Rifling Deterioration?

WapitiBob said:

This is a 7stw using N570 and Berger 195's.
Knowing the powder eats barrels, I measured the throat at 80 shots, then measured again at 432. I burned away .100" of the throat in those last 350 shots, never firing more than 2 then using the chamber chiller. I rarely shoot more than 15 shots in a range session.
At 432 I set the 195's out to the new lands ( 3.275 CBTO ) then pulled back .020". With the extended throat I finally got the boat tail above the case neck/shoulder jct, then I re shot my powder charges to see how bad it was going to be.

This rife has never produced the 3 shot groups seen at 81.5 and 82, which have been replicated. So far, it seems the bullet placement in the case has more influence on this rifles groups than fire cracking and throat erosion.

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highfinblue said:

I know it's apples and oranges, but my 28 Nosler loved 81.5 grains of N570 and Berger 195's. Don't know much about an STW, but I find that kind of odd. No other load comes close in my 28, 0.020 off lands. I'm at around 200 on the round count and it really is a tack driver. I only use it to hunt, so hopefully it will last a while. I have lots of littles and a 7 PRC to shoot often.

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Greyfox said:

While the rifling in the photo looks pretty rough, my guess it's likely not that much different then it was before you experienced a loss in accuracy. The section of the barrel that you should be looking with your bore scope is the erosion at the throat, and/or a carbon ring that forms where the free-bore and the rifling begins. It is the heat(fire-cracking), and erosion of the powder burn/bullet entry into the land that causes this wear which migrates with time. Accuracy loss begins with the change in bullet seating depth to the lands. The military has specs for the the amount of rifling erosion permissible at the throat before replacement (ie, .010" throat erosion/1MOA accuracy loss). The other area of barrel wear that can cause an abrupt change in accuracy is at the crown(entry point at the end of the barrel). It can usually be seen with the naked eye. It should be even and square. This damage can be caused by physical contact that damages the crown and/or improper cleaning methods.
What may be cause an accuracy change over time in the main section of the barrel, and observed with a bore-scope, is carbon(carbon slick in the lands), and/or copper build up…..easily seen and can often be removed by cleaning.
When I was competing I would have to monitor barrel wear, replacing barrels as much as 1-2x per season. Barrels were subjected to high heat in the warm summer months and accuracy/precision demands were pretty stringent.
This cutaway is an example of a shot out barrel. First place to bore-scope the barrel. It has much more than .010" of throat erosion/rifling loss which is about when you should start checking accuracy/precision and about a new barrel…..Or perhaps decreasing the seating depth of your bullet to get a little extra accurate barrel life……IMO
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mray190 said:

I was cleaning one of my AR-15's tonight along with a couple others. This one in particular I've had for some time and have been having trouble getting a load to work. The original load consisting of 24 grains of Varget over a 77gr HPBT just seemed to stop shooting well after years of sub moa performance. I contributed it to mounting up a suppressor and the change in barrel harmonics. After some load development I found a node that seemed accurate. However after loading 10 of two charge weights I got some really wild ES's with one being 100fps. So tonight after cleaning it I got my borescope and was kind of puzzled by what I saw. The edges of the rifling were nearly impossible to see vs another one that I cleaned. This rifle had been a shooter and my go to for coyotes for years. It's a Larue Stealth that I purchased as a complete upper and stripped lower. Last photo is of the second AR, both 223's.View attachment 627892View attachment 627893View attachment 627894

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