This subject comes up often and can leave you feeling cheated or like a lottery winner. We use a Lab Radar to collect our velocity reading, and list our average velocity attained throughout testing of that load. We use rifles that match what represents most of the rifles chambered in that cartridge. For example, most magnums have a 26" barrel, but some have a 24" Some standard rounds have 22" barrels, or we list velocity from multiple barrels or average the velocity from multiple barrel lengths. We try to match what is common. There is no reason to list 450 bushmaster from a 26" barrel when the vast majority are 16-22" on the market. Likewise, a 20" 300 RUM barrel listing would be just as silly.
24" 6.5 PRC, 26" 338 RUM plus brake, 16.1" 450 Bushmaster plus brake
You may be wondering what to expect from your rifle. Most customers have velocity readings very similar to our results, since we are testing in real world conditions at an open range in all weather conditions. However, we have had some instances when the numbers just don't seem to match. Throughout all our testing, we are seeing some patterns emerge with velocity differences. The first, and most obvious, is barrel length differences. The way barrel length effects velocity is not linear. That is to say, the increase or decrease in velocity from 20" barrel to 22" barrel is not necessarily the same as from a 24" to a 26" barrel. There could be more or less of a change when the starting length of barrel is different. The type of powder being used also makes a big difference when the barrel length changes. For example, when testing 450 Bushmaster in our 16.1" Ruger barrel and moving up to a 22" custom barrel, we only gained about 60 fps over 6" of barrel. The 450 Bushmaster rounds use a very fast burning powder. Contrary, some of our 6.5 Creedmoor loads are showing 200 fps differences between our 26" barrel test rifle and our 22" field guns. The powder used is considered a slow burn rate rifle powder. The latter example we are seeing the classic 50 fps per inch of barrel.
Wyoming's Bighorn Mountains 2019
The next easy to explain cause is ambient temperature and temperature of the ammunition. We develop new loads in all types of weather and try to test in all weather while getting an average. All powder is temperature unstable. It just happens that some powder is more stable than others. A load we develop in January in Michigan is going to have a different velocity than running in the summer months down south or in the southwest, and vice versa.
Nevada Desert 2021
Round count down the barrel since it was new, matters as well. Most barrels speed up 100-200 fps after they have 100-200 rounds down them. Again, this is just an average, but we have witnessed this many times with our in house rifles, as well as with customer rifles we are doing custom ammunition for.
Some barrels are just simply faster than other barrels. For no good explanation, other than rifle voo doo, some barrels are just faster, or slower. I have an older Winchester M70 in 7 Rem Mag that is just plain slow! My standard load clocks in at 3050 fps whereas the same load clocks 3210 in a Browning, and many customers have reported 3180. All the same load!
It all boils down to clocking the loads in your rifle to be sure. Having access to a chronograph helps solve the puzzle for longer distance hunting applications. That being said, in most situations at traditional hunting distances, 100 fps one direction or the other, is not going to break a hunt or cause a miss. If you plan to shoot long distance, we recommend thoroughly understanding all the dynamics involved.
Below is a list of Cartridges and their barrel lengths we test with:
6.5 Creedmoor 26", 22"
6.5 PRC 24"
7 Rem Mag 24", 26"
7 SAUM 22"
28 Nosler 26"
300 WSM 23"
300 Win Mag 26", 24.5"
300 PRC 26"
300 RUM 26"
338 RUM 26"
350 Legend 22"
450 Bushmaster 16.1", 22"
Typical range session. Developing custom 338 RUM
We enjoy seeing extremely low ES and SD
Let us know what topics you would like to see covered in our blog. Don't forget to subscribe to our newsletter and Like and Follow us on Facebook.