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

Tikkas are notoriously slower than most other rifles. That’s my only knock against them. Think its due to the longer than average throats.[/quote]

In some cases such as the 257 Weatherby, Military cartridges and I suspect some others too, the CIP specification calls for a longer throat, leed, or free bore than SAAMI specs. In many cases the two or the same, where they differ significantly the reamer makers will usually list both as an option.

In some of the loadings with the longer throat the European loadings can be slightly hotter than US although for the most part they are the same. Some Euro factory loads were too hot to shoot in my short throat-ed 6.5x55.

This makes sense..

Which is also why Butch Lambert isn't seeing the velocity increase. He smooths the throats in the rifles he puts together with fine steel wood BEFORE he shoots them the first time. I also know another custom gunsmith who uses a similar method, and is well-known for the accuracy of his rifles.

Will keep looking at my range notes. Something tells me there just might be an article in this!

Thanks,
John

I was hoping you would say that.

I don't think it is widely known or talked about. How many folks take a rifle out of the box and check velocity immediately? Then, after load development go back after 100 rounds being fired, check velocity again? Another 75-100 fps might not be noted on the same original drop chart if shooting out to 4 -5 hundred yards.

If velocity is checked for a different bullet, well data is lost. Same with a different lot of powder.

Checking for velocity increase has to be purpose driven from the first shot, after X amount of shots, with the same bullet, same lot of powder under the same weather conditions and same brass, loading procedure etc.

Hell, only some loony would do that. LOL

Puzzling, to say the least.

That sometimes-yes / sometimes-no behaviour does not stand well against the law of physics and look more like a thing of medieval alchemy. I would tend to take your chronograph readings with the handload with a grain of salt, and I really hope you can shed some more light on the subjetc as I am very curious about it at this point.

Thank you!

Alvaro

Alvaro,

I've done some more investigating in my range-notes. It gets even more interesting--and confusing!

But hope to make some sense of it all with more research.

Good hunting,
John

I linked an article a while back, DoD research, I think.(might have it saved) where at a certain temperature (the higher of two flame fronts) copper oxide, and lead oxide deposition on barrel (rifleling/lands) was promoted.

My thought was that this deposition works like leaded gasoline providing a bit of lubrication.. it would likely fill in, preferentially, small pits and divots associated with throat erosion.
Perhaps corners as well..

I will look around for the article.

2002, NATO/OTAN " Combustion of Solid Propellants", France, presented at RTO/VKI special course on "Internal Aerodynamics in Solid Rocket Propulsion" held in Rhode-Saint-Genise, Belgium 27-32 May 2002.

It's a bit technical, rocket science after all, but if you spend some time starring at and thinking about the phase diagrams it starts to make sense...
But you might pick it up quicker than I did...

Is it possible that a well broken in barrel looses velocity not because of wear induced increased tolerances but the barrel is too smooth? I have experimented with barrels that were either fire lapped or had lots of JB used on them and to recreate the original longitudinal scratches taking a couple of passes with a stainless steel bore brush. This only on barrels that were close to heading to the tomato patch anyway. So far I can't document any changes.

In my experience (and that of some barrel makers) a "too smooth" barrel often results in more velocity (and copper fouling) because there's more contact between the bullet and bore, that in a bore lapped with not-too-fine grit--resulting in higher pressure and velocity/

Can immediately think of at least two examples.

Thanks MD. Seems like the counterintuitive is the rule and the exception is the norm when it comes to this ballistic gak stuff. I am still learning how much I don't know about these things, it keeps it interesting though.

In medium size cases ( 260, creedmoor, 6.t-284...) i have found that i get 30-40 fps INCREASE in velocity once the velocity stabilizes. This usually takes 70 rounds or so.

This correlates with what a lot of match shooters see- guys that blow through a couple barrels a year. IF you clean a lot, this can be different
Once i get 75 rounds down a barrel i only clean every 100-200 rounds.

I shot another 20 shots this past weekend and the average speed of the last 5 is now 8 fps higher than the first batch I shot, and 18 higher than the previous range session.

All seem well within SD and chronograph tolerances.

For reference, barrel is a custom 1:8" 22-250 and load is 36.5grs of H414 with a magnum primer (RWS 5333) pushing 77gr TMKs and 75gr ELDMs.

