Help Understanding Throat Erosion

I'm not completely ignorant. I understand that heat and pressure from normal firing will erode every throat. Rapid firing, hot loads, inadequate cool-down all increase the speed of erosion. This erosion causes excessive leade, wears (and as it progresses may chip) rifling, and contributes to accuracy degradation.

I have a remmy 5R that only has about 500 rounds through it. It would have more if I hadn't had problems after the gun had some work done (re-cut chamber, action blueprint, trigger work, bolt lapped, etc.) The gun was unfired prior to this work. The issues were corrected by the original shop and it shoots decent now.

The problem is, when it was in for the checkup a bore-scope revealed "slight firecracking". Truth be told, I was getting 16" groups with this gun at 100 yards on initial sight in. There were times where I was pissed and may not have allowed much of a cooling period. But I wasn't shooting more than 5 rounds at a time before walking down range so I cant imagine it getting too ungodly hot.

In any event I literally have 476 rounds through the gun. All have been factory ammo. Maybe 100 rounds were shot during the frustration phase where I may have fired 5 rounds faster than I should have.

I looked at the fire-cracking and it looked more like tool marks to me but I didn't really see the firecracking much if at all. I'm a noob with all this bore-scope stuff so my opinion on what I saw is worthless.

Is slight firecracking normal for this round count in a 308?

Does this mean my barrel will wear more rapidly from here?

Who did the initial work on the barrel and what did they say was causing 16" groups at 100 yards? Why didn't you just send it back to Remington? It may have had a crap barrel from the start. I don't know of any 5R's that don't do .75 moa right out of the box. 500 rounds should show very minimal barrel wear. I shoot 5 shots in fairly quick succession from my .308 without worry. Now if it was 20 or more then I would worry.

Good questions Russ.

tss1004 said:

....I have a remmy 5R that only has about 500 rounds through it...

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That's about a two good weekends and an inconsequential round count in the life of a .308 that hadn't been machine gunned.

It would have more [erosion] if I hadn't had problems after the gun had some work done (re-cut chamber, action blueprint, trigger work, bolt lapped, etc.) The gun was unfired prior to this work. The issues were corrected by the original shop and it shoots decent now.

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"The issues...". What issues? The issues described below, or other issues prior to the existing problem?

The problem is, when it was in for the checkup a bore-scope revealed "slight firecracking".

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This is not surprising, I have seen firecracking in less than 100 rounds with calibers like the 6.5-284 and .300 WinMag.

FWIW, those unfamiliar with a bore scope will typically scare hell out of themselves looking into rifle bores. About the only ones that don't actually look BAD straight from the maker are custom barrels like Krieger, Broughton, Bartlein, Lilja, Schnieder and another half dozen excellent barrel makers. I have yet to look into a Remington, Winchester, Savage and the other main makers and see a bore free of rotary tool marks, gouges running parallel to the lands and throats that are straight enough not to look crooked.

Truth be told, I was getting 16" groups with this gun at 100 yards on initial sight in.

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This is not a firecracking issue. This is something drastic. Is this still a problem?

There were times where I was pissed and may not have allowed much of a cooling period. But I wasn't shooting more than 5 rounds at a time before walking down range so I cant imagine it getting too ungodly hot.

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Rapid fire of an occasional magazine is not quite "normal wear", but nor is it outright "abuse", but this approach will clearly take you in the wrong direction for best performance.

In any event I literally have 476 rounds through the gun. All have been factory ammo. Maybe 100 rounds were shot during the frustration phase where I may have fired 5 rounds faster than I should have.

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Again, the overall round count is a non-problem. Rapid fire in an abusive way (more than occasional strings in practice or matches) is, of course, a good way to not improve things and does contribute inordinately to wear.

I looked at the fire-cracking and it looked more like tool marks to me but I didn't really see the firecracking much if at all.

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You're probably seeing both, and both probably look worse to you than they really are. I am constantly amazed at how bad factory barrel can look inside and can still shoot OK.

I'm a noob with all this bore-scope stuff so my opinion on what I saw is worthless.

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Not worthless, but certainly of limited value. See above. Otherwise, it is probably almost impossible for any of us to tell if a barrel won't shoot by how (bad) it looks. I have Marc357's old Krieger .300 WinMag barrel in my shop and it has over 5,000 rounds through it. Rifling is difficult to find, and ugly when you get there. It was originally extremely accurate, and was still shooting right around 1 moa all the way up to the bitter end. Go figure.

Sometimes you can see the problem, especially if the chamber is cut incorrectly. I often see rifles with rifling that starts very short on one side and very long on the other, indicating incorrect chamber reamer alignment (see Gordy Gritters for the correct way to chamber a precision rifle barrel). Rifles chambered incorrectly like this typically have limited potential.

Is slight firecracking normal for this round count in a 308?

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IMHO: Yup.

Does this mean my barrel will wear more rapidly from here?

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Of course, even if you don't machine gun it. Wear is the nature of the beast, and normal to expect.

The good news is that if you handle it sensibly, you will probably get quite a few thousand rounds through it before accuracy goes south.

