Temperature Effects on Zero

Posted June 24 2011

For over three hundred years man has endeavored to find his target with the first shot from his rifle, whether it be man or beast, in conflict or at peacetime. This skill in the art of marksmanship is one of the most pondered and talked about subjects in the Military & Law Enforcement communities today. There are facts, opinions and myths covering a wide range of elements in the gap between the point at which the firing pin strikes the cartridge primer, to when the projectile strikes its target.

There is no escaping the simple fact that this subject is scientifically founded. However, this mathematics and physics soaked area can be explained and broken down into understandable segments that allow the Sniper, Marksman and Precision Shooter to dramatically increase the probability of achieving the ‘First Shot Hit’ whether it be from 200 metres or 2,000 metres.

This paper examines the effect that temperature has on bullet placement from a rifle. More to the point, it examines the effect variations in barrel temperature has on the rifle itself, not the ambient air temperature/air density relationship, and not the ambient temperature/powder burning rate relationship, but purely the effect that barrel temperature may have on rifle-zero.

1. Introduction

The rifle used was an expensive, high-grade sniper rifle utilizing a match grade barrel and an inert pillar bedded or ‘bedding blocked’ stock. The trigger has near zero Lock Time and the telescopic sight is arguably the best that money can buy. The ammunition was either precision hand-loaded or marked match grade by a reputable company and line. The ammunition has been chronographed at specific velocities over a wide temperature range. The rifle was zeroed from the 100yd mound at the local rifle range and consistently prints tight groups. The average ambient temperature was 30 °C (86 °F).

At the rifle range three weeks later, when the same rifle/scope/ammo configuration was used at the same distance, using exactly the same shooting position, the first round struck the target low by about 36mm (1.42 inches). The second round printed roughly in the same place and the third a little higher. The barrel had not been cleaned between shots.

When this happens, it can be very easily written off by the shooter as the result of one or more causal factors, such as, the sights must have been knocked, it must have been a bad shot or my shooting position must have been off.

However, what if the scope wasn’t knocked, and it was a technically fine shot and the shooting position was exactly the same. After several years of this type of erratic shot placement, one would have to start questioning the weapon and ammunition rather than the shooter.

In this day and age of increasing technology, many urban myths are now able to be confirmed as either having significant underlying truth, or simply debunked as random opinion. People are now starting to ask questions such as, ‘I hear you telling us that your first shot from a cold, clean barrel prints low on the target, but what have you done to demonstrate that this is really the case?’

The most common answer is ‘Every time I come out to the range, the first shot I fire is at 100 yards from a clean/cold barrel and it shoots somewhere here. The second shot goes closer to the bull, and the rest of them go straight through the bull from then on’. Does this sound familiar? This person probably hasn’t recorded, to the millimeter, every cold shot for the previous 50 shots, recorded the ambient temperature each and every time that shot was made, nor chronographed the same batch of ammunition at various temperatures to be able to interpolate a powder burning rate.

2. Temperature and Trajectory

A change in temperature can affect the trajectory or ‘flight path’ of the bullet in two well-known ways:

So long as altitude, barometric pressure and humidity remain constant, an increase in air temperature will cause a flatter trajectory due to a lower air density (less collisions with ‘air particles’ per unit length of flight path).

The same increase in temperature also causes the nitro cellulose based powder inside the cartridge to burn at a higher rate, producing approximately four times the Point of Impact (POI) shift than just air temperature alone.

Just how much does an increase in temperature affect the powder burning-rate? Some powders are more susceptible to temperature effects than others and will burn faster than others. Some powders will experience a burning-rate

increase of 3.5 feet per second (fps) for every 1 °C (1.8 °F) increase. Others will be more resistant to heat and may only have an increase of 1.5 fps/1 °C.

The .308 Win. Federal Premium cartridge with a 175 grain (gn) Sierra Matchking (Gold Medal Match) fired from a Blaser R93 Tactical 2 Sniper Rifle will show a muzzle velocity increase of approximately 2.5 fps for every 1 °C

increase in cartridge temperature. If there is a 10 °C (18 °F) temperature increase, this will equate to a muzzle velocity increase of 25 fps. This increase in velocity will change the POI at 100 yds by approx 4mm (1/6 inch), not counting the lower air density.

