Barrel speeding up

[BallisticPrimate]

I've noticed reports suggesting that it is common for barrels to pick up speed after a couple hundred rounds. I'm interested in what kind of speed increases people have noticed and at what point in their barrel life? The reason I'm asking is that I used my friends Magneto Speed to record the initial velocity of my 6.5x47, it now has 150rds down the pipe and my dope (using drop scale factor) suggests nearly a hundred fps increase (2862 - 2959 with 130JLK's). This seems significant and I'll remain doubtful of the results until I'm able to use the Magneto again.

 

[Seandradamus]

I would say average increase for 6.5x47 is from 40 to 80 fps within the first 150 rds.
my 6.5 saum gained 150 at about the same.
6 creed gained about 60
223 gained 50
this is what I have experienced.

 

[Delicatessen]

My 260 picked up 50-75 fps (higher pickup on the heavier 143/147 gr than 130 gr)

 

[Bacarrat]

My Bartlein 6.5 SAUM picked up about 100 FPS after a couple hundred rounds through the barrel.

 

[Juggerxxx]

Tikka 223 increased 50fps when I finally had a chance to check again after 300 rounds.

 

[HKpirate]

After a couple hundred rounds my 6 comp match picked up about 75fps. Really tossed a wrench in my system as I was working up a new load for rdf's as much as I'd love to run 105s at 3250 it's not feasible for prs.

Sent from my SM-G935V using Tapatalk

 

[FourT6and2]

My 6.5 Creedmoor LOST about 25-30fps according to Magnetospeed. But DOPE at distance didn't change. So somethings screwy going on there.

Bartlien barrel.

 

[samb300]

I just broke in my new 24" Bartlein 5R in 6.5x47L. My "break-in" load went from 2635fps to 2675fps over the first 150 rounds, so +40fps. I won't be shooting that load anymore, but I'm interested to see if my match load (2725fps) speeds up at all from here.

 

[Vargmat]

My Shilen barreled 6,5x47L has been very stable. Ive fired close to 800 rounds so far and there really is not much of a difference going on.

 

[lash]

My .260 increased almost exactly 100 fps...suddenly...during a Chrono session at about 130 rounds. It has stayed fast since then with over 1000 rounds.

 

[BallisticPrimate]

My .260 increased almost exactly 100 fps...suddenly...during a Chrono session at about 130 rounds. It has stayed fast since then with over 1000 rounds.

Interesting, what brand of barrel, mine is a lothar Walther

 

[lash]

This one is a Rock Creek. One I bought and hung on to for a couple years before spinning it in.

Barrel speeds vary, even within the same brand and caliber. Just the small sampling above is a taste of what to expect.

I suggest that you verify the speed you now have, tweak your load if needed now, and enjoy the shooting.

 

[lash]

Matter of fact, I was also surprised and a bit unsure when it happened, because of the suddenness of it. I even started a thread similar to this. Not because it sped up, but because of how quickly and it happened and how much the jump was.

 

[BallisticPrimate]

Matter of fact, I was also surprised and a bit unsure when it happened, because of the suddenness of it. I even started a thread similar to this. Not because it sped up, but because of how quickly and it happened and how much the jump was.

I'll certainly take more speed for the same case pressure - do we have a theory as to what causes the speed 'jump'?

 

[FourT6and2]

I'll certainly take more speed for the same case pressure - do we have a theory as to what causes the speed 'jump'?

I have no idea how guns work. But my best guess, based on absolutely zero knowledge, is that maybe a little copper and carbon fouling builds up, and decreases the inner diameter of the bore. This maybe increases pressure a little bit? I have no clue. If this were the case, though, it would be that when you clean the bore and get all the build up out, speeds would go back down. But I don't think that's happening, right?

 

[BallisticPrimate]

I have no idea how guns work. But my best guess, based on absolutely zero knowledge, is that maybe a little copper and carbon fouling builds up, and decreases the inner diameter of the bore. This maybe increases pressure a little bit? I have no clue. If this were the case, though, it would be that when you clean the bore and get all the build up out, speeds would go back down. But I don't think that's happening, right?

Yes that is what is interesting about the phenomenon - once the speed increase occurs it seems to plateau. I wonder if there is a difference between button and cut rifled barrels. Perhaps one of the barrel makers on this forum can chime in

 

[Niles Coyote]

I've noted an increase of 2-4% across many of my barrels after 100-200 rounds and a couple cleanings. Both cut and button rifled.

 

[Delicatessen]

I am pretty sure they why is due to basically micro polishing small imperfections left in the machining process of the barrel. The reason it reaches an equilibrium is that once done any more wear would just worsen the pressure seal effect and slow the barrel (but the bullets don't keep eroding like that because the nature of the tolerance fit and bullets have finite plasticity)

 

[FourT6and2]

Maybe I'll reach out to one of the barrel manufacturers and ask this question. Maybe they'll write a short article about it for the main SH page.

 

[dirthead1]

I have a 7mm Rem Mag with 25" Krieger barrel. At 30 rounds down the barrel, it shot 180 Berger Hybrids over 71.5 gr Retumbo at 2977 fps. At slightly over 100 rounds, it shoots the same load at just a hair over 3100 fps. Velocity is the same with virgin, 1 x fired, and 2 x fired brass. No pressure signs with virgin brass, very slight pressure signs with 1 x fired, and severe pressure with 2 x fired. Needless to say the load is too hot and will be backed down, but the barrel gained about 130 fps over 70 rounds.

