A look at how rifle barrels speed up: Measuring the increase of muzzle velocity in new barrels

Schmidt Bender PM II 5-25 review AICS AX Chassis 6.5 Creedmoor Spuhr Proof Barrel 16

A look at how rifle barrels speed up: Measuring the increase of muzzle velocity in new barrels

Seasoned shooters and barrel makers will often tell you new rifle barrels speed up. Normally they say something like, “barrels will gain speed for the first 200 rounds or so then hold steady for the remainder of their service life.” Some shooters report gains in given loads exceeding 100 feet/second.

The reasons for the increase in speed are debated. Barrel makers will tell you that the manufacturing process has the tools running in line with the bore. When you chamber the barrel, the reamer runs perpendicular this axis, creating a rougher finish. When you shoot your barrel you are effectively lapping out the chamber’s throat. Others, typically not the barrel makers, think the barrel is being “broken in” and firing removes tool marks from the barrel.

Most of the data sets that support these claims are built on chronographing loads at different times through the same gun and comparing them. I’m unsure how well this accounts for environmental conditions as well as other factors, such as how the bore was cleaned, was it left wet with oil in it, etc… all factors that would effect the recorded muzzle velocity of the given round.

Additionally, I had some shooters claim that my barrel length and velocity posts, in particular the post I wrote on the 6.5 Creedmoor, were flawed since I used a new barrel and didn’t account for the increase in muzzle velocity. In this post I’m going to attempt to measure how much a barrel speeds up, at what rate this occurs, and how it may influence the data in the barrel length and velocity posts.

Plan A was to borrow the new rifle from my friend Paul and shoot it before he got a chance. He didn’t like that idea, so I went for Plan B and built a new gun. For testing purposes I selected a Proof Research stainless steel Sendero contour barrel. This cut rifled barrel has a 1:8″ twist. I installed the barrel on a Remington short-action receiver and chambered it in 6.5 Creedmoor. The barrel was finished at a length of 26″ with a .420″ dish crown.

Pretty cool looking rifle, isn’t it? The parts are as follows:

I wanted to use the same load for testing so I ordered factory Norma 6.5 Creedmoor 130 gr Hybrid ammunition from Brownells. Norma is known to make extremely high end factory loaded ammunition.

After talking to a number of shooters I respect, we decided I would fire 10 rounds through the barrel, clean it, then fire additional rounds. This would be the only time I clean the barrel until the end of the test. Since that would yield hundreds different data points, I decided to group each 10 round interval into a series, which would provide a more meaningful basis for comparison at the conclusion of testing. To see if a clean bore would impact results, after I fired the last round, I would clean the rifle and fire another 10 shots. To prevent the rifle from getting too hot, I waited 3 minutes in between each series of shots. I used a Fluke IR thermometer to ensure barrel temperature was in the mid-80s prior to shooting each series. At the end of testing I would have data on the rounds I fired through the barrel grouped into different series.

This test is controlled for temperature (air temperature was 80F during testing), barrel, and ammunition. All rounds were shot in the same session with the same environmental conditions.

I fired all rounds prone, from a bipod with rear bag. Muzzle velocity data was recorded with a MagnetoSpeed V3 barrel mounted ballistic chronograph. I am absolutely confident in this tool and find it superior to the Oehler Research 35P proof chronograph. To read about how they compare, click here.

Results are posted in the table below, please note the following abbreviations in the table; Series= 10 shot group, CRC= Cumulative Round Count, MV= Muzzle Velocity (feet/second), SD=Standard Deviation, CHG MV Last= change in MV from last series, CHG MV Initial= change in MV from initial series

I also plotted the change in SD.

Comparing the first series (#1) to the last (#22), the muzzle velocity increased 18 feet/second. Series #20 showed the highest increase in muzzle velocity with an increase of 28 feet/second.

How did the results match your expectations? I had expected a greater increase in the muzzle velocity for the first two hundred rounds. At about 120 rounds I started texting guys, “I think the internet lies”. I had envisioned a pretty rapid rise in muzzle velocity based on some of what I read, but that isn’t what I observed. Keep in mind though, this is a small sample size of only one barrel.

Why didn’t you shoot more rounds? I had originally intended to shoot 300 rounds. I wanted to capture an increase in velocity and then a plateau. At 200 rounds I didn’t see enough of a gradient to support this, so I stopped testing. Shooting all those rounds prone and then waiting in between each series was pretty time consuming. I hit the point of diminishing returns. Looking at the data I did gather, I think this upward trend in muzzle velocity would continue.

How do this relate to the barrel length and velocity posts? I think it shows that while new barrels do speed up, that change in rate is gradual. Take a look at the 6.5 Creedmoor post I wrote, in it, the 142 SMK load was faster at 26″ (2,677 ft/sec) than it was at 27″ (2,663 ft/sec). I think this was based on sample size rather than the barrel being new. At that point in the testing the barrel only had 20 rounds through it. If you look at the data above, you’ll see a negligible change at this interval.

Did cleaning the barrel have a significant effect? I cleaned the barrel between series 21 and 22. Note the average velocities are identical and SD was within 2.2 feet/second. I had assumed there would be more of a change. There wasn’t.

How can precision shooters apply this data set? Well, I think it is a good reminder to periodically check your muzzle velocity, something that more shooters are doing regularly when they arrive on a new range or conditions change. This is clearly best practice.

My data is different, you are wrong! I’d be happy to share any data sets shooters have. Shoot me an email from the contacts page!

What do you think of the Norma ammo? I like it! I’ll post a stand alone review. In the interim, check it out here.