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.
You may also be interested in the results we obtained for handgun calibers: http://www.ballisticsbytheinch.com/
Cheers!
Jim D.
Also what is shows is the effeciancy of the BR case. Adam
Although interesting, many of us shoot barrels shorter than 28″, this test should have continued until the barrel was 20″, as I would be curious what the loss of velocity is between a 26″ and a 20″ barrel.
At 3000 fps,it takes the bullet about 28 micro seconds to go one inch. This extremely small increment of time is insufficient for the pressure inside the barrel to have much of an influence on the speed of the bullet at the end of the barrel, so these results are not surprising. I think intuitively, folks would expect the velocity to be higher but the math supports the results. It would be interesting to see results of a 20″ barrel but based on the small amounts of time we are dealing with, I expect the speeds still would not be that much different, and certainly not much influence on energy or killing ability on game you would be hunting with a gun with a 20 inch barrel. I doubt if you would have 100 fps difference between a 26″ and a 20″ barrel.
Bruce
I have several Krieger barrels in both 20″ and 26″. The average fps difference with a identical load is 18 fps. I believe the fps change is reduced beyond 27 or 28″. This seems to be the case with my 30″ LR barrels.