i.e. shouldn’t the statement of a fast vs a slow barrel should be correlated against pressure generated for the same fps?

Just using the same ammo seems to be too much of an assumption that pressure and therefore FPS should have been equal and it was just the barrel that was fast.

EDITOR: Your point is well-taken, but IF there is a pressure differential that further proves the point that each barrel is different. As long as each barrel is not exceeding max pressure (and none did here as we were using conservatively-loaded factory ammo), the fact remains that some barrels were faster than others — GIVEN the SAME LOAD. There can be a number of reasons — different land/groove configurations, different levels of internal smoothness, different bore diameter, different amount of chokes. But the fact remains that with the same ammo, different velocities were recorded.

You might say — oh well, all this proves is that I need to add more powder to “equalize” the slow barrel. In fact, in the real world, we have found this usually does NOT work out. Slow is slow. The slow barrels (chambered in 6mmBR) we tested could simply NEVER achieve the velocities of the “fast” barrels, when both were loaded to max acceptable pressures. Meaning, even when we adjusted the charge upwards for the slow barrels, they could NOT shoot as fast as the fast barrels. With our handloads, we tried to get all the barrels to deliver 2890 fps with 105-107gr bullets. We simply could NOT get there with the slow barrels — they would max out around 2850-60 fps before there were clear pressure signs. By contrast, the fast barrels could shoot 2900+ before we started to see ejector marks, cratering, or other signs of pressure. In specific examples, we have one 28″ barrel which can run 2910 with 30.3 grains of Varget, and another 28″ barrel that hits the wall at 2865 with 31.0 grains of Varget, and you can’t get any more speed from that slower barrel without ruining the brass.

We can get into an elaborate discussion, but the bottom line here is that some barrels just deliver fewer FPS (at max acceptable pressures) than other same-length barrels (at max acceptable pressures). If you have a slower barrel, you need to accept what your barrel can give, and don’t pursue dangerous pressure levels to try to make the slow barrel deliver the velocities of the faster barrels. That’s sort of the point of the story.

EDITOR: You folks don’t seem to get it. We are not claiming that one brand of barrel is alway better than another. In fact we stated clearly that speed variances are “barrel specific, NOT brand specific.” What we have found, empirically, is that different same-length barrels chambered for the same cartridge will show a significant variance in measured velocity, with the SAME LOAD. That is a fact, and any one who has built many rifles will attest to that. It doesn’t matter if there are many underlying factors in the speed variances. We are only stating that the end result — different speeds with the same brass, bullets, and powder amount — is a “fact of life” that shooters should understand when they change barrels and when they work up loads.

We often get questions from readers asking “why can’t I get the velocity it shows in the load books?” or “why does my buddies’ Savage shoot faster than my [same-model] Savage?”, or “Why is my new barrel slower than my old barrel”. The point of this article is to provide the simple truth that some same-length barrels, with the same load, are faster than others. That may seem obvious to the critics, but there are many shooters who are not aware of this phenomenon, or who didn’t realize the variance in velocity can be 100 fps or more. Once they learn this fact, they will not be tempted to boost their powder charges to unsafe levels in order to read a velocity that can only be achieved in an inherently faster barrel.

I believe when using a slower powder you will see a proportionately larger change in fps per barrel inch. For example the change would be larger if you used 2000MR instead of Varget or RL15. The test was done cutting from 33″ to 28″. If you cut from 26″ to 20″ I believe the spread would also be larger then 8 or 10 fps. Interesting discussion.

Get the above barrels listed in the bulletin measured for starters.

Different bore and groove size variation and groove width all effect the overall bore surface area and will impact on the velocities and pressures.

Send the barrels here. I will measure the bore and groove sizes and will post the data and send the barrels back.

That’s the only way you will learn anything to start with.

If brand PN has a groove size that is say .0004″ bigger than brand S than I’m going to say that is the difference but with out measuring we’re all guessing!

Later, Frank
Bartlein Barrels

EDITOR: This is NOT true for the 6mmBR: We did a 6mmBR cut-down test, starting with a 33″ barrel. We cut it in 1″ increments down to 28″. Total change in velocity was just 40 fps, very consistently about EIGHT (8) FPS PER INCH. Among the barrels referenced in this story, the length variation was only ONE INCH.

SEE Test HERE: http://bulletin.accurateshooter.com/2011/12/barrel-length-and-velocity-barrel-cut-down-tests/

EDITOR: It’s not pointless to have this discussion, because people often post (in frustration) that they can’t reach velocities published in load manuals or on our Cartridge Guides. If they don’t understand that every barrel is different (even from the same maker), they may experiment with excessive loads in an effort to try to “hit the numbers”. Additionally, this shows the importance of developing loads specific to each barrel. When you change out a barrel, you can’t just lift a load from your reloading log and expect that it will be ideal for your new tube. Some folks, new to reloading, don’t realize this.

Editor: We have done a cut-down test on 6mmBR barrels (slicing it down at 1″ increments) and found the average difference in speed per inch (from 33″ to 27″) to be 8 fps. Hence the article states: “Roughly 10 fps can be attributed to the slightly longer length (27″ vs. 26″), but otherwise this particular barrel was simply faster than the rest.”