Notwithstanding these observations, there are also other factors that cause a much greater change in the POI at this range with changes in ambient temperature.

3. Factors

Two of these possible factors were eliminated from the equation by means of field trials involving three different rifles (chambered in .308 Win.) held in May 2007 at the WA Police Forensic Ballistics Section, Midland Western Australia.

The first factor to be addressed was the belief in some quarters that ambient temperature variations caused changes in the internal diameter of the bore of the weapon and therefore caused changes in in-bore friction, back-pressure and burning-rate, all of which impact upon muzzle velocity.

The second factor concerned possible velocity changes between a clean bore and a fouled bore.

An Accuracy International AWP, Blaser R93 Tactical 2 and a Remington 700, all chambered in .308 Win. were subjected to a series of tests.

4. Measuring Equipment

Equipment used in these trials for determination of muzzle velocity as well as the temperature of air, weapons and ammunition were as follows:

4.1 Kestrel 4000

The US made Nielsen-Kellerman Kestrel 4000 weather station (S/N 499206) was used for measuring the ambient air temperature and also the temperature of the environment that the test items were placed in.

4.2 LiMiT T90 Infrared Thermometer

The Scandinavian made LiMiT T90 infrared thermometer (S/N 7058160) was also used for measuring the weapon systems and ammunition. This device was used in a manner whereby several readings were taken from a weapon system to avoid incorrect readings from shiny surfaces.

The Kestrel 4000 and the LiMiT T90 can be seen in Fig. 1 below. The Kestrel has an accuracy of +/- 1 °C and the T90 has an accuracy of +/- 2 °C.

4.3 Oehler 35P Chronograph

The Oehler 35P was used in a 50m indoor test tunnel and was placed 8m in front of the muzzle of each weapon when tested. Muzzle velocities were calculated to the muzzle using Sierra Factory Ballistic Coefficients and JBM Small Arms Calculation tables from the web. (http://www.eskimo.com/~jbm/ballistics/traj/traj.html).

The Oehler 35P can be seen in Fig. 2 below.

5. Hot/Cold Test

All three test rifles were heated in a calibrated Forensic oven for 45 minutes to reach a temperature of 50 °C (122 °F). Three rounds of the same ammunition type were

fired through each weapon and their velocities recorded by the Oehler 35P Chronograph.

This test was then repeated with all three weapons at room

temperature (21 °C/70 °F) and again at -20 °C/-4 °F. The lower temperatures were achieved by using ‘dry ice’ and these temperatures ranged +/- 5 °C.

All ammunition used in this range of tests was kept at room temperature, which ranged from 19 – 21 °C. The ammunition was fired within a 3 second period after being chambered to avoid heat transfer between rifle and powder charge.

The results showed virtually no change in muzzle velocity with the same room temperature conditioned ammunition fired through these weapons, at three very different temperatures. The velocity changes were +/- 6 fps, sometimes in the opposite direction. This change could only be absorbed by the error allowed in the Oehler 35P Chronograph and the variance within the same batch of ammunition.

6. Clean Bore/Fouled Bore

Due to time constraints, only the Remington 700 in .308 Win. was used in this test. This rifle had been fitted with a custom Stainless Steel Heavy profile Diamond Lapped barrel from a reputable Australia company.

This rifle was first cleaned thoroughly using Hoppes Number 9 Solvent before the test. Ten rounds were then fired through the weapon in order to foul the barrel.

Ten more rounds were then fired through the Oehler Chronograph and logged. This rifle was then cleaned for approximately 25 minutes in order to remove all powder and copper residues from the throat, grooves and lands.

A further five rounds from the same batch of ammunition were then fired through this weapon at the same temperature, utilizing a bore scrub and two solvent patches between each shot.

The results show a difference of 6 fps on average. This would equate to a 1mm change in POI at most at 100 yds.

The results from these two tests indicate that there is no appreciable change in muzzle velocity when the weapon has either been cleaned, or subjected to a massive change in temperature.

