Theoretical Evidence of Throat Erosion

[Howard Roark]

With the topic of how long will XYZ caliber barrels last I recently checked a couple of barrels for throat erosion.

First, my .223 AR match rifle started loosing accuracy at the 300 and 600 yard lines. After shooting an 8 on the 600 yard NRA MR target I decided to check things out. The barrel was a SS Douglas with about 3300 rounds down it. The throat had eroded .138 thou. Time for a new barrel. I rebarreled with a new Hart and have not shot the 600 yard line yet but on a 600 yard target reduced to 300 yards it shot a 199-12x. I'll call it fixed.

Second, I checked the throat on my .260. After 1000 rounds it has eroded .007 thou. 500 of these rounds were shot with VV N-160 and after finding that IMR 4350 shoots much better groups I switched. The erosion most likely will accelerate due to 4350 being a bit of a hot burning powder. The increase in accuracy is worth it.

Edited to add:
I didn't post this as wonderment of erosion, but to put hard numbers on real world testing.

 

[GRIZ22]

A friend who shoots service rifle, high power rifle, etc and used to place regularly at Camp Perry told me he replaces barrels every 3000 rds on ARs. He said after that they're no good for shooting matches. Don't know what barrel, powder or load he uses but I guess your AR barrel wear is not unusual.

 

[nbkky71]

Most HP shooters will rebarrel when the rifle won't hold the X-ring any longer and that is generally around 3,000-4,000 rounds. That number will vary among barrel manufacturers.

However, it doesn't mean that the barrel is 'no good' any longer. Often times a such barrels are perfect for shooting reduced courses.

I'm shooting a Wilson 1-7 barrel in my AR15 service rifle and have logged 2,700 rounds so far. I haven't measured the throat yet, but I shot a 190-9X on an MR63 (reduced) at the last HP match. She should do fine for the EIC match at Camp Butner in a few weeks.

 

[granite]

"...it has eroded .007 thou...."

How do you measure erosion of less than a micron?

Is there some optical interferometer used?

 

[lipadj46]

"...it has eroded .007 thou...."

How do you measure erosion of less than a micron?

I have a micrometer that reads to the 1/10th a micrometer with a vernier scale. I believe it is from the 50's so I am guessing there are gauges now that do that.

 

[granite]

You can get a micrometer with a one micron resolution .

But this is not measuring a piece of shimstock.

How would you use a micrometer inside the bore?

 

[lipadj46]

How would you use a micrometer inside the bore?

A micron caliper I would suppose. Or we could suppose the OP made a typo and really meant 0.007" not 0.000007"

 

[Howard Roark]

.7 seven tenths
.07 seven hundredths
.007 seven thousands
.0007 seven ten thousands
.00007 seven hundred thousandths
.000007 seven millionths of an inch

No interferometers or green light waves needed. Just the ability to read a regular old vernier micrometer.

Uhhh...I don't understand, .007 THOUSANDTHS

 

[jerkface11]

We're supposed to believe that 7 millionths of an inch is enough erosion to to be detrimental to accuracy?