Rifleshooter continues his series on velocity versus barrel length with different rifle cartridges (.300 Winchester Magnum, .223 Remington/5.56×45, and .308 Winchester/7.62×51), this time with a rifle chambered in 7mm Remington Magnum:
Ballistic data was gathered using a Magnetospeed barrel mounted ballistic chronograph. At each barrel length, the rifle was fired from a bipod with rear bag. Five rounds of the Federal 150 grain load (load #1), four rounds of the 160 grain A-Frame (load #2) and 175 grain GameKing (load #4) and three rounds of the 165 grain GameKing (load #3) were fired at each barrel length. The number of rounds fired for the hand loads was limited by the components available. Average velocity and standard deviation was logged for each round. Since I would be gathering data on 36 different barrel length and ammunition combinations, and would not be crowning the barrel after each cut; I decided to eliminate gathering data on group sizes.
Once data was gathered for each cartridge at a given barrel length, the rifle was cleared and the bolt and magazine were removed. The barrel was cut off using a cold saw. The test protocol was repeated for the next length. Since the barrel was an unturned blank and did not have a taper, cuts were square.
Temperature was 57F.
Results of experiment are found below:
Average velocities and standard deviation (SD) were calculated by the Magnetospeed barrel mounted ballistic chronograph.
As the barrel was cut from 28″ to 20″, average change in muzzle velocity for the four cartridges was 239.5 ft/sec. Average loss of velocity per inch was 26.6 ft/sec.
At 23″ in barrel length the rifle became noticeably louder. The 20″ barrel was unpleasant to shoot, the 165 grain GameKing load became very smokey, and the 175 grain GameKing load exhibited a lot of flash (I wrote “flash!!!” in my notes).
The velocity drop from 21″ to 20″ is anomalous, and I would be hesitant to identify “21 inches” as the ideal 7mm Remington Magnum. However, based on this data, it seems apparent that the 7mm is happiest at barrels 24″ or longer. Considering the round’s high relative capacity, this is not surprising.