Freebore Boost in Rimfire Pistol

Longer gun barrels generally create higher velocities because they give a bullet more time to be accelerated by the propellant. (Detail post on the physics of this here.) Only in rimfire cartridges do we ever see the limit of this reached: After about 18″ of barrel a standard .22LR cartridge, which contains less than 1gr of powder, will start to decelerate as the friction of the bullet with the barrel exceeds the dwindling pressure produced by the small volume of propellant gas.

We’ve mentioned freebore boost before: This is the extra velocity that a bullet picks up passing through a suppressor. A suppressor is sort of a leaky barrel, so an inch of suppressor doesn’t boost velocity as much as an inch of barrel, but it can still add something. But does it provide boost even with the low propellant volumes seen in rimfire cartridges?

Here is a picture of the internals of a typical suppressor (here an Element 2 by AAC): It’s a series of baffles that vent the propellent into the enclosure as the bullet passes through. This .22 rimfire suppressor is just over 5″ long.

AAC Element .22LR Suppressor

I chronographed shots through a Buckmark pistol in three configurations:

  1. 4″ barrel
  2. The same 4″ barrel with 5″ suppressor attached
  3. 5.5″ barrel
Buckmark .22LR Pistol with 4″ barrel (installed) and 5.5″ barrel

I tested two different cartridges: CCI SV (which has 0.6gr of powder behind a 40gr bullet) and Gemtech (which has 0.8gr of powder behind a 42gr bullet). Here are the results:

Cartridge4″ Bbl4″ bbl + 5″ suppressor5.5″ bbl
CCI SV883 ± 4.7 (n=30)904 ± 3.7 (n=69)930 ± 5.3 (n=30)
Gemtech897 ± 9.3 (n=20)907 ± 3.9 (n=55)941 ± 4.3 (n=30)
Muzzle Velocity in feet per second (fps), with 90% Confidence Intervals

In both cases the freebore boost was significant. With CCI SV ammo the suppressor added about half as much velocity as lengthening the barrel from 4″ to 5.5″. With Gemtech’s load it added a quarter as much.

1 thought on “Freebore Boost in Rimfire Pistol

  1. RickB

    Intersting on two counts. That a longer barrel can actually reduce velocity. And that a supressor, despite being leaky, still provides extra barrel length to increase velocity. (Subject to the constraints mentioned above.)


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