Inner Barrel Length and FPS Rule of Thumb?

[AnthonyG]

So, with all else being equal, a longer inner barrel yields a higher FPS than a shorter inner barrel of the same make and model. This is basic physics: the longer the bb is in the barrel, the more time it has to accelerate (assuming the correct cylinder volume), thus the higher the FPS.

My question is, has anyone figured out a general rule of thumb for estimating change in FPS per length of barrel? ie) Every 25mm of additional length yields an increase of 5fps, for a 6.01 barrel etc...

Right now I use two barrel assemblies for my primary to switch it from indoor legal (370fps) to outdoor legal (395fps), but both of my barrels are different makes and have different bores, and I don't have access to enough barrels of the same make to actually perform these tests.

[cetane]

Only works as a constant if you run 0.2's. Your indoor barrel probably out joules your outdoor barrel if you check with heavy bbs

[pestobanana]

Joule creep is barely noticeable on AEGs unless they are specifically built extended GBs with short barrels.

There is no rule of thumb because it is highly dependent on your cylinder voluming.

[AnthonyG]

My joules are consistent for both barrels.

If I assume a V2 gearbox on full Cylinder and 6.03 TB, does that change anything? It doesn't have to be a hard and fast rule, even just ballpark figures; what can be expected on the high end vs low end.

[lurkingknight]

there are a lot of complex factors in trying to calculate joule creep/overvoluming, so it's more time saving to just do it with trial and error. The progression is not linear and highly dependent on cylinder volume vs barrel volume, projectile weight and a lot of different factors on how the air charge is delivered. I've read about people trying it by just changing barrels and the chart was definitely not linear and they couldn't account for all the variables in the math to equal the end result values they were seeing. The only thing they could replicate was similar results, but not predict exactly what the results would be, so they know the effect exists, but don't have the math to solve it.

It comes down to if it shoots the correct fps with one barrel for one use, and the correct fps for another use, then you are ok, don't ask anymore questions unless you want your brain to explode.

[ThunderCactus]

If you want to switch from indoor to outdoor, run a 400fps spring and a barrel that runs at that muzzle energy.
Then for indoors just run a bit shorter 6.13 bore barrel to drop 50fps.
Much easier to drop fps by bore than length.

And the BB only accelerates more over a longer distance so long as theres pressure behind it. SO you can overvolume a short barrel and properly volume a longer barrel. But easier to just run a wider bore since the barrels will be closer to the same length.