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The Bullet Tumbling Factor

Much has been made of the fact that on occasions there is evidence of some bullet tumble .... sometimes evident from a target when the ''soot smudge'' around the hole is other than uniform.

The photograph below was one taken at the end of test #3 when testing expansion in wetpack. Initially I was puzzled most with regard to the apparent uneveness of the rifling engraving around the bullet driving area. It certainly seemed that this bullet impacted the wetpack slightly off axis, thus one ''petal'' failed to peel back - the rifling idiosyncracy however I was unsure about initially.

The answer came quickly from Rohrbaugh when I brought the matter up ... that is written up below this picture.

A probable tumble candidate

Explanation for the above.

After a discussion with Karl Rohrbaugh it is now quite clear why and how this can happen.

The design of the barrel is such that there is a significant freebore* of 0.250" - the reason for which being pressure control. As we all know I think, the 9mm Parabellum is a high pressure round, even at standard pressures. This small delay before bullet engagement with rifling permits the immediate pressure peak to dissipate to a safer level - sparing the barrel/slide mechanism some extreme trauma. This is both desirable and necessary with the design of such a small gun using this round.

Further to this - it can probably be imagined therefore that not every bullet of every type will engage the rifling with absolute symmetry, per the example above. This then also explains why there are occasions also where a perceptible degree of bullet tumble is seen after exit from the gun.

I have stated more than once - this gives me no cause for concern, seeing as the likelhood of long range engagements is remote. It being much more likely that ''things'' would be happening within ten feet or so. At this distance I doubt if an assailant is going to make much complaint if he is hit by a 100% stable bullet - or one which started to tumble a little!

*''Freebore'' - the distance from chamber end to beginning of rifling grooves and lands .. a distance the bullet must ''jump'' from cartridge to onset of engagement and engraving. Necessary on all firearms but usually a smaller figure. With rifles, often only perhaps 25 thousanths of an inch.

A word on tumbling and ammo in general ........

For the most part, Speer Gold Dot is the ammo' par excellence for this gun, also included is CCI Blazer with aluminum cases - it uses the Speer bullet also. Functioning is best with these choices and also in my experience tumbling is very infrequent.

Some FMJ ammo used for practice does seem slightly more inclined to tumble at times .... as do some other JHP varieties. Again, this is infrequent enough for me to not be overly concerned.

I will point out though that there is considerable variation in ammunition I have sampled from different sources. For a start I find all too many bullets mic' up at nearer 0.354" than the desirable 0.355-0.356", the barrel major diameter being 0.355". Even this will reduce engraving, the lands being a minor diameter I believe, of 0.352"

9mm ammo examples and OAL's

Look also at the rounds shown in the image above - you will notice quite a variation in OAL. In fact, the CCI Blazer is the longest ... that dimension being almost identical also with Speer Gold Dots (not in this pic' I am afraid). Bullet ogives are not majorly that variable it seems and probably it is seating depth mainly governing these variations.

In my book, the longer OAL is less likely to provoke much tumbling - compare 1.124" of the Blazer (or Gold Dot) with the Winchester White Box 147's, which are 1.085" - the difference is all but 40 thou - that is in effect increasing the freebore which is already large.

This is enough I maintain to probably increase the tumbling possibility, as the bullet jumps the effectively longer gap before engaging the rifling, the case headspacing of course is a constant. Whether a bullet exits a case totally axially every time I very much doubt, and the longer it has to destabilize before entering rifling, the more likely is an asymmetrical engagement.

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