Subsonic Trail Boss Loads
Date: Jul 31 2013
There has been a lot of chatter on Internet sites lately about centerfire loads that fire bullets at subsonic velocities. The advantages of such loads, the posters type, are low noise and an absence of recoil, and I’m all for that. So I started a search for subsonic loads for a few of my centerfire rifles to see if there was any substance to bullets at subsonic speeds. For the sake of argument, let’s say 1,100 fps is the speed of sound, and any bullet flying slower than that is considered subsonic.
Powders suitable to attain such a slow velocity are very limited in cartridges such as the .223 Remington, .243, .30-30 and .308 Winchesters and .30-06. Light amounts of relatively fast burning powders intended primarily for handgun cartridges, like 2400, Unique and Red Dot, come close but do not quite generate that velocity level. Loading heavy bullets with these powders, and a few others, lowers velocities even more, but then there’s the matter of suitable rifling twist to stabilize such heavy, long bullets at subsonic speed.
IMR’s Trail Boss is the only powder I’ve found that works to achieve subsonic velocities in a variety of rifle cartridges. The powder was designed primarily for handgun cartridges shooting lead bullets at low velocity. The powder also works well to produce slow velocities in rifle cartridges because its circular kernels with a hole in the middle are fluffy, and a charge occupies a good portion of a powder capacity in a rifle case. For instance, a minimum charge of Trail Boss for the .243 Winchester takes up about 60 percent of the .243’s powder capacity and about 75 percent of the .308 Winchester’s. That works as a safety factor, too, because it would be next to impossible to load a double charge of Trail Boss.
The Hodgdon Powder website (hodgdon.com) lists Trail Boss loads for rifle cartridges. I pretty much used the minimum charge weights for my five rifle cartridges to keep bullet velocities below 1,100 fps. The lack of noise was the first thing I noticed shooting Trail Boss in the .223 Remington. The report was not much louder than high-velocity .22 LR cartridges. In fact, the .223 was a bit easier on my ears, because it lacked the crack of the shock wave of a supersonic bullet. Recoil barely bumped the scope’s crosshairs off the aiming circle.
Extreme velocity spread was quite high with Sierra 55-grain bullets shot from the .223 Remington. One bullet with a low velocity of 951 fps hit about 2 inches below others of normal velocity, and one bullet with a high velocity of 1,118 fps hit about the same amount high. Bullets with velocities close to 1,000 fps, though, made a fairly tight group, under an inch, at 50 yards. Five of these Sierra bullets, however, will land in .75 to .50 inch at 100 yards when they are shot at a regular velocity of 3,200 fps with a maximum amount of Benchmark powder. That indicates these 55-grain bullets are at the ragged edge of stability fired from my Savage’s one-in- 9-inch twist barrel. According to the “Sierra Bullets Infinity” ballistics program, the 55-grain bullets at a velocity of 1,000 fps require a rifling twist of at least 10.82-1 to stabilize, so the spin imparted to them by my Savage’s 9-inch twist was enough; however, the spin from a regular one-in-12-inch twist of a .223 Remington is not sufficient.
Proper twist rate for subsonic velocities was really evident from the poor results with the .243 Winchester shooting 100-grain bullets. My Cooper Model 22 .243 has a one-in-10-inch twist barrel and always groups Nosler 100-grain Solid Base bullets under an inch at 100 yards when shot with a muzzle velocity of about 2,900 fps. With a muzzle velocity of 950 fps, the bullets hit the target sideways at 50 yards. According to the Sierra ballistics program, my .243 Winchester would need a twist of one in 7 inches to stabilize the Solid Base bullet at that slow speed. A lighter and shorter bullet, like a 75-grain hollowpoint, would just stabilize in my 10-inch twist barrel.