Why do some barrels create higher velocity than others

[degreen]

I asked the same question only mine was on a factory Savage. I am getting just over 100 fps over what the reloading manual states.

 

[J E Custom]

Bore finish can only account for around 50 FT/SEC from rough to smooth. In fact you can get to smooth and lose velocity. The bore needs to be finished with a brush/honed surface to seal and not drag on the bullet. Don,t get me wrong, finish is important but not worth 200 ft/sec unless the slow barrel is realy bad. I have a realy bad barrel that you would have to see to believe, and I will show pictures as soon as i mill the barrel open for pictures and it was about 200 ft/sec slower but there is a lot more wrong with it than a poor finish. (More to come when I get the pictures).

Barrel finish has more to do with how fast a barrel will brake in and reach its maximum velocity with a particular load. One of the slowest (good)barrels i have seen had the Black Star treatment (polished so much it Looks like chrome Plating).

J E CUSTOM

 

[cohunt]

Has anyone mentioned the land to groove ratio? You can have the same twist, same #of lands/grooves but each barrel maker usually has their own "proprietary" land to groove ratio. Not all "5r" barrels are created equal.

Also, the heat treat/hardness can come Into play, and other "treatments" such as melonite can increase speeds too.

 

[Alex Wheeler]

Its in the geometry of the lands/grooves. Thats all I will say about it, but there are fast barrels and slow ones. You can tell by looking if you know what to look for. Top accuracy can be had whether fast or slow.

 

[Pdvdh]

Its in the geometry of the lands/grooves. Thats all I will say about it...

You're no fun, leave us hanging!

 

[cgarb]

boat racing comes to mind...people used to polish the bottoms of their boats to a mirror shine to reduce drag, that smooth flat surface was actually more surface area on the water...a dimpled finish like a golf ball created a turbulent "boundary layer" and reduced the surface the water came in contact with thus reducing drag and the boats went faster...not anything at all to do with gun barrels but the remark about a barrel being too smooth...sort of the same, just on a microscopic level.

 

[Duhawk]

Everything mentioned above can factor in....but what about powder burn rate? I've loaded to the book before and found velocities faster then expected. I adjust that in Quick-Load after shooting through a chrony…..and naturally that all changes once again when I run out of powder and change powder lots.

 

[Susquatch]

Many variables I would look at long before considering the barrel.

First off, no two rifles are likely to be the same. Also, reloading tables are just guides. If you have two manuals that both have the same velocities for the same load, I want to see them!
1. Scale accuracy
2. Brass water capacity
3. Chamber Dimensions
4. Primer
5. Bullet - theirs or some other manufacturer
6. Powder lot
7. Sizing & neck tension
8. Throat length and lead angle
9. Seating depth
10. Ambient Temperature
11. Twist rate
12. Rifling design (5R, number of lands, land/groove size ratio, land height, etc)
13. Bore dimensions
14. Moly vs Copper bullets
15. Chronograph Calibration and Setup
16. Bore condition (lapping, defects, etc)
17. Dozens more I can't think of just right now.....

Frankly, Rifles are like lovers. Every one of them is different. Vive La Difference! I wouldn't worry about it. Just enjoy!

 

[Deleted member 123574]

I have 3 brux barrels, I'm not sure why, but all three run a little faster than I thought they would. I'm guessing bore finish has something to do with it. Brux and Bartlein have been my favorites. I prefer cut barrels for no other reason than my limited background in metallurgy leads me to believe this is a better system. Maybe it has something to do with it.
My cut barrels seem to run a little faster than my buttons, but this is purely speculation, I have no idea if the process has anything to do with the speed.
All three shoot really good, as do all of my custom barrels, but I do have a couple barrels that seem really slow. As J E said, it could be a number of things.

I don't think that cut barrels have much to do with velocity, may be the manufacturer. I have a 30-06 shilen 25" barrel and 150 speer btsp bullet over 61gr. of h4350 is running 3184 fps. This load work up was online using imr reloading data.

 

[cohunt]

At least the thread is only 4 years old this time ;-)

 

[dogz]

At least the thread is only 4 years old this time ;-)

true that, however it's a good reminder that there is no free lunch when it comes to velocity....

as one of my best friends would say "pressure equals speed and speed equals pressure"