Which leads us to the real question of whether it shoots OK.

Does it?

If so, stop messing with it and go shoot. If there was nothing wrong with the original barrel and the gunsmith did a decent job at everything, the rifle should shoot less than 1/2 moa.

If it won't shoot within 1/2 to 3/4 moa with Federal Gold Medal 168s or 175s (the average off the shelf performance in my own experience with folks bringing them to class), there is something wrong with it and it needs to go back to whomever did the work for you so they can fix it.

Are you sooting the rifle off a bench rest?
I allways sight in and test accuracy from a bench rest that holds rifle steady opposed to shooting off sand bags, bi pod, ect.

The rifle was fired from a bench mostly but also from a bipod with a rear bag from prone. It had nothing to do with my ability or my setup or shooting style. I'm not an expert and I'm certainly not on par with Mr. Shell, but I'm not a novice either.

The issues were headspace issues and bolt issues that included slight binding from the front action screw as well as boltface contact with the barrel. The 16" groups were resolved. The rifle now shoots. 75-1 moa. To be honest I expected better but I also haven't had the time to develop a load for this rifle. The sighters were factory and I tried everything from 150gr federal fmjbt to 168gr smk. The brass was all reloaded with 41gr RL15 under a 175smk and federal match primers to a coal of 2.8

I don't want to name the smith. He made it right and by all accounts is a decent guy who knows what he is doing and this was a fluke. I couldn't send it back to Remington because it had been modified prior to shooting and the problem surfaced after the mods.

I have other heavy barrelled bolt action 308's that I'm very pleased with. Some remingtons and some "salvages". I had such high hopes for this 5r. But ive learned so much more BECAUSE of the problems that it hasn't been that bad. My biggest concern was does a barrel wear appreciably faster once "slight firecracking" is noted? Mostly because I want to know if I would be wasting my ti.e with a factory barrel or if I should just have the damn thing rebarrelled.

Thanks for the answers. I think I'm going to continue with load development on it and see if I can get it to shoot as well as my salvage.

So a smith basically trashed it and now it isn't living up to your expectations? Hardly Remington's fault.

Russ D said:

So a smith basically trashed it and now it isn't living up to your expectations? Hardly Remington's fault.

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What the **** are you talking about Russ? I didn't start a thread to bash anything. Not the rifle. Not the smith. None of it. A simple question about throat erosion. How the **** did it turn into me blaming remington for anything?

I'm pretty sure I also said I haven't played with finding the right load for the rifle yet. Therefore I'm admitting that I haven't done my part to see the rifle perform at it's full potential. But yes, I did expect better from a rifle that was supposed to have some accurizing. But never did I suggest that Remington did anything wrong. Hell, I wasn't even mad about the smith.

You've got me really steamed Russ. I didn't say anything was Remingtons fault at any point. I happen to own several dozen remington 700's ALL OF WHICH SHOOT BETTER than this particular specimen. Get off your ****ing high horse.

I told my ex... oh wait a min different meaning of this thread :-)

"firecracking"?? I have never heard this term before. Justin, hate to hijack a thread but what is this condition?

I guess I should have read the way that was going to sound before I posted it. My point was really that the smith who messed it up should be the one trying to make the rifle what it should be. 5r's should shoot very well, if it doesn't do that now then the responsibility for making that happen should't fall on you or Remington. Sorry for the way it came out I sometimes post shorter responses then I should when I'm using my phone with the bad N key.

BTW barrels are consumables like ammo. Even if it did need a new barrel prematurely you wouldn't be wasting your time by getting everything you can out of this one.

E.Shell said:

I have Marc357's old Krieger .300 WinMag barrel in my shop and it has over 5,000 rounds through it. Rifling is difficult to find, and ugly when you get there. It was originally extremely accurate, and was still shooting right around 1 moa all the way up to the bitter end. Go figure.

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Marc's .300WM is finally dead? That thing just went on and on. Well, at over 5,000 rounds from a magnum, he certainly got his money out of it! Guess that why he wasn't at the last Mayberry match.

TSS1004 -
stick with the rifle thru some more load development. Chances are good that it will end up shooting for you. Don't worry about that throat - it's not likely the problem right now. If the rifle doesn't shoot for you, a rebarrel with a good smith and a premium barrel ain't that expensive. You might also want to get the action blue-printed and trued. A good truing job can help a lot.

Best of luck!

mikec said:

"firecracking"?? I have never heard this term before. Justin, hate to hijack a thread but what is this condition?

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This might answer your question.
http://forum.accurateshooter.com/index.php?topic=3329507.0

Dave, thanks.

mikec said:

"firecracking"?? I have never heard this term before. Justin, hate to hijack a thread but what is this condition?

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I've been doing so much reading and listening of late, on subjects related to accuracy. Everything from barrel variance due to bulges of a thousandth of an inch caused by lubricant in the bore to well, the info out there is mind boggling really. There is so much that can affect accuracy the endeavor to figure it all out is daunting and consuming. There really is so much out there it almost takes the fun out of shooting because accuracy has so many variables most of us don't really think of.

