Chapter 0 — Start Here
Precision shooting produces data. The question is not whether to track it — it is which numbers actually matter and what they tell you. After thousands of rounds across hundreds of barrels, the precision shooting community has converged on five metrics that together paint a complete picture of barrel health, load performance, and when action is required.
Muzzle velocity is the most fundamental number in your session log. It tells you how fast the bullet left the barrel, and — when tracked over time — how that speed changes as the barrel ages.
A healthy barrel with a consistent load produces consistent velocity. As the throat erodes and the bullet’s engagement geometry changes, velocity typically decreases — sometimes gradually, sometimes in steps. A barrel that has lost 40–60 fps from its baseline velocity is usually approaching the end of its useful life for precision work, even if groups are still acceptable at shorter distances.
Velocity is also your dope foundation. A muzzle velocity 50 fps lower than your calculator assumes will produce meaningful point-of-impact errors at distance.
Extreme Spread is the difference between the fastest and slowest shots in a string — a measure of how much your load varies from shot to shot. A low ES (under 15 fps for a well-developed precision load) means your load is producing similar pressure and velocity on every shot, which translates to vertical consistency at distance.
A high ES (30+ fps) means rounds are arriving with different velocities. At 1,000 yards, a 30 fps ES difference can produce 10–15 inches of vertical spread from velocity variation alone, before wind or technique factors in.
ES tends to increase as a barrel ages, as throat erosion creates less consistent bullet-to-rifling engagement. Tracking ES over barrel life lets you see this trend before it becomes a competition problem.
Standard Deviation is a statistical measure of how tightly clustered your velocities are around the average. Where ES captures the extremes, SD describes the distribution. Single-digit SD (under 10 fps) is achievable with well-developed loads in good barrels. Under 15 fps is solid for most precision applications. Above 20 fps, velocity variation is contributing meaningfully to vertical dispersion at distance.
Like ES, SD tends to climb as barrels age. The combination of rising ES and rising SD, alongside declining average velocity, is often the clearest signal that a barrel is approaching retirement.
CBTO drift is your direct measure of throat erosion — how far the lands have moved since your Day Zero baseline measurement. As the throat erodes, the bullet seated to the same depth now has more jump to the lands, changing pressure dynamics and potentially affecting accuracy and velocity.
Tracking CBTO drift tells you when to adjust seating depth to compensate. Most competitive shooters make this adjustment when drift exceeds .030”–.040” from baseline, though the threshold varies by cartridge and load.
CBTO measurements are only meaningful when taken with the same comparator insert and caliper every time. Changing tools invalidates your drift data. Borely requires you to record your tools with every measurement.
Round count is the simplest number and the most fundamental. It is the X-axis against which everything else is plotted. Velocity trends, ES trends, CBTO drift — all are only interpretable in the context of how many rounds have been fired.
Round count is also the basis for maintenance planning. Tracking rounds since last cleaning lets you identify whether your barrel shoots differently dirty versus clean, and what interval produces the most consistent performance.
Finally, round count is the clock your barrel runs on. Knowing you are at 800 rounds on a 1,500-round barrel is very different from knowing you are at 1,200. One means you are in the middle of your barrel’s productive life. The other means you should be planning its replacement and starting your baseline on the next one.
No single number tells the complete story. A barrel with stable velocity but climbing ES may have a load development problem, not a wear problem. A barrel with significant CBTO drift but stable velocity and ES may still have years of competitive life with a seating depth adjustment. A barrel with dropping velocity, climbing ES, and significant CBTO drift is telling you something unambiguous.
The five numbers work as a system. Tracked together over the life of a barrel, they are a complete diagnostic tool that tells you not just where your barrel is today, but where it is headed and what you should do about it.