Methods
Tests were run with 77 grain Sierra MatchKing bullets to approximate a match load that I use with Varget. Loads of different powders were estimated using QuickLOAD V.3.9 (Hartmut G. Broemel) to achieve similar muzzle velocity and not exceed SAAMI pressure maximum (55,000 psi) for 223 Remington. Similar tests were run with 55 grain Hornady FMJ-BT bullets with two powders to approximate a practice load that I use with IMR 8208 XBR. Details on the powders used and QuickLOAD data are HERE.
Case Preparation
Once-fired Lake City 5.56 NATO brass (military range brass purchased in bulk; stamped 02 to 10) was prepared by primer removal, primer pocket swage with a Dillon Super Swage 600, clean/polish for 4 hours in Frankford Arsenal treated corn cob media in vibratory cleaner, anneal with Annealing Made Perfect Mark II annealer (cycle 0125), lubricate and resize with RCBS SB XL full size die for 223 Remington, trim case length to 1.76 inches and chamfer. Cases were primed with CCI 41 primers (21st Century Shooting hand press). Case volume was 30.56 ± 0.10 (Mean ± SD) grains H20 (n=10). Cases used in accuracy loads were Lapua 223 Match cases reloaded 6 times. Preparation used annealer cycle (0136), no swage, and volume of primed cases was 30.29 ± 0.10 grains H20 (n=10).
Powder Measurement and Bullet Seating
All extruded stick powders were dispensed/measured with an AutoTrickler V2 system (Adam MacDonald, New Brunswick, Canada) on an Entris Sartorius Balance. The AutoTrickler delivered powders within a 1-2 granule range of the target weight. One exception was dispensing IMR 8208 XBR for 55 grain bullets because the fine grain sticks jammed the older style dispenser system during the run. A Redding BR-3 powder dispenser was used for this one load (25.5 grain average; 25.4 to 25.6 grain range). Spherical powders (H335 and CFE 223) were measured/dispensed with a RCBS Charge Master 1500. Bullets were seated with a Redding Competition Seater Die for 223 Remington with a Redding T-7 Turret press.
Temperature Control
Five (5) cartridges each were enclosed in water-tight 50ml centrifuge tubes and brought to target temperature by placing tubes in Yeti Rambler 36oz vacuum insulated bottles containing water at desired temperature for at least 2 hours.
1°C: Centrifuge tubes containing cartridges and magazines were placed in sealed plastic bags and placed in a -10°C freezer overnight. Two hours prior to firing, centrifuge tubes (2 to 4 each) were transferred to the Yeti bottles that were filled with ice cubes and water that had cooled overnight. Magazines (metal – 20 round) were transferred in the bag to an ice container to keep cold until firing.
18-19°C: Centrifuge tubes containing cartridges at room temperature (about 24°C) were transferred to Yeti bottles and filled with water at 18°C. They were held for 2 to 2.5 hours to equilibrate, and water temperature was measured again with a mercury scientific thermometer (Fisher Scientific) immediately before removing the tubes to load and fire the cartridges.
38-39°C: A similar procedure to the 18-19°C procedure was used except the Yeti bottles were filled with water at 41°C
54-55°C: A similar procedure to the 18-19°C procedure was used except the Yeti bottles were filled with water at 61°C
Rifle, Chronograph and Accuracy Tests
Cartridges for temperature-sensitivity tests were fired through an AR-15 on a bipod with a 20 inch 1:7 twist White Oak Armament Varmint profile barrel chambered in .223 Wylde with an attached MagnetoSpeed Sportster chronograph using Hi 1 setting.
Accuracy tests at 200 yards were run at ambient temperature (80-81F) with a service rifle upper manufactured by White Oak Armament using a Bartlein 5R 20 inch 1:7 twist barrel with target crown. The Rock River Arms lower had a Geissele DSM trigger set to 4.6 pounds. The rifle was fired off a front rest with rear bag. The 4.5X service rifle scope was replaced with a Bushnell Elite Tactical 3.5-21X50 rifle scope with anti-cant level to minimize aiming error. Accuracy (MOA) was determined by eight (n=8) replicates of 3-shot firings at 100-yard bench rest targets (I.B.S. - The National Target CO.) at a distance of 200 yards. MOA (minute of angle) was calculated by measuring the widest shot-to-shot center for the 3 shots and adjusting for 100 yards. Mean and standard deviation of the 8 replicates was calculated for each load tested. Ten shots were chronographed immediately before or after the accuracy tests with the same ammunition..
