Throat cracking question

[Keith Glasscock]

I have a total of zero experience and have to ask.

I recently had a 3 groove barrel stop shooting prematurely (1800 rounds in a 308 win). I spun it off and gave her a look to see if it would be worth setting back. I do not have a borescope so I use a magnifying glass and a light from the muzzle. With that I can see the leading edges of the lands are worn down and there are some lengthwise cracks in the throat. A few of the cracks go through the ledge from the freebore diameter to the groove diameter. There also appears to be a small chunk missing from the corner of one land. Considering the amount of magnification I am using, that missing piece is about .001-.002" wide and .060" long. I'm going to run the barrel down to my smith to look at when he fits the new barrel. I'll let him decide if the old one is worth setting back.

The throat wear isn't as bad as I thought it would be. The bullet touch point had only moved up .010" from new, but the chamber cast is telling me that the errosion of the leading edges of the lands is not even. Two lands show a significant amount of errosion (.125") and the third shows almost none.

Would I have a 3 groove barrel again - from this manufacturer? You bet! It was the best shooting barrel I've ever had, and even if the life is shortened by its love for fast, hot loads, I would do it again!

Now the question:
Does fire cracking normally run lengthwise of the barrel? I've seen pictures of cross-wise cracking, but none this direction.

Thanks,
Keith

 

[fdshuster]

Keith: Very interesting topic. I do have a Hawkeye and have been using it ( a lot) for almost 5 years now, getting up close and personal with a lot of chamberings.

1800 rounds with a 308 is not a very high count. Years ago Kevin Thomas did an exhaustive, controlled & documented test ( article in "Precision Shooting") comparing 4 or 5, 308 chambered barrels. They all began to have accuacy issues at around 3800, by 4000 they were ready to be pulled off & replaced.

Yes, longitudinal ( front to rear) firecracking is normal. I've seen a lot, especially beginning at the mouth of the chamber, directly in front of the case mouth. At the beginning they look like cuts made with a razor, gradually grow longer, and the alligator skin patterns begin to form, very faintly at first, then becomming more pronounced.

When the alligator cracking gets really bad, small chunks of steel will begin ripping out of the surface of the bore, and these "holes" begin to fill with copper jacket material, making copper fouling removal more of a chore.

I watched it happen, from beginning to end with 2, 243 Winchester custom barrels, and it was not pretty to watch. They both started to have accuracy issues at around 800 rounds, ( I keep a written record of every round fired), and at 1300 to 1400 rounds neither would hold 5 shots in 1 moa.

As far as firecracking perpendicular to the lands: no, I've never seen that. Reamer & tool marks run across the lands & grooves, but the firecracking that I've seen is either parallel with the lands and/or the alligator skin.

For the difference between the start of 2 lands, compared to the third being at a different starting point, I've seen that with new chambers from the day they were cut. I find it to be really common with a lot of mass produced factory barrels.

At the other extreme, I've scoped my newly chambered barrels, done by my 'smith, using my new PTG reamers & the throat & leade is square, parallel and true. They look like an engineers drawing.

Just recently found an ugly front to rear crack in one of the grooves on one of my custom barrels, chambered in 6 BRX. No alligator type damage ( 1035 rds. fired), located in the leade. The rest of that area looks as new, so I have to write it off as a defect in the barrel steel. I have plans to setback the heavy contour, straight taper 26" barrel & have the chamber re-cut with my reamer, to put me in front of the damage.

Just what little I've learned, thanks to having the borescope, one of the most valuable ( to me) purchases I've ever made. Keep in touch & let us know what you learn.

 

[RottenII]

FDShuster...interesting write up. Thanks for passing it along.

 

[Keith Glasscock]

Thanks fdshuster,

I am disappointed that it went away when it did. I had expected it to do the wear the throat out thing. The fact that it won't shoot a string without going silly might have a lot to do with the fact that the barrel is used for F-class. Between the warm loads it has endured, the high rate of fire, and the resultant heat, it has been there and back more times than I can count. I suspect that the smith may try setting it back an inch or more. If it shoots half as well as it did when it was younger, I'll be tickled pink.

The funny thing about this barrel is that it has become heat sensitive to an extreme. If I shoot a 5 shot string and let it cool completely, it returns sub 1/2MOA verticals. By the 8th shot, it is opening up, and by 12, it is throwing a high or low flyer. That would be fine if I had an unlimited amount of time, but 22 shots in 22 minutes doesn't work.

I did have a friend borescope the barrel 1200 rounds ago. He also allowed me to borescope it. At that time, the throat was dead even and as close to perfect as we could tell. It had been done well. I think that the asymetric wear might be an illusion. I'll know for certain when the barrel gets to the smith. Until then, all I know is what little I can see from the chamber end.

 

[lpreddick]

using my new hawkeye, i saw rear to front(longitudinal) grooves(gouges) in the lands of a 22 br that was previously a 22-250 that had been shot a LOT. didn't set back the barrel enough. so, i had 4 in lopped off and rechambered in 22 br and the lands are now sharp and crisp and the gun shoots 1/4 moa or better. if i didn't know about the gouges i would have spent a lot of time adjusting loads to no avail. i love my borscope!