7mm Remington Magnum Velocity Versus Barrel Length

7-rem-mag-with-20-inch-barrel

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:

Test Protocol

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.

7 rem mag saw and generatormagnettospeed on 7 rem magcutting 7 rem mag barrel by the inch

Results of experiment are found below:

table 7 rem bbl length data all cartridges

7 rem mag barrel length chart

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.



Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.


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  • Andrew

    Quite interesting. Thanks for the post.

  • TVOrZ6dw

    These tests provide excellent data.

    I wonder how long a barrel needs to be before you start to seeing either the velocity plateau or start dropping off?

    • Blake

      with 7mm rem mag, really really long, especially if the load is optimized for it.

      bullpup designs really help keep rifles this long portable.

  • Frank Young

    Very interesting indeed. Thank you very much

  • Swiss Pete

    Ross Seyfried did that back in the 80s in Guns&Ammo. Just sayin’ and feeling pretty old right now :-/

  • Pseudo

    I love these. Nothing like solid firearms data. Please keep it up. I’m sure you have tons on your plate but I’d be super interested in pistol calibers’ performance vs barrel length.

  • Blake

    Thanks for the data.

    The wiki article on 7mm Rem mag states that if you’re going to run one with less than a 24″ bbl you might as well be using 270 Win, as the point of 7mm RM is high capacity & long burn time.

    But with 180gr match bullets in a 26+” bbl, it’s seriously impressive.

  • Riot

    That is a hell of a drop on the 175 grain between 22 and 21 inches.

  • MR

    Was hoping to see low light photos of muzzle flash from a 16″ barrel. 🙂 Other than that, good article. Really like the look of the rifle with that unprofiled barrel. Lugging it around on a spot-and-stalk hunt? Maybe not.

  • WCC

    I am curious to know why the 160 grain bullet was faster than the 150 grain bullet in all respects. Is this a result of the powder loads used? Can you provide specifications for the four loads used in the test?