Firing/Chronograph Sequence
Rounds were fired beginning at 8 am on the range between August 1 and August 19, 2021. Ambient conditions during tests ranged: 76-84F; 82-91% humidity; 29.81- 29.95 mm Hg at 147 feet elevation. Each day two rounds were fired through the rifle to prepare the barrel and test chronograph function.
1°C: A magazine at ice temperature was removed from the ice cold bag. One tube at a time was removed from the Yeti bottle, opened, and cartridges were dumped on a plastic lid. Rounds quickly were loaded by a Nitrile-gloved hand into the cold magazine and loaded in the rifle. Five shots immediately were fired within 12 to 15 seconds verifying each shot was measured by the MagnetoSpeed chronograph. The mean and SD of the 5 shots was recorded and each shot speed was recorded. Memory was erased after recording, and the next five cartridges were removed from tubes and loaded into a freshly unbagged cold magazine. Ten rounds were fired in 5-round batches for each powder-bullet combination at each temperature over 4 days.
18-19°C, 38-39°C and 38-39°C: Similar procedures were followed except magazines at ambient temperature were used.
Barrel chamber temperature was measured during and after each 10 round firing sequence for many groups and varied between 86 and 127°F.
High Temperature Barrel Chamber-Induced Changes in Velocity in Simulated Fast-Rate and Slow-Rate Firing Sequence
Barrel chamber temperature was measured with a ThermoPro TP-16 Meat thermometer. The metal probe was inserted as far as it would go into the chamber, and the tip was pressed against the chamber wall until the temperature reading was stabilized. Cartridges used were at 19°C (66°F) using the method listed. Cartridges were loaded with either CFE 223 or AR-Comp powder as described for the other parts of the study.
Rapid-Firing Sequence was fired at 1 shot per 5 seconds with the cartridge loaded in the chamber for 5 seconds. Slow-Firing sequence was fired at 1 shot per 30 seconds, with the cartridge loaded in the chamber for the full 30 seconds. An initial 10 shots were fired through the rifle at rapid rate before the rapid- and the slow-firing sequences were shot and muzzle velocities measured. This allowed simulation of the second 10-shot rapid-fire sequence at 300 yards and a mid-20-shot sequence of a slow-fire sequence at 600 yards. Because the test with CFE 223 was run first and the test with AR-Comp was run second with only a 20 minute barrel cool-down with chamber cooler, the temperature of the second test was run at approximately 15°F higher temperature than the first. Comparison of velocity effects was normalized as change in velocity (FPS) per 10°F increase in barrel chamber temperature above the cartridge temperature (66°F).
In order to estimate what the real world effect this would have on differences in bullet impact points at 300 and 600 yards, velocity difference were entered into a ballistics program as described below. Velocity difference between the baseline mean minus one standard deviation and the rapid-fire mean plus one standard deviation was calculated for CFE 223. Velocity difference between the baseline mean plus one standard deviation and the slow-fire mean minus one standard deviation was calculated for AR-Comp. This maximal variation in velocities for the two powders based on temperature-sensitivity and variation in velocity (SD) had effects on ballistic program predicted impact points.
Data Analysis
Hand recorded ambient and chronograph data was transferred to Excel spreadsheet to calculate initial means and standard deviations for muzzle velocity data (See example of Excel spreadsheet for 34°F). For each powder-bullet combination, muzzle velocity was compared among the three temperatures by one-way analysis of variance (ANOVA) with Holm-Sidak comparison test to identify significant difference in muzzle velocity between temperatures. SigmaStat V.3.5 was used for statistical analysis and SigmaPlot V.10 (Systat Software, Inc) was used to display data in graphs.
Impact points at 200, 300 and 600 yards for 55 grain and 77 grain bullets were estimated with a ballistics program (Ballistic: Advanced Edition 6.5.3, 2020 Peak Studios LLC) for velocities measured for each bullet-powder combination at velocities measured at 34°F and 100-102°F. The zero point was set at 10 yards in the program and bullet drop at 200, 300 and 600 yards was recorded for each bullet velocity.