Home Forums Hunting & Shooting Ask The Gunwriters New barrel vs a broken in barrel and changes in velocity

Alvaro,

To be clear, I'm not making a definitive statement about the issue, one way or the other. For me to have a high enough level of confidence to make a statement on the matter, I'd have to witness the phenomenon in a statistically significant sample size of barrels where the pertinent variables were controlled, such as barrels that were as identical as possible, loads that were as stable as possible, etc. So I'm just reporting my observations, not stating that they are more or less valid than anybody else's observations.

Jordan,

As noted earlier, I'm interested in this partly because I'd never heard of it before--and partly because I dunno why it wouldn't happen with factory barrels, some of which are of VERY high quality these days.

Am going to look at more of my loading notes and see what's there, with both good factory and custom barrels.

John

I appreciate your effort John B.

Because I experienced this in 2 different rifles, both new and both shot at least 150 rounds, I thought my chronograph got out of whack as stated in a different thread.

I started to Google search the velocity increase and came across many instances where a velocity increase was "claimed". I am anxious to see what your written records show.

Here is an example of my Google search. There are others but it is easy to cherry pick.

http://bulletin.accurateshooter.com/2018/06/tech-tip-velocity-increase-in-new-gun-barrels/

Sakoluvr,

Thanks for the info, especially the link.

Don't understand why the "smoothing" of the throat would result in higher velocity, because in general less resistance to the bullet's passage (especiallyl in the throat area) results in LESS pressure, and hence velocity. They are pretty much tied together, due to basic physics.

But have done considerable research in various writings on benchrest accuracy. One guess is that the increase might occur due to bullet seating-depth changes as the barrel's broken in--which affects pressure/velocity considerably. This isn't mentioned in Speedy's article, but it's apparently pretty common. All my tests used exactly the same load, including seating depth.

The other factor that might apply is how much many powders vary in velocity with temperature. This isn't as much a factor in benchrest shooting as it used to be, due to far more temp-resistant powders.

I’ll offer this - how a bullet is imprinted by the rifling actually occurs in the throat area and different bullets imprint differently based on hardness and other factors, so a new barrel with a rough throat ?

Well, yeah--but why would a "rough" throat result in lower pressure/velocoty? Why would a smoother throat result in more velocity, which according to the laws of internal ballistics means higher pressure?

And again, am baffled about why this would only apply to "custom" barrels and not factory barrels.

I’m not saying this is it BUT in a freshly cut throat the edges of the lands would be nice and sharp and imprint the bullet easier (cut vs. press) than a broken in throat where the lands were more rounded. Somewhat like a serrated knife blade cuts a softer surface (like tomato) vs. a non started blade which pushes on the surface but doesn’t cut and displace as well as the surated.

So the pressure to imprint could be lower with a sharper / ruff land than with a rounded off one...

Again - just postulating

Me neither, and I really value your systematic approach.

I just wanted to point out how different people would share so different opinions or observations on the same subject. I say opinions or observations, not scientifically driven conclusions after examining a statistically relevant sample size and I apologize if my post read different.

Alvaro

Please keep us updated, John.

Just a quick thought, similar to what Spotshooter mentioned, if the freshly cut throat has edges/tooling marks that minimize surface contact with the bullet, I would expect to see lower pressure caused by bullet engraving. As the marks in the throat wear and become more smooth, surface contact would increase, thereby increasing pressure and velocity. As the throat wears toward the end of the barrel’s life, the bullet is jumping so far to the lands that pressure from bullet engraving drops, and velocity starts to decline with it.

I don't see where it is stated that it only happens in custom barrels. The link I posted happens to be with custom barrels. My Google searches indicate it happens to both factory and customs.

I am convinced it happened to both my Bartlein and my factory Hells Canyon.

Edit: I see where Jordan said it was more pervasive with customs. Maybe folks with customs tend to be more apt to check velocities, hand load and keep up with such things.

I said I haven’t noticed the effect as much with factory barrels, but that could partly be because I haven’t tracked them with good record-keeping as closely as I have my custom barrels. That’s not to say that I haven’t noticed it at all in factory barrels, but not as often nor as much of a velocity increase as I have seen in my customs.

But again, these aren’t hard facts, just my casual observations.

