We’ve had a couple articles in the past on the importance of Chronographing Barrel Break In. This is especially true when breaking in a new barrel. The first several rounds through a barrel will likely see an increase in velocity. As the small burrs and imperfections of the bore are worn in by the passing projectiles there is often a velocity increase. That matters because if you chronograph a new barrel and head to a match you’re in trouble. Chances are excellent that at some point in the first 150 rounds or so the muzzle velocity will pick up and your DOPE will be off as a result. I’ve had it happen to me at a match, no bueno. I wanted to dig into this topic a bit more, so let’s go!
Chronographing Barrel Break In
I’ve recently acquired a shiny new Bighorn Arms TL3 custom action! It’s beautiful and it’s my new match/practice rifle. I thought it also makes for an excellent opportunity to test and gather data on muzzle velocity by chronographing barrel break in. Barrel break in is a topic all unto itself. So here’s the quick bullet points. I don’t subscribe to the benchrest break in methods. I’ll put 20-50 rounds through it and give it a brief cleaning with some wet patches followed by some dry ones. That’s about it. I continue to shoot and practice with the rifle until I’m approaching 150 rounds. However, I got to thinking about how that’s a bit arbitrary.
So this time around I’ve chronographed my first 50 or so rounds across multiple strings of fire. For the next 100 rounds I’m going to chronograph five round strings of fire every 20 rounds. So when I get to 70ish rounds I’ll chronograph five, when I get to 95ish I’ll chronograph another five, etc. The idea is to track muzzle velocity across those first 150 rounds and observe the velocity and it’s stability. By chronographing barrel break in I hope to find a less arbitrary and more data based number of rounds. Hopefully this will give us a better idea how many it really takes for a barrel to “break in” or “settle in” with regard to muzzle velocity.
The First Five Rounds
So here are the first five rounds through the new barrel. I want to point out that by utilizing our reloading by chronograph method with this caliber I’ve already developed a “pet load” for my 6×47 Lapua barrels. The same load has produced single digit SD numbers across three barrels, two different contours, and two different twist rates. All were Criterion Barrels and I think this speaks to the consistency of their product. The same load performs well in barrels of the same caliber, length, and twist rate. The first five rounds produced an average velocity of 2996, which is within 30fps of the expected final velocity of 3025. There was an SD of 7fps and ES of 20fps which is excellent.
The Next 22 Rounds
Here we’re seeing some velocity increase already. We’re now up to an average of 3021fps with an SD of 8fps and ES of 34fps. I am still thrilled with the consistency of the velocity. However, we are seeing the speed pick up a bit and the ES is getting wider. Meaning the span of velocities from slowest to fastest. I expect the ES will tighten up again as the barrel settles in. An SD of 8fps across a 20+ round string of fire is very very good. So I think that speaks to the validity of our reloading by chronograph method.
More Chronographing Barrel Break In Data
The ES and SD fluctuated a bit across the different strings of fire and number of rounds fired. What I’m already seeing is some crazy consistency in the muzzle velocity. This same load on the last barrel had an average speed of about 3026fps. So it quickly sped up to that point and leveled off already. I’m going to put another hundred rounds through the barrel ahead of the Mile High Shootout this coming weekend. I’ll check velocity and SD/ES every 20 rounds or so and see if there’s any more discernable changes. I’ll report back with those findings next week. With an after action report on the Mile High Shootout to follow!
So final thoughts here, my own arbitrary number of 150 rounds for break in and muzzle velocity stabilization may be way higher than it needs to be. That’s the point of this experiment so we’ll see what happens over the next 100 rounds. Wouldn’t it be interesting if the average velocity and ES were stable in the first 50 rounds? That would certainly save some powder and components. Obviously, this is just one gun and one set of experiences but we’ll see what happens. I may repeat the test with barrels in the future to verify the results and conclusions we make here.