The fun thing about all of this is the number of variables at play, every time you squeeze the trigger.
However, before we start;
1. The throating: To the end, SMLE barrels carried over the long throat necessary to accommodate the old Mk6 “torpedos”.
2. The internal dimensions of the barrel: As noted previously, as per the specifications on the original drawings, the BORE spec was tightly controlled around the nominal .303” figure. However, GROOVE diameter could run out to .320 and STILL be in spec.
So, we have a bullet that is smaller in diameter than the possible groove diameter; thus what we potentially have is what is called in modern parlance, a “bore-rider”. Yes, the bullets DO certainly engage the rifling, as can easily be seen by the fact that recovered projectiles are clearly engraved with the five lands and the fact that you generally hit your target (or at least give it a nasty fright).
HOWEVER, it is a bit more “interesting” than that.
Upon ignition, the bullet is bashed forward into the start of the rifling. With any luck, this engagement of bullet and rifling will be concentric.
Now, if your bullet of choice is like the classic Mk7, the slightly concave, flat, but open base WILL expand slightly to provide some improvement to the gas-seal.
If, however, you have chosen unwisely and taken the boat-tailed “pill”, things are a little different. The short cone of the boat-tail directs a much greater proportion of the super-heated gas AROUND the sides of the bullet. Now, if the bullet were a close fit to the nominal GROOVE diameter, this would be a lesser problem. HOWEVER, given the difference between the GROOVE diameter and the actual BULLET diameter, there will be gas “leakage” and thus gas CUTTING of the throat and leade.
If the bullet construction has less than PERFECT concentricity of both the jacket thickness and core insertion, its centre of mass will not be coincident with the notional aerodynamic centre. Inside the barrel it will tend to spin around a centreline somewhere between that of the bore and of its own mass.
Once the bullet gets to the muzzle, things start to get interesting. The “pointy end” passes the muzzle without any drama, BUT as the rear of the bullet exits, things get exciting.
A flat based bullet exiting a perfectly cut crown will cause the propellant gases to erupt in a “perfect circle” around the base of the bullet. At this precise time, these gases will not just flow sideways at “ludicrous speed”, but being a fluid (with mass), rush FORWARD, past the bullet, thus potentially affecting its stability to some degree.
In the case of a boat-tailed bullet, this gas “blow-by” takes place over a comparatively longer time AND acts on the wedge-shaped boat-tail. Thus, there is potentially more scope for destabilisation. This is on top of the fact that boat-tailed bullets have, for a given mass, a shorter bearing surface than a flat-based bullet, so their initial alignment in the bore is more “fraught”.
What happens next is that after the bullet clears the muzzle, the tiny (one hopes) difference between the actual centre of mass, the aerodynamic centre line and the ACTUAL rotational axis come into come into conflict. In a vacuum, the bullet would rapidly settle down to spin on its centre of mass, regardless of shape. Once you add the real-world annoyance of the air we breathe, it gets more complicated.
Aerodynamics come into play rather unsubtly. Oh, and then there is GRAVITY.
At launch, the bullet, if supersonic, forms a compression cone at the front and another, smaller shock cone at the rear. These are the principal sources of drag in supersonic flight.
Under the influence of Gravity and Drag, the bullet is losing speed and altitude from the moment it leaves the muzzle. The trajectory is pretty much a parabola.
However, all is not (generally) perfect. The previously mentioned precession caused by tiny discrepancies between the various “centres” now comes into play. The bullet is NOT traveling on a “perfect” parabola, but rather, precessing (spiraling around this nominal flight path). The “size” of these spirals is a part of the explanation of “groups” vs “one-holers”. Given that a “typical” rifle bullet is spinning at several hundred thousand revs per minute, it is a miracle of engineering that they hold together at all.
Finally, consider that whilst the rotational velocity does NOT decrease very much as the bullet goes downrange. LINEAR velocity does.
Now comes the fun part.
We now have a bullet spun fast enough to achieve stability at any practical temperature and air density on the face of the planet. Due to drag, as the range increases, "linear" velocity decreases. The outrageous initial rotational velocity does not decrease much at all, and thus, the spinning bullet tries to stay aligned with the original line of departure at the muzzle. What this means is that, at SEVERAL intervals along the "parabolic" trajectory, the pointy end will no longer be exactly aligned at a tangent to the curved trajectory.
Eventually, aerodynamics wins a brief struggle with rotational inertia and the bullet, already "precessing" jumps back into line (more or less).
The new "stable state" is very likely to NOT be exactly on the original trajectory: it could be to either side of it. The more "excessive" the rotational stability, the longer it will take to re-stabilise.
If your barrel was elevated 15 degrees to land bullets at some particular range, then the bullet will try to keep pointing at 15 degrees, until such time as the aerodynamics overcome it. If this happens in tiny increments at short intervals, the relative grouping deviations will be small. If your bullet is OVER stabilised, the corrections will be fewer and MUCH bigger as you travel down-range.
Mk7 bullets are only spun at their speed because of their length. The bullet is within a whisker of the same length as the heavier, round-nosed Mk6 that preceded it. The higher muzzle velocity of the Mk7 means, of course, a correspondingly higher rotational velocity, and thus a completely different set of down-range dynamics. I suspect that sticking with this “traditional” length was deliberate and meant that there was no urgent requirement to re-engineer EVERY .303 weapon in the inventory to reliably feed a shorter projectile.
(Anorak back in closet)…………
