Rifle/barrel effects on BC.
Yes, it's possible. The important thing to keep in mind is 'how much' effect is possible/likely. The barrels I test from are all heavy barrels with match chambers/crowns with twist rates adequate to stabilize the bullets being tested. I use these kinds of barrels because they are most likely to launch the bullets with minimal initial yaw and adequate stability. In other words, I'm giving the bullet the best conditions to fly with the highest BC it can fly with.
You can read in chapter 10 of my book (pg152) a case where I measured the same bullet to have a BC ~13% lower in one barrel compared to the test barrel. This was a deliberate test done to illustrate the consequence of inadequate stability on measured BC. The .243 caliber 95 grain VLD flies with a G7 BC of .249 when fired from a 1:8" twist 6mm BR heavy barrel, which produces an estimated stability factor of 1.67 (comfortable). The same bullet flies with a G7 BC of .218 when fired from a 1:10" twist lightweight hunting rifle barrel that only produces a stability factor of 1.07 (quite low). So this is one case where inadequate stability can produce a lower BC.
That's quite irrelevant for the most part though, because we tend to shoot bullets with twists equal to or greater than the recommended twist, which always impart adequate stability.
Is it possible (you might ask) to increase the BC of a bullet even further by spinning it even faster? NO! Once a bullet is flying with comfortable, adequate stability (SG = 1.4+), the BC of that bullet will not improve any more by spinning it faster. In other words, when the bullet is adequately stabilized, it's making it's way thru the air in the most efficient orientation (point forward, zero yaw). Spinning it faster can't make it fly with less than zero yaw, or lower than the minimal drag.
Point of this is that the BC can be 'less' from a given barrel if that barrel doesn't impart adequate stability. But the BC for a properly stabilized bullet cannot be improved further by any mechanism related to stability.
Another question that's been touched on: "Can a bullet have a lower BC over long range by being over-stabilized (locking it into a nose high orientation)? Answer: not by a measurable amount. First of all, a bullet with an excessive amount of stability will fly with an increasing nose right (for right twist barrels) orientation called yaw of repose. This is the mechanism that results in spin drift. The nose right orientation is only a very small fraction of a degree, not enough to change the retained velocity of the bullet by even a single fps over 1000 yards. It's barely enough to steer the bullet (drift) by less than 1 MOA in 1000 yards, so it's effect on drag is negligible.
To summarize stability effects, we can say that: once a bullet is adequately stable, you cannot reduce or increase it's drag by spinning it faster. You can only hurt the BC of the bullet by not having adequate stability.
For further reading on this subject including Sierra's tests, refer to the bottom half of this article:
exterior ballistics
So how else could a barrel affect BC? If a given barrel imparts more initial yaw for a bullet, that could lower the BC, right? Sure it could, especially in the first 50-100 yards that it takes for the bullet to dampen out the yaw (go to sleep). Again, don't put too much emphasis on this, especially for long range shooting. First of all, most of the rifles we use for LR shooting are well made with heavier than average barrels, good crowns, and tend to launch bullets with minimal yaw so their effective BC's will be about as good as possible because there's not much yaw to dampen. A soda straw thin barrel firing a heavy bullet might induce significant initial yaw for that bullet. It might even increase the drag, and reduced the measured BC by 10% for that bullet in the first 100 yards of flight. However,
after 100 yards, the yaw of that bullet will be down to near zero, and it will fly the rest of it's trajectory with a BC closer to the max value. This is a big reason why I advocate against testing BC's at close range (In addition to the greater sensitivity of the test to uncertainties of every variable compared to longer range tests). Even if you execute the test perfectly at 100 or even 200 yards, it may produce a lower BC for that bullet than what the average, effective value is over long range because of the yaw effects that only exist up close. In other words, using a BC that's measured in the first 100 yards isn't representative of the bullets true average over long range if the bullet was launched with significant yaw. All of this is covered in the article I posted, which I'll repost here:
http://www.appliedballisticsllc.com/index_files/AccurateSpecificationsofBallisticCoefficients_3.pdf
To sum up effects of initial launch dynamics:
It is possible for a barrel to produce a lower measured BC in the first ~100 yards if that rifle launches the bullet with a high level of initial yaw/yaw rate. However, that BC will not be a true average of the bullets performance over long range after the initial yaw has damped out.
