How does chopping a barrel alter velocity nodes?

[itchytriggerfinger]

I understand that. What I'm trying to figure out is if there is any data on what is the most efficient length of tube for a given cartridge/barrel length. At point point does pressure peak and start declining causing diminishing returns on barrel length?
What would it benefit me to carry a rifle with 26" of barrel if I could get the same velocity out of 20"or 22"?
I knew when I cut off 4" of my 308 that I was gonna give up velocity but since I was dedicating that rifle to subsonic use that losing 4" would be irrelevant. I also know that were I to add 4" to barrel length it would not necessarily mean that I would see an increase in velocity once powder was consumed and pressure was on the downward, so where is that point? Can it be calculated?

 

[steffen707]

I can tell you I couldn't figure that bullet out in my 7 rem mag. I may try a different powder because I want that 168 to shoot. My tikka 7 mag is digging the 162 eldx bullets.

nice! that was probably my 2nd bullet choice. I heard the eldx doesn't hold together at short distances....only heard one report of that so guess i should do more research before shun it.

 

[steffen707]

Correct. Someone can get lucky and have their old load work, but in general, when you make a change like that it's starting over on load development.
because the barrel was shortened I'm not going to be in that same velocity node because the projo is not moving in that velocity range anymore. Also the barrel harmonics and complelty different as well.

that's what i had figured before I posted my question, but thought maybe there was a chance it was more like a particular powder charge that caused the node. This is going to be a fun foray into hand loading I think. I'm going to work up a load for the 24" barrel, then chop it to 18" and do a new load. If it performs satisfactorily, then i'm getting a new 18" barrel threaded for 5/8-24 and then i'll have to redo the 18" load a little for the new barrel/chambering.

 

[steffen707]

I understand that. What I'm trying to figure out is if there is any data on what is the most efficient length of tube for a given cartridge/barrel length.

Poke around on this site, they have chopped several catridges. I think they discuss your exact question on their 308 chop.

7mm Remington Magnum (7 Rem Mag) Barrel Length Versus Velocity

7mm Remington Magnum (7 Rem Mag) Barrel Length Versus Velocity Since its introduction in 1962, the 7mm Remington Magnum (7 Rem Mag) has developed a strong following, becoming the most popular metri…

rifleshooter.com

I believe they have articles on all these catridges
7.62x39
223
224 valkyrie
308
6.5cm
6cm
243 win
7mm rem mag
300 win mag
338 lapua

 

[Doom2]

Considering only pressure and velocity the bullet will continue to accelerate until the force exerted by the gas pressure equals the frictional and drag forces acting on the bullet. The length varies for each caliber and bullet type as well as powder and for all practical purposes is longer than the barrel lengths that are manageable.

For practical considerations a shoulder fired rifle in excess of 10lbs begins to become unmanageable, and longer barrels also unbalance the weapon making it muzzle heavy.

So in practice the most efficient length is determined by factors unrelated to the velociy/pressure relationship.