Talk:Freebore

Delayed onset of additional sources of resistance

Bentriplett suggests: I think you mean to write "rotational momentum" not "rotational inertia". Inertia is synonymous with mass, and rotational inertia is similar. Engaging the rifling adds no mass and doesn't change the shape of the bullet enough to change its moment of inertia about its spin axis. Now, even talking momentum instead of inertia, very little of the resistance force acting on the bullet is from change of rotational momentum as a fraction of total resistance due to friction and deformation, so little th (the rest was unfortunately lost)

I concur both that I might have selected better nomenclature with internal links, and with the earlier editorial comment that further revision is needed (to avoid limiting discussion to friction.) Conceptually, there are a number of possible sources of resistance causing increasing pressure as the propellant reaction proceeds:

• 1 - linear acceleration of the bullet mass and gaseous propellant products
• 2 - sliding friction of the bullet against the neck of the cartridge case, and possible case deformation to remove a crimp
• 3 - rotational acceleration of the bullet mass
• 4 - deformation of the bullet by the rifling
• 5 - sliding friction of the solid bullet against the solid barrel
• 6 - lubricated friction within portions of the barrel where the solid bullet is separated from the solid barrel by either bullet lubricant, propellant gas, molten bullet, or molten barrel; and possibly
• 7 - deformation of the bullet to conform to barrel groove diameter

1 and usually 2 will be present with or without freebore. 3, 4 and 5 can be delayed by freebore. 6 may be present within a freebore diameter exceeding bullet diameter, within a groove diameter exceeding bullet diameter, or where temperatures exceed melting temperatures of bullet or barrel materials. Temperatures may vary with distance from the chamber, and molten bullet and barrel materials may be indistinguishable from 5 on a macroscale. Bullet deformation occurs as rifling lands displace the bullet jacket or core. In theory, deformation may also involve displacement of core material as an oversize bullet is constricted to barrel grove diameter, or as an undersize bullet expands to barrel grove diameter through centripetal acceleration and through differential linear acceleration along the axis of bullet movement. I suggest 1 and probably 2 should be mentioned as unchanged, while 3,4 and 5 should be discussed with appropriate source citations as to their significance. I would also suggest comparing the advantages and disadvantages of freebore. That comparison may involve some aspects of 6 or 7 depending upon the types of bullets and range of bullet velocities considered. Discussion on the talk page may minimize editorial conflict. Thewellman (talk) 22:00, 6 May 2019 (UTC)

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I think 6 should be omitted. The definition of freebore indicates that it always has diameter that exceeds the freebore diameter, this is according to SAAMI which I think makes it definitive. If the diameter does not exceed the bullet diameter then it isn't particularly "free" except of rifling. I think freebore has to be free of bullet resistance.

The intended purpose of freebore ought to be discussed. Discussion of advantages and disadvantages is much more subjective and I'm not 100% certain about how to proceed with this. Certainly greater freebore delays onset of bullet resistance for a given cartridge of defined OAL, like factory ammunition, but freebore might be added in order to allow longer OAL for hand loaded ammunition, or to allow bullets with longer ogives to be used etc.

I have also found better images for use on this page to help explain freebore. I have gotten permission from one copyright holder for one image, and I'm waiting for final word from SAAMI for permission to use part of their .308 Win chamber drawing. I will include these if and when I get permission and have time to upload the images. Bentriplett (talk) 22:57, 6 May 2019 (UTC)

Also... forgive my wiki-fu, I'm a newb when it comes to editing wiki articles in any significant way or using these Talk pages for discussion. Bentriplett (talk) 22:59, 6 May 2019 (UTC)

Forgot to mention... while rotational acceleration technically will result in some bullet resistance, for any typical or even reasonable twist rates, this effect is negligible. I have computed the effect of bullet twist and you can compare the rotational energy with the linear energy and you find that the rotational component is basically in the noise. I claim this is true for any rifling slower than 1:5", maybe even faster rifling as well.

I think this article should also discuss the leade/throat as people tend to get these confused. According to SAAMI Leade and Throat are synonymous and are the first part of the bore of the barrel after the freebore, but I've seen reamer manufacturer drawings refer to the freebore as the leade (not right) and the leade as the throat (which is ok). Bentriplett (talk) 23:16, 6 May 2019 (UTC)

Welcome aboard, and please forgive me if my tone seemed unwelcoming. That was not my intention, since I consider Wikipedia improved by encompassing divergent perspectives.

I'm enthusiastic about your proposed illustration improvements. I concur with omitting discussion of lubricated friction (6) except as it may be necessary to describe sliding friction. However, the volume of propellant gas escaping around the bullet within the freebore (potentially with accompanying erosion by gas at high temperature and velocity) may be perceived as disadvantages. With regard to advantages vs disadvantages, I suggest the primary advantage is allowing increased flexibility in ammunition dimensions -- specifically cartridge case length (for cartridges headspacing on the rim, belt, or shoulder) and bullet seating depth with respect to the forward extent of the full diameter portion of the bullet rather than the bullet tip conventionally described by overall length. I suggest the primary disadvantage is diminished accuracy resulting from potentially variable conditions between the onset of bullet movement and stabilization within the rifled bore. With reduced ammunition tolerances available to handloaders, initial propellant reaction pressure uniformity can be achieved by seating the bullet at a depth to engage the rifling as the cartridge is chambered rather than relying upon the case crimp; although this may leave the bullet stuck in the bore if an attempt is made to unload a chambered cartridge prior to firing. (I have some experience with similar loading and unloading procedures with naval artillery.) Delaying the onset of full resistance reduces steepness of the leading edge of the pressure curve increasing the margin of safety for larger charges of similar propellants with potentially higher velocities from identical case dimensions. As you suggest, handloaders using the freebore to decrease seating depth of identical bullets functionally increase case volume for similar velocity advantages. I suggest the subjectivity of these advantages and disadvantages may be explained. Any stated reservations SAAMI may have about exploring reduced freebore dimensions would seem powerful arguments for the advantages of freebore.

I don't disagree with your assessment of the relative magnitude of rotational acceleration, but that assessment might fall within the discouraged realm of original research without published source citations for theoretical calculations or empirical evidence.

I agree definition of other associated chamber dimension terms might be useful to resolve potential ambiguity about whether the leading edge of lands might be tapered from groove diameter to land diameter, or that taper might uniformly extend to full neck diameter, or there might be various cylindrical lengths with unspecified transitions to smaller diameters. It might be useful to describe anticipated changes through throat erosion over the useful life of a barrel. It might also be useful to compare the benefits of freebore with such alternatives as gain-twist or paradox rifling. Perhaps the paradox gun is simply an extreme example of freebore? Thewellman (talk) 05:44, 7 May 2019 (UTC)