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.