To avoid fuzzy math and to get solid data:

1) Chronograph the first 5 or 10 shots on day 1 of the brand new rifle/barrel.

2) Shoot at least 100 rounds of the same load or play around finding a load.

3) Go back to the exact same load that was used for the first 5 or 10 shots and chronograph them from a squeaky clean barrel.

4) compare velocity from day 1 with the brand new rifle/barrel and the same load after at least 100 rounds were fired.

The problem that I see is not chronographing the very first shots on day 1 from a virgin barrel. + or - 20 fps is insignificant.

While not a new barrel (#6 Benchmark 7.7 twist, 26.0625" long) it is a rechambered one. 6 Dasher to 243 Win, barrel has about 1500 rounds on it before rechamber and 0.0625" taken of the crown for a clean up.

To fire form the new brass (Lapua) I jam it 0.010" with a 105gn Hornaday BTHP. This brass is out of the box no prep at all.

Using QL the velocity should be 3074 ft/s, so after operating the soft ware I arrived at 43.06gns of RL 22. This gives me an average of what was predicted, 3074 ft/s. ES is high (22 ft/s) due to the brass not being prepped at all.

This fire form load goes right ~0.750", 5 shot at 109 yards for the last 45 rounds.

There has been no detectable change in velocity as the round count goes up and averaged from the start. However, there is 55 rounds to go.

Some thoughts and questions: Is there enough gas leakage to 'wash' the barrel ahead of the bullet ? Is there enough change in the throat profile to alter the resistance of the passing bullet? Does the thin layer of copper/carbon deposited on the bore make more or less resistance as the bullet passes? Or a combination of all?

I have only had one barrel ( an 8 twist, #3 Benchmark in 280ai) exhibit increasing velocities as round count went up, so just kept reducing charge weight to stay on a barrel time.

To add another data point, I just received one of my match rifles back from the 'smith with a new JC Custom 7" twist 4G MTU contour barrel chambered in 6mm Creedmoor with 0.100" throat. Loaded up twenty 105AM with 42gr of StaBALL. Fired fifteen of them between curing the DBC with 10 shots and fouling the bore, post-clean. Average velocity was 3010 fps with an SD of 15. Saved the other five to chrono after shooting 100+ rounds down the barrel. Between testing the 115 DTAC, 112 MB, and 108 ELD, round count is 45 right now, with another 20 loaded for testing. I'll likely have 100 rounds down the barrel in the next week or so, and then I have a local PRS-style match on the 24th, with another ~100 rounds. I'll try and chrono those original five 105 AM loads either before the match or after. I'll update once I've done that.

Jordan, you fired 15 rounds BEFORE curing the DBC? You said between curing.

Assumption being that DBC has no affect.

Looking forward to your result

Applied DBC before firing the rifle. Fired 10 rounds to cure (naked 105 AM). Average velocity 3012 fps. Cleaned the bore. Fired 5 more rounds (moly coated 105 AM). Average velocity 3007 fps. I have another 5 rounds loaded with moly coated 105 AM that I’ll chrono after 100-200 rounds down the bore.

All velocity measurements taken with LR.

i wonder if harry pope had these same problems?

The commonly used Chromium Molybdenum (chrome moly) barrels are so individual I don't know that any facts can be determined. I certainly would not expect it.
I have a bunch of barrels from brand new to severe alligator cracking in the throat and that barrel is still one of the more accurate barrels I have as is the newest one with less than 200 rounds down the tube.

Many people have noted an improvement in accuracy after a barrel has a few hundred rounds through it but because I am perpetually testing loads, bullets and powders all of different batches over time, it makes it more difficult to be definitive.

You would need to make some rounds and document a barrel accurately for round count and then on similar days in terms of temperature and humidity, take sample groups with the original ammo etc. All variables apply from shooters consistency to target range to test ammo management and housing between usage. Alternatively you could put the same handloading items aside and reload the same combinations fresh before use.

What ever you do, it wont mater to the target, whatever it is. Most people will just develop a load and stay with it until the barrel noticeable goes off in accuracy then visit the 'smith.

Any updates Jordan?

The match got cancelled so I went to see if any bears were out and about instead. I hope to put another hundred rounds through that rifle in the next week or so.

As another data point, a friend has the exact same barrel that I just had installed on my match rifle. After approximately 300 rounds we checked velocity on his initial load, and it had increased by about 65 fps.