Q I enjoy reading the cartridge test reports in Clay Shooting and I have bought some of the ammunition mentioned in the past; there is just one point that I’d like cleared up please.
On the box of one type you tested, it said the velocity was 1,450 feet per sec, but when you tested them your chart recorded 410 metres/sec (1350fps). Why were your figures different from what it says on the box?
Dick Manning, Lincoln
A The reason for this is simple; the velocities are measured at different distances from the muzzle.
We use Birmingham Proof House Laboratory reports for our cartridge test information, where ammunition is tested to full international CIP approved standards. The agreed distance for velocity testing under CIP rules is 2.5 metres from the muzzle, as we mention in the information printed with our test results.
Some cartridge makers state velocities as measured at the muzzle, which will always give a higher figure due to the rapid slowing down of the pellets as they emerge from the gun barrel and hit the atmosphere. The amount of difference between MV and 2.5 metres will vary a little according to the velocity and shot size being measured, with higher velocities dropping the fastest initially, and likewise smaller shot; larger shot retains its velocity (and hence energy) longer than smaller shot. However, it is mainly the greater distance from the muzzle that explains why the figures are different. Richard Atkins