December 20th, 2011

Barrel Length and Velocity — Barrel Cut-Down Tests

Many of our readers are in the process of building 6mmBR rifles, or re-barreling their guns to this efficient, accurate caliber. Project builders often ask us: “What length barrel should I use — How much velocity will a 30-inch barrel deliver vs. a 28-inch?” Well, here’s the answer, based on actual field tests (conducted a few seasons back).

Velocities at Different Barrel Lengths
At this website’s invitation, respected Benchrester Jackie Schmidt fitted a 6BR barrel fitted up to his Rail Gun. One of the first tests was to see how barrel length affects velocity. Jackie started with a 33″ barrel–the full length of the blank after chambering and fitting. He cut the Krieger barrel back in 1″ increments down to 28″ length. Tests were done with Sierra 107gr MatchKings, Fed 205m primers, and a stout load of Vihtavuori N140. (Jackie selected the fastest burning powder among our test brands to ensure a full burn in the barrel length). 15-shot strings were fired over an Oehler model 35 chronograph with proof channel.

Velocities Varied 8 FPS Per Inch
RESULTS: At 33″ the barrel’s average velocity was only 40 fps faster than at 28″. On average, velocity was about 8 fps higher for each inch past 28″. (However, the 29″ Average was actually lower than the 28″ because we had one real slow shot at 2932 fps in the string). Comparing the 29″ Max Vel with the 28″ Max Vel we saw that same 8 fps difference. And comparing Average Velocities at 28″ and 30″, the difference was exactly 16 fps (again, 8 fps per inch). Bottom line–figure on gaining just 8 fps per inch past 28″ length. That’s not a big gain. Jackie feels that’s not enough to justify the weight/flex associated with something longer than 30″. Furthermore, the best ES was at 28″ (ES of 24, SD of 7), while the worst ES was at 31″ (ES of 61, SD of 16).

Barrel length 28″ 29″ 30″ 31″ 32″ 33″
MAX fps 2966 2974 2982 3005 3001 3011
MIN fps 2942 2932 2950 2944 2960 2974
Average fps 2952 2948 2968 2973 2982 2992
Average Vel vs. 28″ Same - 4 +16 +21 +30 +40

If we cut the barrel down even further, the variance in measured velocity might have been greater. We’d expect that you’d lose more than 8 fps/inch by going from 28″ to 25″. Still Jackie’s results show that the velocity loss was less than you’d might expect. Keep in mind though that results could vary with other powders, and the numbers would certainly be different with other chambering (such as the 6.5-284) and slower powders (such as H4350). Still the results are interesting.

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