Fortunately "Firecracking" is nearly synonymous with throat erosion and is the most straighforward of variables. When a round is fired the metal heats, expands, then contracts. This will obviously fatigue the metal itself. The result is the metal becomes brittle and hardened. When viewed through a bore scope or a barrel that's been sectioned the condition of "firecracking" has been described as "alligator skin" or looking like a "dry lakebed". It's mottled, and flaky. This condition worsens in ALL rifles the more they are fired. Certainly, the more rapidly it is fired the faster the condition is worsened, but it does happen to all bores (this is also affected by the burn rate of the propellant, the pressure of the load, the mettallurgy of the barrel steel, cleaning methods, hell you name it, back to the daunting task of really "understanding" these variables).

I have attached two pics. Neither are mine. One is actually from E. Shell on a barrel he sectioned that shows throat erosion and firecracking and the other shows the tool marks on a virgin bore. When you look through a bore scope though, at least the one used on my rifle, it's actually quite dark. The scope doesn't "see" much and the image is illuminated with a scant, tiny, yellow old-school flashlight bulb. To someone like me who doesn't look through a bore scope very often, it's sorta like finding a 3mm tumor on an MRI. Difficult. But the right Doctor, or in this case, gunsmith can see it way better than I can.

As shooting continues this throat erosion starts to increase the length of the throat (moves the smooth portion of freebore farther forward along with the engagement surface of the rifling and in some cases, pitting and deforming the shape of the rifling itself). Because rifles tend to shoot the most accurately when the chambered bullet barely touches the rifling, or arguably a thousandth or two from the rifling, this condition is detrimental to accuracy. It also decreases accuracy due to the irregularities in the profile of the engagement area of the rifling. In addition, this throat erosion can become so abrasive that it actually tears the bullet jacket as the round is fired, which copper fouls the bore, and destabilizes the bullet. Again, daunting.

Shooters will sometimes lap the throat to smooth it out. They will also "chase the lands" and handload their ammo so that the bullet once again touches or "almost-touches" the rifling, which nets increasingly longer cartridge overall length. Some guns shoot great with a ton of throat erosion. Some guns give up the ghost once throat erosion is seemingly minimal.

My question was geared towards the "firecracking" being noted at such an early stage AND if that factor indicated that throat erosion will now occur exponentially faster than if "firecracking" was not readily apparent. I knew I was probably splitting hairs, but I thought and still think it's a good question.

Sorta like, we all know that our brake pads are going to wear out the rotors on our vehicles, and this increases the more we drive. But when we go through the pad and down to the metal backing on said pad, we will wear through the rotor exponentially faster. So is this firecracking just the normal wear of the pad, or am I down to the steel backing? Not sure if this is the best analogy, but it's all I could come up with. I'm also NOT an expert on this stuff and this was my best attempt to share what little I do know.

Russ D said:

I guess I should have read the way that was going to sound before I posted it. My point was really that the smith who messed it up should be the one trying to make the rifle what it should be. 5r's should shoot very well, if it doesn't do that now then the responsibility for making that happen should't fall on you or Remington. Sorry for the way it came out I sometimes post shorter responses then I should when I'm using my phone with the bad N key.

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I'm glad you put it that way, and to be honest, this is what I would expect from you. I accept your apology and appreciate your opinion. I'm not really upset with anyone over it. I'm sure group size will shrink when I work up a good load for this rifle. None of this has been for nothing. The education has been worth it. The smith is by all accounts a reputable guy. I may have just had some bad luck and caught him at a bad time.

Russ D said:

BTW barrels are consumables like ammo. Even if it did need a new barrel prematurely you wouldn't be wasting your time by getting everything you can out of this one.

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Yeah, that's sorta where I've been leaning. I really love everything about this rifle except it's past if you can understand that reasoning. Even though the inconsistencies are relatively ironed out, it's hard for me to "trust" it. I mean, 1 MOA is okay, but when I had a few hundred rounds netting 16+ inch groups, it really soured the fuzzy feelings for me.

I will keep this tube, try to wring out as much accuracy as I can, and go from there. As much as I shoot, I'll need a barrel next year if this becomes my go-to .308 bolt gun.

The funny thing about accuracy is that sometimes you just have to forget about all of the variables and just shoot. A good example is handloads. You can spend hours and hours on case prep and never really know if you are getting any returns on your time investment. Sometimes instead of expecting gains you are merely eliminating variables. I shoot a bone stock SPS tac to .5 moa with generic handloads using Lee equipment and full length resized FC brass. I'm not saying it's the best, just that sometimes you gotta shoot it and see. Back to your rifle, I remember a passage in the Tsao Te Chung about how the worst student is the best. Basically because you learn the most through struggle. This seems like a good example if you are looking for a bright side.

sbarber7973 said:

I told my ex... oh wait a min different meaning of this thread :-)

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I was thinking the same thing.






I was also afraid Clandestine would already be in the thread explaining exactly how throat erosion works lol.

r3t1awr3yd said:

I was thinking the same thing.



I was also afraid Clandestine would already be in the thread explaining exactly how throat erosion works lol.

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At least we know were not the only sick focks around here lol