UPDATE:
First, here’s a bit more info:
My throat erosion calculations are based on measurements using the Stoney point OAL gauge with a modified case to fit. I also have the Hornady version of the OAL gauge with the nylon/plastic bullet pusher. I use both to confirm my numbers and they are always within one thousandth of each other. Yes I’m anal, but I haven’t met an F-Class shooter who isn’t.
Original CBTO with 200.20X and new barrel from 2017: 2.360
CBTO with 200.20X bullet this week: 2.375 (bullet lot from 2020)
OK, today I’ve been searching this and other forums for more opinions and one experienced shooter said something that hit me: “make sure when doing throat erosion measurements you use the same bullet (or at least a bullet from the same lot) and use the same modified case.“ Also, Ned posted in a thread last year: “different Lots of bullets can give different readings“ and he touched on this above when he said he uses the same set of ten bullets for his measurements. Also, Medic505 mentioned the use of bullets from the same lot.
This got me going. I searched my bench drawers and found a 200.20X bullet from the lot I used in the original measurement in 2017. I had set it aside cause the meplat was not uniform when I did the pointing. USING THIS BULLET MY CBTO reads 2.366 !! Clearly in line with the numbers that several guys posted above and in the vicinity of .001 per hundred rounds.
With Berger’s reputation for uniform bullet dimensions, I had assumed uniformity between lots. Wrong!
MY LESSON LEARNED: when doing throat erosion measurements USE THE SAME BULLET or BULLET FROM THE SAME LOT. Ned
I suggest when you do your initial throat measurement put that bullet I an envelope and mark it with date and use that exact bullet for future erosion calculations.
Thanks for your valuable comments. This forum is Tops.
I also measure distance to "touching" using the Stoney Point tool (Hornady OAL Gauge). I select 10 bullets at random from a Lot of bullets and generally hang on to that measurement set for the life of that Lot of bullets, and usually for the life of the barrel. I measure using all 10 bullets and take the average. I do it that way to minimize the impact of variance in my measurement technique, bullet dimensions, etc. Even doing that, I consider my measurements over time to be an estimate only. If you switch to a different Lot of bullets, the measurement at a given round count may often be different, even if not hugely different.
Realistically, making an accurate claim of land erosion of .001" or .002" over a just few hundred rounds fired is a difficult thing to do using the Stoney Point tool, due to the inherent limitations of the tool and how it is used. It is probably a stronger assumption when measuring the difference between a barrel that has 2000 rounds through it to when it was new, especially if you make the same observation with multiple barrels over a long period of time, using the same bullet measurement set for the two measurements with each barrel, of course. Even that has a caveat in that by comparing measurements taken at zero rounds to a couple thousand or more rounds, you can't necessarily claim the erosion was perfectly linear with respect to the number of shots fired. In my hands, there is usually a measureable increase in the distance to touching after the first hundred rounds or so, then it seems to slow down a bit after the barrel has been broken in. Like Dean observed, I might not measure a statistically significant difference in the distance to touching the lands for the next 200 to 300 rounds after that. Eventually though, the distance to touching number will measurably increase in my hands. I suspect this is an effect of smoothing out any rough or sharp edges/corners in the throat that affect measurement via how the bullet actually touches the lands during the measurement process with the Stoney Point tool immediately after the barrel has been chambered, but that is only my guess.
Regardless of the rate of land erosion someone experiences, the most important thing in my mind is using the information as an indicator to determine whether some adjustment to the load is necessary. Unless I was seeing something like .010" erosion per 100 rounds in a .308 Win, it wouldn't bother me too much except as a consideration for whether I needed to re-check seating depth. Barrels are consumable items. As much as I would like to get 5000+ rounds of
competition accuracy barrel life out of my .308 barrels, I recognize that it's probably only going to be 2000 to 3000 rounds, depending on the barrel and the loads fired through it. An early Bartlein barrel on one of my .308 F-TR rifles is currently at 3400+ rounds. I used a relatively mild Varget/185 Juggernaut load through it for most of its life. It has substantial fire-cracking and land erosion, and coppers up relatively quickly as a result. Nonetheless, 5 out of the last 6 F-TR MR (600 yd) matches I shot with that rifle were 200 point cleans with good X-counts. I ran out of the Lot of Juggernauts I had been using and it just didn't shoot the same with the new Lot, so I spun it off. I kept it for sentimental reasons and still have it.
