TERM: Free Bore Boost
AKA: Velocity Increase
APPLICATION OF USE: “That silencer caused free bore boost!”
DEFINITION: When a silencer is used on a weapon and causes an increase in velocity.
INTO THE WEEDS: One of the most common questions I get about silencers is how they’ll change the gun. Usually the question is more in depth and revolves around accuracy changes, but generally I answer “every gun, every can, every barrel, every bullet is a little different and will react differently.” Some don’t like this answer, but it is what it is. One thing that usually occurs, however, is free bore boost. Free bore boost isn’t guaranteed, and will be experienced with some cartridges more than others.
Note the gasses that have blown in front of the bullet.
Free bore boost is a term used when talking about the increase in velocity of a bullet with a silencer attached. There are two working theories about what causes this increase in velocity.
- The first states that essentially what happens is while the silencer is doing its job of capturing the hot expanding gasses, these gasses are still acting on the bullet by propelling it forward and causing an increase in the velocity. This theory was coined by Doug Olson some years ago and is in my opinion the more feasible of the two (or at least it’s the one I can wrap my head around easier).
- The other working theory is that the gasses actually cause drag on the bullet and when the bullet and gasses exit the barrel into the silencer the drag is eliminated allowing the bullet to achieve a greater velocity. The best analogy I was given was to think of a boat on water; the boat glides along the water and causes a wake, which in turn is drag. If the wake/drag was eliminated the boat would run along the water seamlessly and achieve a faster speed. One thing that remains the same for both theories is that the bullet no longer has the effect of friction from the barrel.
In many cases Free Bore Boost is negligible, usually in the 15-30 feet per second (FPS) range. I have seen velocities increase substantially with certain cartridges, most notably .300 Blackout. The most extreme results reported were 100ish FPS.
It is important to chrono your rounds to know the difference in velocities with and without your silencer.
The best way to understand free bore boost is to test out the theory yourself. Use a chronograph and shoot with and without your silencer to get the average increase in velocity. For a hand loader who is trying to control as many aspects as possible, this information can be very valuable. If you’re shooting factory or issued ammunition it’s helpful to know an average increase, especially when calculating trajectory.
The last noteworthy point is that different silencers can also have different results. Differences in baffle design, mounting options, and silencer size all play a part on Free Bore Boost. One could also theorize that shooting a silencer larger than the actually bullet diameter (i.e. .30 caliber silencer on a 5.56 weapon) Free Bore Boost may decrease because the gasses blow past the bullet at 2-3x the bullet velocity and straight out the silencer.
IN SUMMARY: Free Bore Boost is an increase in velocity of a bullet when a silencer is used. It still may not be fully understood why this happens but it’s important for a shooter to know it happens.
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About the Author: Alexander Crown was the littlest big mistake Gemtech ever hired–and they even manage to continue to promote him to higher positions. Prior to his early retirement (caused by an enemy winning a marksmanship award at the cost of his hamstring), Alexander served in the 3/509 PIR out of Ft. Richardson and spent a little time in Iraq. In addition to dabbling in the world of silencers and science fiction, he has a BS in biology and is an avid gardener — like, the kind of gardener Hugh Farnham would appreciate and Alec Holland would envy.
Freebore is worth discussing and understanding within its technical context. Freebore is that portion of a rifle’s chamber between the case mouth and the lead/rifling. It is a critical factor in power generation with some cartridges, e.g. the .308 Winchester and most of the Weatherby family.
Freebore is easy to understand. Combustion builds pressure while the bullets “jumps” from the case mouth to the lead/rifling. The initial lack of friction as the bullet traverse the freebore results in a slower rise to peak pressure. The reverse is seating a bullet into the lands/rifling – this can substantially increase pressure for the same powder charge versus a bullet that “jumps.”
A real world example of this is .308 Win versus 6.5 Creedmoor. .308 Shooters commonly regard Varget as THE standard powder. Creed shooters often regard H4350 the same way. Varget is a faster burning powder and the .308 has roughly 56 grains of case capacity to the Creed’s 52.5 grains. The .308 uses its freebore to burn a larger quantity of faster powder at roughly the same operating pressure as the Creed.
Within the context of a suppressor, whether you get a boost and to what degree will be strongly dependent on residual pressure at the muzzle and suppressor design. Shorter barrels and less efficient powder burns result in a higher pressure at the muzzle during bullet exit. A suppressor moderates gas expansion upon the bullet exit and maintains some of the pressure within the system as the bullet traverses the “freebore” of the suppressor.