambering, threading and crowning, I would just replace the barrel with a new pre-fit barrel. Either way, it sounds like one heck of a good shooter!

 

[BillB204]

It is a good shooter MtnCreek. It's a 30" Benchmark; 1:9. 300-400 shots ago it wasn't unusual for me to get .25" -.35" groups. Now it's more like .45"-.55". And thinking more about it over the last couple of days, I'm inclined to agree with you that I could probably get a new barrel for only about $100 - $150 more than setting this barrel back.

 

[Jim K]

When I discussed throat erosion before on this site, I was asked in a PM what caused it and how it could be prevented. With your indulgence, I will repeat the answers here.

When a rifle is fired, the first thing that happens is that the pressure expands the thin metal at the case neck. The chamber must be of the size to allow that, or pressure builds to excess quite rapidly.

But the bullet does not move. Kept back by its own inertia, it sits there while high pressure gas and particles of burned and unburned powder rush around it. Eventually, as milliseconds go, the bullet decides to get up out of its easy chair and move into the barrel, shutting off the flow of gas past it.

But that gas rushing through a small area has acted like a cutting torch, burning and eroding the steel of the barrel.

Can throat erosion be prevented? No. Some powder/bullet combinations will reduce it, but the chamber throat has to be cut to allow the bullet to chamber easily, so that gap is going to exist.

Why does erosion cause inaccuracy? Because when the bullet does decide to enter the barrel, it finds an area larger than itself and tends to skew or try to turn sideways. That scrapes and distorts the bullet and accuracy suffers even if the bullet does straighten up and fly right when it hits the rifling.

How long will a barrel be good for? There is no definite answer because it depends on the load, powder, bullet, barrel metal, etc. Certainly using a Stellite liner (as machineguns often do) would allow a barrel to remain accurate for most shooters' lifetimes, but it would also be very expensive and cost more than several ordinary barrels with installation. It is probably safe to say that with most rifles in most calibers, with factory ammunition, a barrel is good for at least 1000 rounds, probably close to 2000.

Since that is 50-100 boxes of factory ammo, most barrels will be good for the lifetime of the owner. Those who have the time and money to shoot a lot will, like BillB, find that accuracy suffers. But for an average hunter, even a barrel with considerable erosion will shoot well enough for the purpose.

FWIW, there is no such thing as muzzle erosion. Muzzle wear comes from rust caused by water entering the barrel or from wear caused by a cleaning rod. The latter is a major problem with rifles like the military M1 and M14 rifles which were cleaned frequently and must be cleaned from the muzzle.

Jim

 

[BillB204]

Thanks Jim. Great description. Something else that hit me while I was thinking about continuing to "chase the lands" is that each time I move the bullet seating out, there is more case space. Please correct me if I'm wrong, but it seems that there would then be less internal case pressure and thus less muzzle velocity. So each time I move the seating out, I'll have to move my powder load up.

Also, to correct my last post, I just checked with my gunsmith on the cost of setting the barrel back and he quoted me $100. Being that I paid almost $500 for the barrel, I think I'll have it set back a time or two before I buy another barrel.

 

[fguffey]

"‘in the past’, not the same as ‘once upon a time’ before the Internet is was common to increase the length of the threat (increase free bore) to take advantage of large case capacity as with the 300 Win Mag, the 300 Win Mag has a short neck, by increasing free bore in the 300 Win Mag the shooter, reloader could move the bullet forward and increase case capacity and use longer and heavier bullets. Let us not forget, increasing the length of the case from the tip of the bullet to the case head, let us not forget, by increasing the OAL the reloader could cause problems with the length of the magazine and the OAL of the case"

As to moving the bullet out and keeping up with two thoughts at a time, moving the bullet out could lower pressure if you do not add powder, moving the bullet out into the lands can cause an increase in pressure.

Again, I am the fan of the running start, I want my bullets to have jump before hitting the lands.

F. Guffey