 

[BallisticPrimate]

Maybe I'll reach out to one of the barrel manufacturers and ask this question. Maybe they'll write a short article about it for the main SH page.

I think there would be quite a few people interested in this

 

[FourT6and2]

I think there would be quite a few people interested in this

Ok, well I'll make the rounds and see what happens

I reached out to Bartlein, Krieger, and Hawk Hill.

 

[FourT6and2]

Frank, from Bartelin Barrels, was kind enough to offer up some insight. I'm posting this with his permission:


"The biggest thing that makes the change [in velocity] I'm going to say is a combination of two things. Some fouling getting built up into the bore of the barrel (it's very hard to get the carbon build up out and why some shooters consider clean and what they see are two different things) and the throat breaking in.

So carbon build up and the throat breaking in to me are the main reasons. That's my opinion and also feedback I get on test barrels.

Each barrel is going to be an individual. How nice the reamer left the throat when the barrel got chambered will determine how long it takes for the throat to polish and break in. To me in a good barrel this is the only real thing that you break in. If the reamer leaves the throat really rough it's going to take longer to polish and break in."


He also mentioned ammo from lot to lot (if you're shooting factory ammo) and/or each individual's loads are big variables. If one hand loads, maybe there's a change somewhere along the way.


"How the gun/barrel is being cleaned. A big problem is shooters are not always cleaning the gun properly and or often enough or maybe I should say on a regular basis. As an example a shooting a gun in .308win. vs. a .300wm. Yes the same bore size but the .300wm has almost double the case capacity. So with more case capacity you will increase the amount of powder fouling. If you don't keep up with the powder fouling it will for the lack of a better explanation make things to an extent make the barrel tighten up especially in the throat/chamber are and this will drive up pressures etc....Same goes with a .308win. vs. a .243win. basically the same case capacity in regards to powder but by reducing the bore size from .30cal. to 6mm you've in effect have turned the .243win. into a magnum type round."

"Bullet sizes/diameter vary. Possibly from lot to lot and from maker to maker. As bullets are made the bullet dies will wear and as they wear the bullets tend to get fatter."

"I'll give you two personal examples. My 6x47 Lapua even when new and after x amount of rounds thru it.....from a clean barrel the first two rounds will be 30-40fps slower then the rest. I attribute this to the barrel wanting some fouling in the bore and then it settles right down. At 100 yards that little velocity difference is virtually unseen on the target but at a 1000 yards if my wind call is on those first two rounds will be a 9 and 10 out the bottom. So at distance I will not make a elevation correction until a couple/few or so rounds are on the barrel.

The next example is my F Class gun in .284win. From day one that barrel has not changed velocity higher or lower as of yet. Weather it's clean or dirty and I've clocked velocity thru a Oehler Chronograph from the very first round fired. Every once in a while I will run that gun thru the chronograph watching for changes. It hasn't yet and has 400 rounds on it now.

Why the difference between the two guns? Your guess is as good as mine. Keep in mind the different caliber and or bore sizes. I'm shooting different types of powder as well as I'm shooting different bullets thrum not to mention different lots.

I will also say most barrels if they are going to change velocity wise will usually do it in the first approx. 100 rounds. Your going to get some fouling build up and also the throat is going to break in as well. Then usually the barrel will settle down. That's not me just saying that but also data I've received back from ammo makers using ammunition test barrels in controlled conditions.

I also on average I try not to let my guns go more then 100 rounds in between cleanings. Most high power rifle type matches each day you are shooting around 50 to 100 rounds thru the gun. I will clean my gun after each day of shooting. So that is my baseline. My gun has to go at least 100 round in between cleanings and hold accuracy etc....​​​​​​​"

 

[Code]

My take on this- when a quality barrel is new, it is lapped smooth and linear with the rifling. As you break in a 6MM SLR (as an example) the throat will become "tiled" or "flame checked", and as this happens what we see as the "cracks" are actually displacing material making the bore and groove dimensions slightly smaller at the same time the friction coefficient is changed by the rougher bore that is the result of the cracks- this effect is worse with overbore cartridges. This in turn creates more pressure as the bullet obturates into the lands and grooves- exaggerated by the powder pressure curve compounding- accruing faster velocities for the first bit of barrel life. The same thing is demonstrated in reverse when adding lube (friction modifiers) to bullets such as moly or boron nitride- lubed bullets are typically slower in velocity with the same weight powder charge.

The barrel will eventually erode enough throat material as the round count increases to start reducing pressures, and resulting velocities fall too. Now there are other factors to consider here too, such as if the barrel was rough to start with, you may see little to no upward swing in velocity- only a gradual decline as the throat gets eroded further. Another contributing factor is case efficiency- you will see much more velocity swing in overbore cartridges such as 264 Win Mag, 6.5x284 Norma, 243 Win and the like- especially shooting bullets with long bearing areas like monolithics. More efficient cartridges such as the 308 Win don't flame check as badly, often eroding smoothly in the throat and never getting rough to create as big of velocity swing.

Just my theory. YMMV