What about the effects of rough/smooth bores? A rough or smooth bore will affect a bullet by the engravings it leaves behind on the bullet's surface. A very rough barrel can even 'drag' some jacket material back past the bearing surface/BT juncture which may get peeled forward on exiting the muzzle, producing little 'fins' which add to the drag. This situation can be improved by laping the barrel, and/or coating the bullets with moly or something else. Keep in mind, this is a way for bullets to possibly fly with
more drag, not less. However, it is a way for the BC of a bullet to be affected by what rifle it's fired in.
To summarize the above discussion, it is possible for a rifle/barrel to have
problems which result in launching a bullet with less than it's potential BC. The biggest effect would be firing a bullet with inadequate stability. But that's not a situation anyone would care about because accuracy would be bad and you wouldn't care about the BC.
The short range BC can be lower if significant initial yaw exists. This is also not likely to be relevant for most LR shooters who use high quality heavy barrels. Furthermore, the long range average BC of the bullet will tend toward it's potential max after the bullet goes to sleep.
One real effect that a barrel can have on a bullet for it's entire trajectory is the roughness/smoothness of the engravings that are left on a bullet. This is mostly a surface roughness effect, which affects the least significant component of bullet drag: skin friction drag.
The barrels I test in give the bullets their best chance at flying with the highest BC, and that BC is measured over long range (600 yards minimum). If bullets are actually flying with a much lower BC (5% or more) from your rifle (especially over short range), I would say it's entirely possible and there's probably something about your rifle that can be improved. If you think that your rifle is shooting bullets with a much higher BC (5% or more) than what I've measured, I consider that an
extraordinary claim, and as MC so aptly put:
...extraordinary claims require extraordinary evidence to back them up.
Another statement that's worth repeating from MC's post, which summarizes much of what I'm talking about is:
It is an astute observation that most of the ways that an imperfect barrel or misformed bullet change the BC is to lower it by increased drag. There are a small number of cases where we've measured BCs 10% or so higher than the bullet manufacturer's claims (Litz also), but we've never had a case where a majority of rifles we tested had consistent BCs and one rifle gave a much higher BC than the rest. The odd rifle out seems to always give a lower BC.
And I would be interested in the details of those small number of cases. I'll bet we tested bullets from very different lots, or there is some other 'good reason' for the +10% out-lier that doesn't actually imply a bullet flying with less drag than what's possible.
To reiterate a statement that I made earlier in the thread; I'm not here to stomp my foot and declare my BC's are perfect. My numbers and I are not infallible. If you have to adjust my BC by a couple % to get it to match your data, that's fine. I would be surprised if a drop test
repeatably suggested the same couple % error. My point is that my testing and results are self consistent, and at this point I've got a lot of historical data to refer to, much of it includes firing the same bullet from different rifles and observing insignificant differences.
My agenda is the following. I'm trying to acknowledge that there are reasons why BC's can be affected by different rifles, without having everyone lose total faith in the science of ballistics. It's one thing if a number is 'off' by 1% or 2%. In most cases that won't result in a miss. However, just by talking about that, it opens a dangerous door. People come away with the conclusion that 'BCs depend on what rifle you shoot' without understanding the magnitude. It becomes a reason to
accept that it's possible for them to be 20% different, and give up on trying to understand how everything really works.
So, in closing, I'll reiterate: if you have a quality barrel (good interior finish), with a good crown, with adequate twist for the bullets you're shooting, and it's bedded properly it will launch bullets with a BC very close to the max potential BC of the bullet, which is what I attempt to represent with my numbers. If you have a rifle like this, my BC's should work for you with very minimal alteration (less than 2%) for the supersonic range of the bullet. My collective experience over the years since I started measuring BC's supports the above statement. The biggest caveat I can think of is lot dependence of bullets. All the bullet makers have lot variation (even variation within a lot), but it tends to be minor. The worse lot variation I've seen is about 5%, and the specific dimensional differences of the lots explains the 5%. This is a big reason why I include dimensioned drawings of the bullets I tested, and their lot number. If your bullet dimensions very a lot from what I tested, it's a legitimate reason why you would find a different BC for that bullet.
-Bryan
) I can assure you that I do. 