From this we can now deduce that a change in weapon temperature itself does not affect the velocity of a projectile enough to warrant a significant change in the POI at 100yds. Neither does the clean barrel versus a fouled barrel.

Figure 3 Results recorded from the Oehler 35P Chronograph at a distance of 8m from the weapon muzzle.

7. Final Test

7.1 Temperature Conditioning

On May 17, 2007 in Perth Western Australia a series of tests were done in order to find out the reason for unexpected changes in the POI shift from zero with temperature changes.

To facilitate this test on a day measuring 18 °C (64 °F), a mobile freezer truck was hired. On this particular day, all test ammunition was held at ambient air temperature (17 ­19 °C). The same Kestral 4000 weather station as mentioned above was used for measuring the air temperature, as was the same LiMiT T90 Infrared Thermometer for weapon temperature. A motor vehicle with the heater set on the ‘recycle’ mode was used to heat weapons to 45 °C (113 °F).

All weapons, when either heated or cooled, remained in the controlled environment for 30 – 40- minutes to ensure thermal equilibrium.

The three weapons used in this test were the Accuracy International AWP and AWF Series in .308 Win. and the Blaser R93 Tactical 2 Rifle in .338 Lapua Magnum (LM).

All three rifles were fired at -5 °C, 7 °C, 21 °C and 45 °C respectively.

Each time the rifles were removed from the vehicle’s freezer, one round was fired from a steady rest (no bipod) within 3 seconds of the round being chambered. This was to avoid heat transfer issues between the chamber and the propellant charge.

7.2 Accuracy International AWF Sniper Rifle 7.3 Accuracy International AWP Sniper Rifle Manufactured: 2003 Manufactured: 1997

This illustrated a significant change in the POI at 100 yards using ammunition of the same batch at the same temperature. The circle used in the target above is 50mm (2 inches) in diameter. The vertical stringing effect is not caused by a change in velocity, but by another factor as shown.

Total POI change of 2.75 Minute of Angle (MOA) or

0.77 Milliradians (USMC) at 100 yds.

This illustrated that the AWP Sniper Rifle also has a significant change in the POI at 100 yards using ammunition of the same batch at the same temperature. The magnitude of the distance between the groups was slightly less, although still very significant. To accurately display this, Figure 7 was constructed as shown below.

Total POI change of 2.40 Minute of Angle (MOA) or

0.67 Milliradians (USMC) at 100 yds.

7.4 SIGARMS Blaser R93 Tactical 2 Sniper Rifle Manufactured: 2006

This also shows the .338 LM Blaser Tactical 2 also has a noticeable change in the POI at 100 yards when using ammunition of the same batch, at the same temperature. The distance between the groups was significantly less than that observed for the AI series rifles. However, the POI

zones from hot to cold weapon was completely reversed. Rounds fired from a hot weapon printed low whilst those fired from a cold weapon printed high.

0.45 Milliradians (USMC) at 100 yds.

Total POI change of 1.75 Minute of Angle (MOA) or

8. Discussion

8.1 Weapon Differences

Before any deductions can be drawn from this testing, some facts should be revealed about the differences between each weapon.

8.2 Accuracy International AWP

The AI AWP Sniper Rifle used in this test was manufactured in the South of England in 1997 and this particular weapon exhibited the following characteristics;

Stock: AI issued Polymer Barrel: 24″ Stainless heavy profile Cut-Rifled Receiver: Standard AI alloy Scope: Nightforce NXS 3.5-15 x 50 Mounts: Leupold Mk4 Ultra

8.3 Accuracy International AWF

The AI AWF Sniper Rifle used in this test was manufactured in the South of England in 2003 and this particular weapon exhibited the following characteristics;

Stock: AI issued Polymer – Folding Butt Barrel: 26″ Stainless medium profile Cut-Rifled Receiver: Standard AI alloy Scope: Nightforce NXS 3.5-15 x 50 Mounts: Leupold Mk4 Ultra

8.4 .338 LM Blaser Tactical 2

The Blaser R93 Tactical 2 Sniper Rifle used in this test was manufactured in the Germany in 2006 and this particular weapon exhibited the following characteristics;

Stock: Injection moulded Polyamide Barrel: 23 ½” Fluted Chrome Molybdenum

Hammer forged Receiver: Barrel extension acts as the receiver Scope: Schmidt & Bender PMII LP 4-16 x 42 Mounts: Custom XTEK Ltd one-piece bridge.

