Why are there fast bbls and slow bbls

[Iron Worker]

Some bbls of equal lengths can shoot same identical load at different speeds ,why ? I have a 7mm wsm that shoots nearly 7mm STW speeds . Just today shooting 168gr Berger VLDs with bullet touching the lands 69.2 grs of Re#25 I was able to average 3250 FPS. No complaints of course just wondering why ? Its a 26" Brux bbl. 70.5grs of WXR pushed a 139gr SST at 3500 fps ( 3 Shots) but 100 yd group was greater then 2 MOA. 168 gr VLD shot under 3/4 MOA at 130 yds.

 

[Broz]

Yes, I do believe that some barrels are faster than others of equal length and twist. With in reason of course. But I think there are more chrono errors than anything fooling people. I have seen chrono's lie to me and by quite a bit. So I feel the difference in barrel speeds can be significant, but not 200 fps like some believe. I have also seen data showing some bullets of the same weight being way fster than others of equal weight. Again, I am not saying the shooters are fibbing, but rather their chrono is a lying little cuss.

Jeff

 

[strombeckj]

How the chamber was cut will also play a role. Custom rifles with tighter chambers have less brass expansion keeping the gases behind the bullet. I have a .223AI that shoots on avg. 300fps over factory or book loads. When I had this gun built I was told It would give 100-200fps more over a standard .223. Broz is correct on the chrono, try running through a friends also.

Jon

 

[Coyboy]

I have a pro crony that is long in the tooth and thought it was giving me bad numbers, and it may have been during bad lighting shooting sesions.

So I bought an alpa crony and set them up back to back,(during good lighting conditions. The alpa read on average 35fps faster than the pro, and very consistantly at 35 fps high.

But when bullets had low ES both cronys showed it, and when they had high ES both cronys showed it. So there acraccy that way was spot on.

Low batterys and bad lighting will play hell on them the pro crony gives you terrible number spreads and the alpa just says error.

as what was said before, chamber dimensions, lot of powder, lot of barrel steel,
land and groove configuration, bore and groove dia. all these and more can have an effect on velocity, and if the conditions are stacked in favor one way or the other a considerable velocity difference can be seen.

 

[Iron Worker]

Well that's was very informative. I know in the directions for my chronograph it was stated that if the screens are just 1/16th off they'll give a false reading. Yesterday I also shot my 22 mag and 44 mag through chronograph and it was spot on. BUT I'm still etching my head over performance with 139gr SST 70.5 grs of WXR produced 3x 3500 fps ? That's screaming fast ,but the accuracy was ( If I fired a few more shots) would of looked like a buck shot pattern. They don't make WXR any longer.

 

[MagnumManiac]

Well that's was very informative. I know in the directions for my chronograph it was stated that if the screens are just 1/16th off they'll give a false reading. Yesterday I also shot my 22 mag and 44 mag through chronograph and it was spot on. BUT I'm still etching my head over performance with 139gr SST 70.5 grs of WXR produced 3x 3500 fps ? That's screaming fast ,but the accuracy was ( If I fired a few more shots) would of looked like a buck shot pattern. They don't make WXR any longer.

I think you might be surprised to find that WXR was and is plain old RE22 just with a different label attached by Winchester, the loads are interchangeable.

 

[Iron Worker]

Yes that's what I heard .

 

[Loner]

I had an single batch of 22-250 rounds jump from 3350 to 3800 all due to setting
a barrel back and the chamber being tight. No real pressure signs but I did have to have
the chamber redone, the reamer was small in diameter just a hair. So with a 1" inch
shorter barrel speeds went through the roof all due to the chamber specs.

 

[Dano1]

Not only is chamber dimentions a big deal when dealing with speed diffrences, but so are diffrences in internal barrel dimentions, from one rifle/barrel to another. All things being equal every rifle is diffrent. Also depends on if the barrel is smooth or pitted. Cut, button or hamme rforged rifleing. there are many variations. Extreme diffrences most often can be the chronograph. Interesting, but true non the less, two chronographs are better than one and three are even better.

Dan

 

[lazylabs]

I think that people run a huge range of pressure differences. How as normal equipped re-loaders how do we tell the difference in 5000 PSI. I know there are some transducers that can be attached to read relative pressures but I have never actually seen someone use them. I believe a lot of wildcat loadings tend to be way over spec. Lapua brass doesn't make any round "faster" it just lets you run the pressure much higher before you see signs of it. So many people on here talk about running pressures up until your getting ejector marks or primer failure...... that's the edge of scary as far as I am concerned.

 

[Bart B]

Not only is chamber dimentions a big deal when dealing with speed diffrences, but so are diffrences in internal barrel dimentions, from one rifle/barrel to another. All things being equal every rifle is diffrent. Also depends on if the barrel is smooth or pitted. Cut, button or hamme rforged rifleing. there are many variations. Extreme diffrences most often can be the chronograph. Interesting, but true non the less, two chronographs are better than one and three are even better.

Having shot both button and cut rifled barrels with the same bore/groove and length dimensions, chambered with the same reamer, chronographing with the same lot of ammo showed only about 10 fps difference in velocity. That may have been caused by how I held the rifle for each setup.

Biggest difference I've seen has to do with the bore and groove diameters. The same lot of ammo with .3082" diameter bullets in a .308 Win. fired in barrels with bore/groove dimensions from .2970/.3065 up to .3020/.3090 showed a 240 fps spread in muzzle velocity.

More interesting is the velocity difference between two people shooting the same .308 Win. rifle/ammo from a bench. I've seen a 90 fps spread in average velocity between them. It had to be caused by how hard each held the rifle against their shoulder.