opinion.
If you work up a load in a 26" barrel, that will give you 3000 feet per second at the muzzle.
shoot that same load in a 28" barrel. how much velocity will you gain?
Ill use a 6MM Br as a reference. 105gr match bullet as an average bullet weight BC .500.
all and any.
Thanks Dave

At least 50 but probably no more than 100 fps.

My guess from the articles that I've read on this subject would be 30-50 fps gain.

In overbore carts. you would see a higher gain but in small cap. carts , like the 6BR, I doubt you'ld see 50fps.

Well, if the 26" tube has a good chamber and a tight throat, vs the 28" tube having a slop hole chamber and a loose throat, I'd expect the see the longer barrel giving up a fair # of FPS.

True story, I gained 250(ish) fps by removing 1" of barrel (from a 26 to a 25) off the breach end. Seems the first "gunsmith" liked to oversize everything, while the second kept everything trim and tight.

I think what everyone is getting at is there will be some change (my guess is ~30 fps/in), but without clocking them you will never know for sure what you are gaining. Some rifles as Tailgunner found have large chambers that make up for extra barrel lgt. Others have rough throats, sharper leades, ect that can alter things. I`ve two 260 Rems. 1 is a M700/24" bbl the other a M84 Kimber 22"bbl the M700 has 2" more barrel but actually gives velocities with the same ammo within 20 fps or so of the Kimbers, sometimes that much less.

Dependent upon the powder and projectile one chooses they could be almost equal .

Current rifle manufactures are coming down in barrel length because newer powders SC in particular are preforming as well in shorter barrels !. Old school barrels 26-28.5" , New barrels are now running 22.5-24.5" . With little if any loss of velocity .

Shoot Straight Know Your Target . ...