I was thinking my barrels got a little faster. After looking at some examples I could find of shooting the same loads when it was new vs. 100s of shots later, it seems if anything it is a little slower. I can't be sure it's apples to apples data with the same chrono, same temp, same powder scale, etc. Very low confidence data. I have a new barrel I'll be working with before long and I'll try to keep good track. Might even load up ~15 rounds as a baseline test where I shoot some when brand new, then repeat shooting a few at some interval, say every hundred rounds or two and measure with the magnetospeed.

That would be a good idea. If you end up doing that, it would make sense to load the bullets a little long, and seat to the given depth just before you're going to fire them, just to avoid any "cold welding" and associated difference in pressure.

Good point. I will plan to do that. Easy enough to do and obtain a few data points. I'm still waiting on the reamer before I send the barrel off for a rebore. Possibly be shooting in the next month and half.

here is my question : what brand chronograph are you using ? new batteries ? warm weather or cold weather ?

I use both the Magnetospeed and Lab Radar. For the purposes of conducting a specific comparison I'll use Magnetospeed with everything at room temperature.

Well, I decided to look at my records for another rifle, this time a factory barrel. It's a Tikka T3 Superlite with a 1-twist stainless barrel, one of aspecial run from Whittaker Guns in Kentucky, acquired in 2016.

I tried a bunch of different handloads, and didn't settle on one until 87 rounds had gone down the barrel: It consisted of the 140-grain Nosler AccuBond and 45.0 grains of Reloder 23, in Lapua cases with CCI 200 primers.

Here's how it chronographed on various dates, all with a ProChrono that I had checked against my Oehler 35P (and eventually against a LabRadar). It matched right up with both. The velocities are all averages with the chronograph 15 feet from the muzzle:

4/17/16, after 87 rounds through the barrel: 2581 fps. (The conditions were 70 degrees and sunny.)
8/20/16, after 103 rounds: 2660 fps (70 and sunny.)
12/8/16, after 137 rounds: 2694 fps. (This was during one of my periodic cold-temperature tests, at 0 Fahrenheit, on a sunny day).

On 2/3/17 I took the magazine apart and lengthened it slightly, because the 140 AB couldn't be seated close to the lands with the original magazine configuration. The load wasn't shooting as well as I thought it should, so tried some loads with the bullets seated out to the lands, single-loading them into the chamber. They did shoot more accurately--which is when I decided to lengthen the magazine.

My last chronographing of the load took place on 10/2/19, with the bullets seated further out, and 150 rounds down he barrel:
On a 40-degree, sunny day, they averaged 2711 fps. Since the load shot a little faster at 0 than at 70 (not uncommon with "temperature resistant" powders), this could have been due to the 40-degree temperature. But seating bullets closer to the lands also tends to raise pressures, and hence velocity.

This seemed to be an instance of a barrel getting "faster" after being broken-in, but during my very first range session with the rifle, on 3/7/16, I shot some Remington 140-grain Core-Lokt factory loads, to sight-in the scope. They averaged 2533 fps on a sunny 50-degree day, so I decided to use some more of the same box to begin testing a Magnetospeed, mounted on a Caldwell rest in front of the muzzle. The temperature was 75 degrees, and they averaged 2545 fps, Due to the warmer temperature of this test, some increase could be expected--but the factory ammo obviously didn't gain as much velocity after barrel break-in. (This test was after 156 rounds down the barrel.)

During the same 75-degree session I also chronographed some new SIG Elite Hunter factory ammo, loaded with a 130-grain plastic-tipped bullet. Over the Magnetospeed they averaged 2758 fps. Later I also tested them over the ProChrono on a 40-degree day, and they averaged 2711 fps, again about what I'd expect, due to the cooler temperature and the velocity being 15 feet from the muzzle, rather than the muzzle velocity recorded by the Magnetospeed.

Dunno why the handload gained so much velocity during barrel break-in, yet the Remington factory load didn't--but it might mean such velocity gains can be bullet-specific.

Could be John. I don't think the barrel i.e. factory or custom has a real bearing on this. It would take a lot of data points to narrow down bullets, but I am now convinced that there is a very real possibility of a velocity increase after a barrel has been shot for a while.

I looked at my meager records and have found my son's Rem 700 CDL SF in .257 WM has also exhibited (indirectly) a velocity increase via a lesser charge giving the same velocity. That make 3 rifles for me that showed the increase. I rarely buy brand new rifles, so I have limited data.

The bullets that were used are the following: Nos Accubond, Hornady ELD X and Barnes TTSX.


Thank you for contributing to this.





I can only wonder if the velocity increase would have been even higher if you checked the velocity starting with the first few shots.

Magnetospeed