Studying Barrel Drift (Structured vs Conventional Barrels)
What is the most effective way of testing barrel drift? Can barrel drift be mitigated without the use of prohibitively heavy barrels? To what degree is heat the largest contributing factor to this phenomenon. Of all the testing that Coastal has done over the last few years, this may be the most important one. Developing a way to test barrel drift in real life scenarios is critical for determining the effectiveness of a precision weapon system. If one minute of angle is required as a standard, that 1.0 MOA should be maintained throughout an engagement or course of fire. A weapon capable of MOA groups that has significant point of impact drift over 20 shots may not be suitable for a given scenario. Quantifying drift may also serve as a significant improvement on evaluating the quality of a precision weapon system.
It is very important while testing barrel or point of impact drift to make every effort to isolate the different contributing factors when possible and identify factors that can't be isolated. For the purposes of this test, heat was one of the primary factors of concern. Care was taken to balance any variance between test weapons to ensure the validity of the data collected. The main variance of concern was the bore conditions of the test weapons.
With this in mind all weapons were cleaned and fouled with an equal number of fouling shots. Initially the test was performed with two 6mm Creedmoor rifles, a Ruger Precision Rifle and a custom rifle with a TacomHQ Structured Barrel chambered by Collier Rifles installed on a Defiance action. During this test the Structured Barrel outperformed the RPR to such a degree that data looked a little unfair and the test was ran again to confirm the results.
For the second version of the test the structured barrel was pitted against a Bartlein 6.5mm Creedmoor. Both barrels were chambered to the same action via a West Texas Ordinance Switch Lug. This removed a great deal of variance from the test. Both setups now consisted of:
high quality hand-lapped barrels
mated to one action
load developed (Berger OTM Hybrids 105gr & 130gr)
consistently capable of <1/2 MOA
Both loads averaged single digit SDs throughout entire test
identical APA muzzle brakes
action remained torqued in one chassis throughout test
Schmidt and Bender PMII remained mounted throughout test
One Jewell Trigger used (14oz)
The procedure for this test was very basic. The weapon was fired from a standard concrete bench from an Atlas bipod and Protektor bench rest style rear bag. One shooter fired for both barrels and the rate of fire kept to one round approximately every twelve seconds with a digital metronome and stop watch. All targets were identical CPC targets with precise bench rest aim points calibrated in MOA grids. Each target had ten points of aim. Five shots groups at each aim point brought the total continuous group round count to 50 rounds. Barrel temperatures were measured with a lab quality IR gun in between each 5 shot group. All ammunition was stored in thick walled styrofoam coolers for the duration of the test. Each group was measured with optical software to determine the precise offset of the group's center from the point of aim. Group size and chronograph data was also recorded to assess any degradation throughout the unusually long strings of fire.
As was the case with the initial RPR v. TacomHQ test, the Structured Barrel outperformed in practically every metric: smaller variation in point of impact, lower sectional temperatures, longer heat cycle, and lower variance in velocity through out the test. The graph below shows the plot for the relative movement of the center of each 5 shot group. The blue and gray plots show the large drift produced by the RPR which amounted to more than an MOA of drift horizontally and more than 3/4 MOA vertically. The orange plot of the medium Palma Bartlein showed nearly 1.0 MOA of vertical drift and just under 1/2 MOA horizontally. The Structured Barrel from TacomHQ showed less than 0.4 MOA drift in all directions. These plots represent four instances of 50 round shot strings and one additional 25 round shot string!
Clearly, the Structured Barrel experienced less drift during these long strings of fire. Within the conditional context of this test it is reasonable to assume that much of the drift is due to heat or thermal dynamic stress affecting the axis of the bore or the harmonics of the system. All things, as equal as they can be, the Structured Barrel setup would see more than half a minute of additional accuracy. During a long string of fire or long engagement, this system would maintain a higher hit probability than a conventional barrel.
Heat measurements were also taken during testing. The conventional barrel reached temperatures of 180°F while the Structured Barrel remained 17°F cooler. What is even more interesting is that the highest temperature observed on the conventional barrel was at the muzzle, while the Structured Barrel's highest temperatures were consistently recorded at the midpoint of the barrel. It is possible that this plays a role in consistent accuracy in longer strings of fire. This may also explain why the structured barrel maintained more consistent SD's in the latter portion of the test.