Longer barrels / increased velocity - is there a general "rule of thumb"?

The thing with pcps is...its tricky. The timing of the valve needs to match the projectiles timing in the barrel. It's not quite plug and play, so there may be some additional tuning required to really make the components sing well together.

Best thing is to chrony everything and write down the results. I have noticed with the increase in barrel length the fps increased but keep in mind this will differ from pellets to slugs. When i used pellets i saw a 30 to 45 fos imcrease, but with slugs I only saw a 5 to 15 fps increase. For the slugs I didn't really have to change anything other than my zero and I just wrote it all down for later use. Good luck on your endeavors. 

The thing with pcps is...its tricky. The timing of the valve needs to match the projectiles timing in the barrel. It's not quite plug and play, so there may be some additional tuning required to really make the components sing well together.

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Exactly. The weight/resistance of projectile has a +- variable for valve timing. 
I myself would absolutely love to see 24" to 32" barrels available on certain PCP's. 
That's never going to happen as I'm one of few who want long, heavy, traditional bolt action, single shot rifles. 

Look at a Crosman Maximus. They added a 26" barrel & a light hammer spring. Gets a ton of shots at 6 to12ft/lbs.

Drop a Discovery hammer spring in one. Add a power adjuster, & BLAMO!

You just made a excellent little hot rod shooter for heavier pellets or lightweight slugs. 
That platform would never be great for what we consider mid/high power due to the barrel to tiny 2 screw receiver. 
Look at a 17" Rainstorm in .22. Man o'live! If they offered the Rainstorm with a 24-28" custom twist barrel, it could send anything in that caliber meant for airguns. 
The reason the Korean power guns have a cliff dive IS that short barrel. 
It has a potential to be a great power gun with a flat shot string. 
It's so loud because it's set up to make big power from a relatively short 17" barrel.

Most airguns that make Rainstorm power levels use 20-24" barrels. 

Like wympanzee said. It's not that easy. You could gain 15-20fps per inch past 17".
Once you hit the 24" mark in a PCP, that's where big gains in power are made. A 26" barrel can gain 50-100fps over a 24" one. 

A springer on the other hand only needs a 10-11" barrel. They're longer for leverage in a break barrel. I cut & crowned a El Gamo 300 at 11" and gained 30fps. 
Now unless you have your valve, hammer weight, spring preload set up for the projectile in a longer barrel PCP, friction would take over at some point. You would see a loss instead of a gain.



I'm wishing someone would devise a repeatable vacuum system barrel. Could you imagine what a 2K-4500psi PCP could do if pellet was sent from a barrel that had no air resistance in it?


Edit: (It wouldn't have to be a ST; but there definitely would need a good seal from back of projectile or TP to muzzle)

It would have to be a Smooth Twist type. Can't have a vacuum without good seals. 
If you don't get what I'm saying just watch this.




https://youtu.be/9ydJXOTf1-4








Looks like I might get my wish. I bet first mainstream AG maker to do this is Daystate. Or FX. Hmmm

Or, just imagine if somebody like Air Arms ROSE UP! Invented a springer that beat most PCP's!!! Woah Dude, radical! 
Hey, I can dream. We might have to insert a new seal at muzzle for every shot but a springer that beats a PCP in power with a self sustaining system? 
Oh .177's would be a waste. We'd want heavy .25's or .30's. 

Best thing is to chrony everything and write down the results. I have noticed with the increase in barrel length the fps increased but keep in mind this will differ from pellets to slugs. When i used pellets i saw a 30 to 45 fos imcrease, but with slugs I only saw a 5 to 15 fps increase. For the slugs I didn't really have to change anything other than my zero and I just wrote it all down for later use. Good luck on your endeavors.

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If you take a 33.95 grain pellet, and the same exact weight cupped dish base slug. You'll still see a higher speed from the pellet.

That's because a pellet has very little surface area in contact with the rifling. A ridge at head & skirt.

A slugs cylindrical shape has much more mass/volume of surface area in contact with rifling. Thus creating more friction/resistance. 
Many guys are experimenting with very thin driving bands from custom molds they, or others are making.

There's been excellent results by doing this. 
Again, I think we will see slugs evolve for AG's. Hopefully we see a agreed upon specs format like SAAMI for AG barrels. Maybe but I doubt it anytime soon. The AG Industry tends to copy, not work together for the betterment of all.

Take off your moderator. If its ear piercing loud you will for sure see velocity gains. On my uragan going from 22” to 28” I gained 100fps with no other changes. On a brocock going from 23.5 to 27.5 I gained about 35 fps. So like others said it depends on your dwell time, etc. 

Ten to fifteen feet per second per added inch.

Over at GTA there is Bob Sterne, he's done some research on barrel lengths and how it affects power — and what the optimum length is for each caliber, and complicated but useful stuff like that. 

[Screen name: rsterne]

If you post over there, I'm sure he'll respond. He's got his own sub-forum, too, he's that good.



Matthias

It's inconclusive because the pressure in the barrel is decreasing.

So different guns have different amounts of Air.The pressure changes in the barrel are different.

So every increase in the same length of barrel. increased velocity will be different.

Generally speaking, the more powerful the gun with the same caliber, the more obvious of increased velocity when increasing the barrel

Ten to fifteen feet per second per added inch.

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There is no definite number

WONDERFUL info thanks

With fx airguns dynamic block, I've seen roughly 100fps per 100cm of barrel all else equal. Obviously test your own, but its been pretty consistent for me, a 380mm barrel firing a 55.5 slug out of the panthrra hunter at 730fps at max hammer on around 165bar. (Yeah I know it's not a heavy slug gun) Flip over to the dynamic 700mm barrel in .25 and we cruised on up to around 1050fps with the same NSA 55.5 grain slugs.

While the answers given have all involved PCP rifles, in the spring powered world they figured out long ago that velocity Increased with a shorter barrel. This is because most powerplants generated just so much swept air volume to push the pellet down the bore, after that friction began to slow down the pellet. 11 inches seemed to be a sweet spot, but could make it more difficult to cock with such a short barrel. I have an HW50S Hunter with a relatively short barrel and a moderator screwed on to the end. The barrel is 12 inches and the moderator adds another 6 inches which makes it easy to cock, but velocity wise it is on par with my FWB 124D, which has a more powerful spring, but a longer barrel. Always a lot of varibles....