Barrel Efficiency, Tech Tip #1
Barrels are only there to accelerate the ball from a standstill
to 300 fps. In theory they also help with accuracy but that's another
post. The ball goes through incredible acceleration on its way down
the barrel. The balls acceleration rate is approx. 50,000 feet per
second to get to 300 feet per second in 10 inches. The entire barrel
travel time is about 6 thousandths of a second and this means the
ball is seeing about 1500 G's when its getting pushed out the gun.
Although this may sound incredible if someone out there would like
to do the math you will see that I'm close.
Air pressure behind the ball is what causes this acceleration to
happen. This pressure varies between the different guns but is generally
between 50 to 125 pounds per square inch at its peak. The air pressure
peaks right when the ball starts moving down the barrel, after that,
the ball moving down the barrel creates a bigger chamber so the
pressure drops. This is why low pressure guns are a myth, in reality
all guns shoot at considerably lower pressure than 200 psi.
Peak pressures above 150 psi tends to break balls down the barrel
due to really high acceleration and G forces. If you don't have
any way to control the peak pressure behind the ball, the only way
you can change it is to go with lower pressure in the air chamber,
hence low pressure guns. AGD uses the precise contour of the power
tube tip to release air in a controlled manner behind the ball to
limit peak pressures to around 60-80 psi..
It is simple to understand that the harder you push something the
faster it will accelerate and get up to speed in a shorter distance.
So what distance do we have to get the ball up to speed? The effective
length of the barrel is from the balls position before it's fired,
to the place in the barrel where the pressure gets released, This
is usually at the first porting holes or the step in the barrel.
Porting is there to release gas pressure!! You are effectively stopping
the acceleration at the ports so your 14" barrel that is half full
of holes only has an effective length of 7".
Now we understand that we need to limit the peak pressure behind
the ball to keep it from blowing up, and that the pressure drops
as the ball moves down the barrel. The next question we need to
ask is, how far down the barrel does the ball have to go before
the pressure gets to low to do anything useful? That answer is 8-10
inches. We know this from looking at the graphs that our gun dyno
puts out. If your peak pressure is higher, say over 100 psi you
can get away with a shorter barrel, if it's lower then you need
a longer barrel. Since AGD is the only gun manufacturer to actually
test their pressures behind the ball you might have a hard time
getting this info for other guns.
So as far as our guns are concerned, the best efficiency would be
had with an 8-10" effective length barrel. Since two piece ported
barrels with an effective length of about 5-6" are the rage right
now you hear a lot of complaints about gas efficiency. Under some
circumstances there is a good reason to use a short effective length
barrel. Short barrels cut off the acceleration abruptly by venting
and this has the effect of tightening up the shot to shot velocity
variation. If you need this at the expense of efficiency then go
ahead. Tighter velocity control usually translates into some improvement
in accuracy due to better consistency.
So if you want the best of all worlds, limit your peak pressure,
let your ball accelerate all it wants, don't follow the crowd and
keep asking questions.
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