When we fire a high powered rifle cartridge, our bullets trajectory(path) will experience three distinct phases. They are Nutation , Sleep, and Precession. To best illustrate the three phases of trajectory think of a child’s toy top. When spun it starts out wobbly(Nutation), then stabilizes and spins cleanly on its axis(Sleep), and finally begins to wobble again and eventually flips over as it destabilizes(Precession). Our bullet in flight acts the exact same way as the top as it starts out wobbly, then stabilizes, and then eventually destabilizes, but unlike the top in Precession, our bullet is never able to completely flip 180° like the top at the end of its cycle. It is important to understand these phases as a Precision Rifleman if you plan to engage targets through barriers such glass windows/doors. The reason being if we shoot through a barrier while our bullet is in the beginning or end phases of Nutation and Precession, then there is no way of accurately predicting which way our bullet will deflect from the impact of the barrier as it is impacting while it is spinning in a wobbling fashion.
In order to engage a target through a barrier such as glass with any degree of accuracy we must ensure our bullet impacts the glass during its second phase of trajectory, Sleep. When a bullet impacts the glass during Sleep, it is spinning cleanly on its axis, makes a smaller more precise hole in the barrier, and deflects less than during the other stages of flight. In order to ensure our bullet is in Sleep when we impact the barrier, we need stand-off(distance) from the barrier and target to engage through it. Most 5.56mm/.223 loads are asleep by 80m/yds, but longer and heavier bullets need more time/distance to go to sleep, so bullets such as the 7.62mm/.308 Winchester go to sleep around 100-125m/yds. Larger calibers such as the .338 Lapua or .50 cal BMG may not go to sleep until around 200m/yds, load dependent.
So the key to shooting through barriers such as glass effectively is that you need stand-off from it, at least 75-100m/yds; you do not want to be close. You also want the target to be within 10ft of the glass if at all possible. At ranges of greater than 10ft from the glass, it is difficult to accurately determine the deflection of the bullet and increases the chance of wounding or completely missing our intended target. Ideally we also want to engage a target through glass at a straight-on(squared) angle, as any angle of the glass upon impact can greatly affect the deflection of the bullet as well. Bonded core or solid ammunition works best when shooting through barriers such as glass, as many non-bonded or expanding bullets begin to open up or separate immediately upon impacting the barrier.
It is unlikely that a Precision Rifleman would have to engage through a barrier during the final stage, Precession, as the bullet begins to destabilize. The destabilization of the bullet in Precession occurs from torque on our bullet in flight by forces such as spin drift. The longer and heavier our bullet is, the more time/distance our bullet requires to go to Sleep, but it remains asleep longer and will also be less affected by spin drift. Lighter and shorter bullets go to Sleep faster but they also go into Precession sooner and are more affected by spin drift over time/distance.
John Hawes is a graduate of five sniper schools and many other military and civilian firearm courses with multiple instructor credentials. He is a highly decorated veteran of Afghanistan and Iraq and currently serves as Senior Instructor in the US Army Reserve. On the civilian side John is a certified gunsmith specializing in rifles. John is also an accomplished competitive shooter and hunter.