Ninth Stage

6 Responses to “Overbore”

1. 1 Josh May 6th, 2008 at 6:19 pm

   Thanks for this - it’s well done. Also congratulations for not flaming the Editor at Accurate Shooter who jumped on the opportunity to put foot in mouth.

   My initial reaction to the Accurate Shooter post was skepticism also. While the method the author used almost seems intuitive, it’s probably more in the category of conventional wisdom.

2. 2 Ninth Stage May 6th, 2008 at 6:37 pm

   Thanks for the kind words Josh. I like Accurate Shooter too much to want to flame anybody there. They do a heck of a job posting every day. I hope that the above post doesn’t come across too snarky either - I’ve misread others’ posts myself.

3. 3 Josh May 7th, 2008 at 10:30 am

   I share your respect for the Accurate Shooter site. It’s by far the best on the web. Unfortunately, like most firearm related publications - print or digital - it suffers from an occasional lack of technical sense. Today, the editor says:

   “Also, a volume passing through an area can be equated to velocity if the time for the expansion is considered. A higher gas velocity, i.e., more burnt powder through a smaller bore, results in higher erosion rates. I think John L’s index is meaningful in this regard.”

   The first sentence is surely correct if flow is subsonic, but I have no idea what the shock waves are like at a cartridge neck during combustion, and through a shock zone, velocity is always mach 1. The second sentence is an unfortunate example of repeating what others have written - that bore degradation is a result of partially burned powder and combustion gases eroding barrel steel, kind of like a silty river wearing away rocks on the bank. While this seems reasonable, I’ve never seen a scrap of data to support it. For many years I worked for DuPont, back when they manufactured propellant powders. One of the powder chemists told me then that bore erosion wasn’t a mechanical phenomena at all, that it was a chemical process. Combustion gases in an extremely hot, nitrogen-rich, atmosphere altered the chemical composition and mechanical properties of a tiny layer of surface steel, and this led to eventual microscopic spalling of the surface. Seems believable, but I’ve never seem published data there either.

   Either way, Accurate Shooter seems firmly behind what’s probably a well-intentioned but maybe not too well thought out article. I guess it’s time to be quiet.

4. 4 Ninth Stage May 8th, 2008 at 10:28 pm

   Well, I haven’t been quiet but everyone there has. Despite the editor saying, “It’s not the “final answer”, but it’s a good start in my mind”, he is obviously not interested in progressing beyond the start or even discussing an alternative. (not to mention his silly equating a mono dimensional change with a three dimensional change).
   I’ll continue to sift through the regurgitated chaff (pressers and what he gleans from “the shooting wire”) but I have to say he’s lost some respect in my eyes.

5. 5 Josh Benin May 9th, 2008 at 10:14 am

   I don’t want to flame anyone in online forums, but I did send this to the 6mmBR Editor. After thinking about it for a while, I’m convinced that you’re correct in adding a volumetric term to the ‘overbore’ index discussed on that Blog.

   Editor:
   I owe you an apology for attempting to take you to task for comments
   which I thought you added to a post concerning your “Overbore”
   Cartridges via Comparative Index” article. You pointed out that I
   misinterpreted the comment, which was added by a reader, not the
   Editor. Sorry.
   You added that you found ‘Ninth Stage’s’ comments confusing. If
   you’ll indulge me for a moment: All the various attempts to define
   ‘overbore’ use a fraction, with case capacity - a surrogate for the
   amount of chemical energy in the system - as the numerator. The
   numerator is in cubic inches
   John L. and Rocky Rabb used bore cross-sectional area as the
   denominator. Sounds good - that’s the size of the hole through which
   the energy must escape. Denominator is in square inches, and resulting
   index is in inches.
   Ninth Stage used a volume in the denominator as a stand-in for
   bullet mass. This represents inertia, or how fast the bullet can get
   out of the way of expanding powder gases, or how fast the volume
   available to combustion can increase. We do know that this affects
   chamber pressure, and probably does change temperature also. Resulting
   index has no units.
   Everybody ignores powder energy content per grain (probably a
   reasonable simplification), work done engraving the bullet (might be
   significant), and, as you pointed out, several other (probably lesser)
   factors.
   So there are two theories ‘defining’ overbore: one divides energy by
   the hole it must expand through, and the other divides energy by a very
   rough approximation of the rate at which it can expand - how fast the
   bullet can accelerate.
   Let’s play with numbers. A 6BR can drive a 70 gr bullet at 3200 fps
   in a 24″ barrel. Guessing uniform acceleration, the bullet sees an
   average of 80,000 Gs. Sounds like a lot. That much acceleration puts
   an inertial force of 800# on a 70 gr bullet - ignoring everything but
   inertia. Does this sound reasonable? Well lets make another guess,
   that the average pressure on the base of the bullet is 25,000 psi during
   its 2 feet of travel. How much force does this mean on a .243 bullet?
   Works out to 890#, pretty close to the first guess.
   So we know the inertial forces at work here are pretty large,
   especially since peak chamber pressures are around twice the average I
   used. There are some simplifications here, but inertia is a major
   factor (well, we already knew that didn’t we?), and it affects chamber
   pressure. We also know that chamber pressure affects powder burning rate.
   It seems reasonable to say that this 6BR cartridge with its 70 gr
   bullet isn’t overbore - as long as we’re using RL-15, Varget or similar
   powders. Our 3200 fps velocity comes from around a case full. It
   certainly is overbore with H110 but maybe it wouldn’t be with a H110 and
   a 10 grain bullet. It’s downright ‘underbore’ (?) with 4831. On this
   basis, I’d say that any definition of ‘overbore’, as related to powder
   capacity, must contain an inertial term - bullet weight. It needs some
   qualification about powder too.
   The above is just physics, but I’m a mechanical engineer, not a
   chemist, so I’m guessing when I say that bullet inertia affects powder
   burning rate, which affects powder combustion temperature, which affects
   barrel life. Which means that an ‘overbore’ term defining barrel life
   probably needs a bullet inertia term too. I’ll guess one step further,
   that barrels are not eroded by being blasted by flaming powder grains,
   any more than any hard surface is eroded by softer articles, but rather
   barrel erosion is more sensitive to peak temperatures than anything else
   (before anybody chews me out: rivers do not erode rocks, and lead laps
   to not smooth barrel bores - it’s always the abrasives in the system
   that do the cutting, not the conveying medium). Maybe you can scare a
   chemist or metallurgist out of the website to add to this.

   Again, my apologies for attributing a reader’s comments to you, and
   again thanks for the website,

   Josh Benin

6. 6 Ninth Stage May 12th, 2008 at 11:48 pm

   Josh -
   Thanks for the support. I’ve been thinking about this too. I’ll post again about this when my thoughts have coalesced further.

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