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Thursday, 19 January 2017

Bullet Sizes (Diameters) v Bore Sizes (Part 1):

Recent friendly correspondence raised the issue of bullet sizes versus barrel bore sizes.

I am NOT going to answer this question because of the SAFETY issue - and the facts are that there are too many variables that must affect the results - but I'd like to point-out some possibilities for our interest.

Some Variations in Expansion Of Fired  Bullets.
There Is A Lot Of Force Involved.
- & Just a Little Soft Cloth Changes Results.

The only acceptable factual findings would need to be sourced from a ballistic scientist  using a properly equipped test chamber.

- However - the issue was raised : Is using nominal .357 diameter jacketed projectiles unsafe in a 9mm barrel meant to fire a bullet that measures a nominal .355"?

I have done this (with a reduced powder charge) with no ill effects.. but this does not mean that it is safe for every instance.

Some of the variable factors are the actual bore size, the strength of the barrels, depth and type of rifling, the 'hardness' of the projectiles, the length of the bearing surface, the weight of the projectiles, the 'peak internal pressures' - 'burn rate' of the propellant and the friction between the moving object within the bore etc.

We are talking INTERNAL BALLISTICS here.

This problem reminded me of an old article I read in PISTOL & REVOLVER DIGEST that was published by DBI Books Inc. back in  MCMLXXVIthis 'Roman' date I interpret using the 'subtractive method' (!!) as being 1976 in "modern" Hindu-Arabic numerals or 'European digits'.

In this article (Chapter 13), the writer Dean A Grennell shows photos of a Hornady .38 Special jacketed bullet that was fired from a revolver without any barrel. The round was caught in a mixture of grease & paraffin that was used back then to test bullet expansion.
Top "Pregnant" Looking Round Fired Without Barrel.

Now, because of its low velocity - being fired without a barrel (but while the cartridge was contained within a chamber) - it has not expanded or opened at the front.

 .. but the main body of the projectile has been 'bumped-up' in diameter by the unrestrained impetus of the high pressure gasses against the inertia of its mass - while its flat base has been blown concave. - He describes it as looking "pregnant".
Base Of Bullet Distorted By Heat & Gas Pressure.

Now, going back to the question whether a .357" pill is dangerously oversize in a .355" sized 9mm barrel - today I noted bullets for a "Thirty-Two" (7.65mm) revolver - said to officially measure .312" - that are listed as being variously .311" , .312" , and .314" - and I bought x 100 Hornady lead round balls described as 32 CAL  .315".

- If a lead pill contained in a copper jacket can be reshaped to such a degree by peak gas pressure on its base - operating over the short distance that follows a bullet leaving a chamber in free air - and the gasses dispersing...??? ( - Isn't this similar to how the copper slug used in measuring 'C.U.P' internal pressures works?)

- Further - If these photos of a bullet distorted by gas pressure do show the amount of 'malleable shaping' that may be present under firing pressure - but normally is contained in the barrel - surely there must be some degree of tolerance in the 'fit' of our bullets ordinarily.

- Now - I'm honestly not trying in my ignorance to prove anything - but it seems possible that the fit between bullet and bore is flexible (within limits) - as an under size pill will 'set-up' - and an oversize one may extrude or squeeze-down to conform with the lands and grooves - which after all - is how barrel rifling works.

According to Wikipedia the formula used to calculate the pressure required to deform solid base bullets is:
Bullet's BHN x 9.80665 N/kgf×106 mm²/m² = [N/m²] = pressure in pascals
Bullet's BHN x 1422 = pressure in pounds per square inch

 - When a bullet 'flares' or 'upsets' under pressure to fill the bore and engage the rifling - it is called 'Obturation'.

I don't - of course - want to be the person who experiences exceeding that upper limit in one of my guns.

- Sadly no answers in this online article about 'Forensics' - but it is VERY interesting, - Link:

Sorry but I'm not a ballistician  - but we do have fun eh..

Life is Good,

Marty K.