The Long Range Effects of Muzzle Velocity Variation
Cutting costs by using a low quality rifle propellant causes variations in muzzle velocity, which unbeknown to many shooters, is often the reason why a good rifle and good bullet loses accuracy as long ranges come into play.
When the mechanics of a weapon system are running like a well-oiled machine, shot after shot, your rifle has the potential to send your bullet on a truly satisfying trajectory all the way from muzzle to target. But why do we find so many rifle owners scratching their heads in puzzlement when long range success is beyond their bounds of possibility?
It often comes down to inconsistency in muzzle velocity from one shot to the next. Even a rifle that is grouping sub-MOA up to 300 meters may experience complete misses beyond 700. The variation in velocity may be too much, as the effects imposed onto the bullet at longer distances become increasingly sensitive.
While there are a number of factors that contribute towards shot consistency, this article will focus primarily on the quality and suitability of propellant. Our aim is to outline the importance of using a high quality and consistent rifle propellant when long range accuracy is desired.
The Effects of an Inadequate Propellant
Now let’s explain how an inadequate propellant (or propellant load) can cause a shooter to chase his bullet in frustration around a distant target with no success in sight.
High quality factory ammunition, or high quality components used during handloading, will produce a standard deviation closer to 0.
Many shooters measure this by firing 5 or 10 shots over a chronograph, and then calculating the standard deviation of that string of shots. A standard deviation closer to 0 indicates that the muzzle velocities tend to be very close to the average, meaning they’re very consistent. A larger spread between muzzle velocities of a shot string will produce a poor consistency in standard deviation, or a larger deviation between shots.
Higher quality propellants designed for long range shots are more consistent in their burn rate, resulting in far more consistent velocities.
The Effects on Trajectory
Now let’s use a real world example to demonstrate the effects of consistent versus inconsistent muzzle velocities as distance grows. The table below has data input taken from two rifles chambered in .308 Winchester, using the same primers, casing, and bullet, but two different extruded rifle propellants. Hylton’s propellant is very popular amongst the hunting community, while Matt’s propellant costs twice the price and is popular amongst long range precision rifle owners. The bullets used during this experiment were Hornady 168gr ELD Match. Both rifles are capable of achieving sub-MOA groupings at 100 meters.
|Shot # 1||2389fps||2416fps|
|Shot # 2||2415fps||2416fps|
|Shot # 3||2424fps||2410fps|
|Shot # 4||2406fps||2416fps|
|Shot # 5||2406fps||2412fps|
|Muzzle Velocity (Average)||2408fps||2414fps|
We will keep the brands of propellant used during this test anonymous, as the cheaper powder that Hylton used is more than capable of providing good results to the majority of hunters who shoot within their range limitations. However, we can see by the above results that Hylton is recording a far greater spread in velocity between shots, which in turn reduces his shot consistency. So what exactly does this mean and why is this important?
Note: the following excludes point of impact deviation due to the rifle’s grouping capability, but rather displays how a new trajectory will form as new muzzle velocities take place. We will assume a target size of 45cmx45cm for this example.
- 100m – the spread will be insignificant. Our ballistic app shows us that at 100 meters, neither Hylton nor Matt will notice any difference in bullet impact caused by their velocity inconsistencies.
- 300m – the effect is still minor, with Hylton’s bullet experiencing up to 2cm of trajectory deviation between shots, while Matt’s will have no noticeable effect.
- 500m – Hylton’s shot may deviate as much as 8cm, while Matt’s may only experience a shift of 1cm at most.
- 700m – achieving hits on a torso-sized target is now becoming very difficult for Hylton, as the deviation is up to 20cm between shots, while Matt is experiencing a maximum deviation of only 1.7cm.
- 900m – consistent engagements for Hylton at 900 meters is now impossible, as he is experiencing an uncorrectable vertical displacement of up to 40cm between shots, excluding the MOA grouping capability of approximately 26cm. Matt’s deviation between shots taken at 900 meters may experience a trajectory shift of only 3.5cm at most under the current conditions.
Let’s take this example one step further. Hylton engages a target at 900 meters, while the bullet leaves the barrel of his rifle travelling at 2389fps. He spots his fall of shot impacting the dirt 50cm below his intended point of aim. He then adjusts his turret to move his point of impact 50cm high, and engages with a second shot, this time producing a muzzle velocity of 2424fps. The shot will now have jumped at least 40cm high of his intended point of impact. We can see how a shooter can very easily end up chasing shots at longer distances when velocities are inconsistent.
Let’s Sum it Up
While this may all sound a little confusing to a newbie, the important take away point is that muzzle velocities need to be consistent, or close to one another between shots, when long range engagements are your goal.
To ensure consistency between shots, there are a number of things to look out for.
- The first and most obvious is the quality of the propellant being used. High quality rifle propellants are typically tubular in shape, and have very consistent burn rates. When fed through a powder dispenser, it can be difficult to produce consistent measurements, and they are therefore best measured manually.
- Cartridge cases must also be of high quality, as cheaper brands can vary in thickness from one cartridge case to the next. This variation in case-wall thickness will alter the internal pressures and affect velocity between shots.
- Lastly, bullets must be of Match quality, measuring the exact same length from base to tip, therefore producing consistent cartridge lengths and internal pressures.