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Author Topic: Big Bore FPS increase from longer barrel  (Read 212 times))

Offline TeddP

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Big Bore FPS increase from longer barrel
« on: May 22, 2022, 11:36:21 AM »
Roughly what FPS increase do you see per inch of additional barrel length, with all other variables constant?
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Offline rsterne

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Re: Big Bore FPS increase from longer barrel
« Reply #1 on: May 22, 2022, 05:39:14 PM »
Roughly speaking the FPE is proportional to barrel length.... so the velocity would be proportional to the square root of the barrel length.... There is no such thing as "fps per inch" because an inch on a short barrel is a much bigger percentage than on a long barrel, and fps increase at 800 fps is twice that at 400 fps, for the same percentage.... For example, if your barrel is 10" and you add an inch, that is 10%, so you should gain about 5% velocity.... However if your barrel is 24", your velocity gain at 25" long will be more like about 2%....

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Offline shortshell

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Re: Big Bore FPS increase from longer barrel
« Reply #2 on: May 22, 2022, 08:32:46 PM »
With my Texan .457 with the stock 840mm barrel with 3 shots each with an average of  330gn lead  gave 800 fps while another barrel of .457 but only 550mm but with a 1 in 15 twist gave an average of 680 fps using an FX radar chrono & air regulated at 2,300psi while tethered as well . The barrel changes took about 5-6minutes between tests. This worked roughly at 4fps per cm. This equates to around 10fps per inch lost. Accuracy with the shorter barrel at 50m was not as good as the stock one but it has a carbon fibre outer sleeve & a slower twist.
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Re: Big Bore FPS increase from longer barrel
« Reply #3 on: June 03, 2022, 01:16:00 AM »
Tedd,

How much velocity you might gain per inch depends on the muzzle pressure with the shorter barrel.  That, and projectile friction.   Pressure has to be more than enough to overcome friction.  The additional acceleration after friction is overcome, follows the formula F = m.a
Or for additional energy, force x extra distance; where force = average pressure down the extra length x bore area

You need to use average pressure over that additional inch, as pressure decays from breech to muzzle.   This leads to, how does one know what muzzle pressure is?  A clue is to calculate the average effective pressure from the FPE of the projectile with the standard barrel.  The average pressure describes the average driving force (force from air pressure - friction).    Then look at the reservoir or regulator pressure required to produce that FPE at that barrel length.  This does not tell us how long the valve is open, or how well it flows, but the projectile FPE provides a measure for that.

If you plot the above pressures on a graph you can get a good idea of what the muzzle pressure is.  If you are good with math, a regression study will  provide a better value.  If muzzle pressure is  1000 PSI, then adding a few inches of barrel should provide a useful increase in FPE.  If the muzzle pressure is 200 PSI, a few inches of extra barrel are likely to disappoint.

Now, even in the last case, you may be able to retune for more valve opening or dwell, to take advantage of the longer barrel, and get a useful boost in power.  This, at the expense of greater air consumption.  If the muzzle pressure on the standard barrel was high, that represents an air waste that you could harness, to achieve higher FPE at the same air consumption.  Or the same FPE at lower air consumption, via a retune.
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