Barrel speed up

[odoylerules]

This subject came up in another thread of mine and I decided to make it it's own subject here. Lots of guys talk about how their barrels will speed up after 100 or so rounds. I'm relatively new to this long range game, so I'd like to understand more about this phenomenon. Does this happen with most barrels or only certain types? Does cleaning regimens have any effect? When this does happen do you guys reduce your powder charges to get back down to the velocity node that you originally developed?

 

[Canhunter35]

This is why it's a good idea to break a barrel in before serious load development

 

[MTLIVIN]

I'm not a barrel making expert, but my engineer brain tells me the only possible answer is the coefficient of friction is slightly increasing up to or around the 100 shot mark. To get higher velocity with everything else the same, you need more friction and thus more pressure. With hand lapped barrels this makes perfect sense to me, cheap factory barrels I'm perplexed. I've experienced this on all of my new barrels now that I actually track velocity on regular intervals.

 

[Schnyd112]

Same experience over three cut rifled barrels. Not so much sped up, and definitely not a linear curve, but the end result was more speed than when I started. The issue has been inconsistency for the first 150 or so rounds.

To combat this, I have started trying several bullets for the first 150ish rounds with mild loads. Your barrel will tell you if it doesn't like the bullet at 100 yards.

Then, by the time everything has settled in, I have a pretty good idea of where to start serious load development and what bullets I didn't like.

Factory barrels I am usually more worried about copper so I use a different approach.

 

[jpd676]

I have a Criterion 7mm mag that went from 3000pfs to around 3150 fps after 150 rounds. I also have a 700 Long Range 300 rum that went from ~2800 to 2970 after 90+ rounds. In the Criterion I did back the charge down because the node at 2950 was super accurate. In the rum I got a node at the higher charge. I have read that cleaning conditions you chamber and barrel and helps speed up the break in process (sorry, can't find the article for reference).

 

[BEEMAN]

My RBROS 6.5x47 did exactly what Travis said it would do. After 200 rounds it sped up 75 fps. It has a Broughton Barrell. I use Shooter for my drops so I just used the correction factor section to bring my drops back to where they should be.

 

[brentc]

It would be nice to say that once the new barrel break in process done, the barrel is settled and ready for load development, but I've never had it happen like that. The barrel will settle when it settles with or without a break in process and it occurs at some point at or before 200 rounds.

 

[brentc]

Here is a blog entry from Sierra that explains it a little, not so much the "why it happens", but more the "It happens. Prepare to deal with it"

https://sierrabulletsblog.com/2017/03/15/velocity-increase-in-new-gun-barrels/

Here is a topic from another forum that discusses it as well.

http://forum.accurateshooter.com/threads/tracking-velocity-changes-in-a-new-barrel.3782963/

 

[Jim See]

I'm not a barrel making expert, but my engineer brain tells me the only possible answer is the coefficient of friction is slightly increasing up to or around the 100 shot mark. To get higher velocity with everything else the same, you need more friction and thus more pressure. With hand lapped barrels this makes perfect sense to me, cheap factory barrels I'm perplexed. I've experienced this on all of my new barrels now that I actually track velocity on regular intervals.

pretty good assessment, physically a NEW lapped barrel has a finish from abrasive that creates many peaks and valleys in the micro finish of the barrel running from end to end. In essence the bullet is riding on the tip of the peaks. As the number of bullets down the barrel increase these peaks are rounded off relatively quickly, until they hit a point of wear that they are less affected by each passing bullet. At this point the surface area of contact between bore and bullet is increased thus increasing the pressure and velocity of the given load assembly. This is happening on the microscopic level of the surface finish. This is also the reason abrasive bore paste should be used sparingly, excessive surface smoothing can create more pressure heat and copper galling.

 

[AZ82New]

pretty good assessment, physically a NEW lapped barrel has a finish from abrasive that creates many peaks and valleys in the micro finish of the barrel running from end to end. In essence the bullet is riding on the tip of the peaks. As the number of bullets down the barrel increase these peaks are rounded off relatively quickly, until they hit a point of wear that they are less affected by each passing bullet. At this point the surface area of contact between bore and bullet is increased thus increasing the pressure and velocity of the given load assembly. This is happening on the microscopic level of the surface finish. This is also the reason abrasive bore paste should be used sparingly, excessive surface smoothing can create more pressure heat and copper galling.

I 100 percent agree with this. I believe I ruined a Rock Creek barrel with Semichone and Bore Paste. I had become convinced that I had the allusive "carbon ring" based on increased pressure signs from a proven load. I turned to the forums and that's where I became convinced of this. I think in all likelihood my neck tension was too high and the brass was giving out after a couple firings. So, on the forums, I was instructed to Semichrome the crap out of that barrel. Which I spent over an hour doing. Things got worse, so I used bore paste the same way.

I was literally blowing primers on factory nosler ammo. My gunsmith couldn't figure it out.

I should also mention I was getting extreme velocity increase, like a 100 FPS in some cases. Accuracy stated relatively the same.

I'll never really know but this explanation seems the most plausible.

I'm extremely vanilla about cleaning now. That bore is sacred ground…..don't go fiddling with it excessively.

 

[BallisticsGuy]

In my experience it's the first 10-20 rounds in a quality aftermarket barrel that's been lapped and what you'll see is a continuous speed up and then things will plateau rather suddenly. The barrel is then broken in. In rougher barrels like you would expect from most factory rifles it stops speeding up within the first 100 rounds. The factory .338LM barrel (Lothar Walther IIRC) on my desert tech took 20 rounds before it settled down. The Columbia River Arms provided 6XC barrels on my match rifles took only 10 rounds before they settled down. The Short Action Customs finished and Bartlein supplied .223 barrel for my desert tech took an astonishingly low 5 shots to settle down. The factory .308 barrel that came on one of my Savage rifles took nearly 50 shots to settle down. It would seem to be at least involving "coppering in" but I would be willing to bet that frictional wear is not involved almost at all given the hardness differences between copper and steel.

 

[manitou]

It would be nice to say that once the new barrel break in process done, the barrel is settled and ready for load development, but I've never had it happen like that. The barrel will settle when it settles with or without a break in process and it occurs at some point at or before 200 rounds.

Yup. My self build 280ai w/Bartlein barrel gained about 75 fps right around the 200 rd mark, like somebody flipped a switch.