AR-15/M-16 and Steyr Aug barrel rifling

[Deadman]

I recently returned from my first 3 weeks of Australian Army recruit training which of course means that I had the oppurtunity to learn about and use the Steyr Aug (or rather the Austeyr to be more exact).

Before conducting my recruit training I was well aware that the Aug wasn't really regarded as one of the best assault rifles out there (especially by TFL members ), however I supppose as long as its kept clean it will do its job.

My question is in regards to the rifling of the M-16A2 and what exactly happens to the bullet for the first 100 meters after it leaves the barrel. The reason being is that in the Aus. Army Austeyr manual itself it states that for the first 70-90m the bullet from an Aug has an unstable path and yaws slightly upwards, after that it levels out and stabilises. This of course requires the shooter to aim slightly lower at closer ranges.
The rifling of the Austeyr is 1 in 7 with a right twist, and I recall reading awhile ago about how the early versions of the M-16 had a rifling twist that caused the bullet to fly in an unstable fashion at times (meat axe effect etc), later versions of course had the twist altered to improve accuracy.
So basically is the yaw of the bullet from a Austeyr normal or not, as I sure as hell don't want to use a weapon that is inaccurate by design if I ever find myself in a hostile situation.

 

[Zak Smith]

I posted this last night, but it was lost:

If the bullet is spinning too slowly, enough to "tumble" in flight, the accuracy would be terrible. It would be like throwing a frisbee like a pie, instead of like a frisbee. Or throwing an American football with no spin. It's not going to work real well. So this is not happening.

If you have a rifle with slow rifling, for example, a 223 with 1/12" twist, heavy bullets are going to be unstable. As a general rule, longer bullets need to be spun faster to stabilize.

Let's assume your rifle is zeroed for 200 meters, and the sight line is parallel to the ground. The bullet starts off about 2" below the sight line -- the barrel is that far "below" the scope. Since gravity's acceleration always acts on the bullet, in order to hit at 200m, it must start with some upward motion, thus the angle of the barrel is slightly upward, compared to the sight line, which is flat.

Once the bullet leaves the barrel, it follows an arced path until it hits something. It will cross the line-of-sight first around 20 or 25 meters, and will cross again at 200 meters since it is zerod at that range. So the point of impact will be below the point of aim between 0 and 25 meters, above the point of aim at 25 to 200 meters, and below the point of aim at over 200 meters. This is why you need to aim "low" at under 200 meters (in this example). If you had the rifle zerod at 100 meters, this hold-under range would be something like 35 - 100 meters.

The last thing to consider is that since the barrel is angled slightly upwards with respect to the sight plane (which is level with the ground), the bullet leaves at, say, an 4MOA angle to horizontal. It is spinning fast, and has lots of rotational inertia, which must be conserved. Thus at long ranges, say 300 or 400 meters, the bullet will maintain this angle with respect to horizontal - think of a gyroscope. But this is really a minor point.

-z