8.5 Telescopic Sights

To discount the theory that telescopic sights may have played a part in the shift in rifle zero, a further testing was conducted on this day.

Each of the test rifles was used to fire a three shot group at 100 yds when initially cooled to -5 °C. These rifles were then fired to the side of the target using 10 rounds of ammunition as quick as the firer could load the weapon. This was done to heat the barrel and receiver up to in excess of 50 °C.

A three shot group was then fired from each weapon. The resulting groups printed in their respective ‘hot weapon’ areas, however, the scopes on each of these weapons were still cold from the freezer truck.

9. Temperature Change Prediction

The resulting data collected from the three rifles tested, was mapped on a temperature scale. To complete these scales, POI shifts were interpolated between the recorded temperatures according to the trends of each weapon.

Zero change for the AI series of rifles was not constant between the hottest and coldest recorded temperatures. In fact, between 26 °C and 45 °C, the POI change was approximately ¼ MOA for every 6 °C (11 °F) increment. Once temperatures dropped below 26 °C, the POI dropped

off quicker. Below the 15 – 20 °C mark, there was a ¼ MOA shift for every 3 °C change.

However, zero change for the .338LM Blaser Tactical 2 rifle appeared to be constant. Although the other way around, the interpolated data showed a 1/3 MOA shift (ca. 10mm) for every 10 °C (18 °F) temperature changes. This equated to 1cm click for every 10 °C.

Temperature/Rifle Zero prediction charts were drawn up for threes three weapon systems based on the observed data.

Blaser Tactical 2 with S&B Scope (1cm/Click)

10. Confirmation

On February 29, 2008, nine further rifles were tested for temperature zero variation at the Holsworthy 300m classification range, Sydney Australia, using the same methodology outline above.

The rifles consisted of two Accuracy International AWP models, six Blaser Tactical 2 models and 1 custom Remington action with a fluted stainless barrel fitted to an

AI stock. All rifles were chambered in either .308 Win. or

7.62 NATO.

The AI rifles displayed exactly the same results as shown in the May 2007 testing and so did the Blaser Tactical 2 Rifles, even though they were chambered in .308 Win. rather than .338 LM.

The custom Remington Rifle displayed similar results to the AI’s (Hot high and Cold low), however not as much spread was recorded.

11. Conclusions

The primary aim of the tests conducted between 2007 and 2008 on the effects of temperature on rifle zero was not so much focused on why this effect happens, but whether an effect exists in the first place.

There could be any number of reasons as to why temperature variations on the rifle body would change the POI on the target, however, a further time consuming series of tests would be required for the actual mechanism(s) to be elucidated. Taking into account the small variation in trajectory with ammunition temperature variation (powder burning-rate), there is a larger and more noticeable POI variation that affects the precision shooter in today’s Military and Law Enforcement Sniper community. The fact is that temperature variations do cause POI variations, and that to some degree, these variations can be interpolated and mapped.

To the best of the author’s knowledge, there is currently no portable ballistics calculation system that takes this factor into account. At the moment, it is entirely up to the shooter to compensate for these variations. It is precisely because of this type of variation that the ‘cold barrel zero’ is practiced by some of the most elite elements.

For the seasonal shooter, or the special operations sniper operating in extreme environments, this can not only eliminate a section of doubt in himself or his weapon, but can also improve his chances of achieving that all important ‘First Shot Hit’.

Acknowledgments

Would like to thank Sgt G. Roberts Western Australia Police TRG Sniper Cell Maylands WA 6051(Author), Senior Constable Clive Roberts of the Western Australia Police Forensic Ballistics Section, members of the Western Australia Police Tactical Response Group Sniper Cell and finally Dr Alexander Krstic MD, Spectre Ballistic Solutions Pty Ltd, Adelaide, South Australia.