How long before a barrel stops speeding up

[Matt_3479]

I broke in a barrel on my new 6mm creed and after 80 rounds of factory I started developing a load. I found 42.5 and 42.8grs of h-4350 both preformed pretty good and settled with 42.8grs at 3137fps. I had a shoot on Sunday and went in with 180 rounds down the tube and left with 260 rounds and noticed towards the end of my shoot my rounds felt higher pressured, heavier bolt lift. I took the remaining rounds from the shoot to the range yesterday and ran them over the chrony again and they came out at 3198fps and groups opened up. I’m now at 308 rounds down the barrel and need to get my accuracy back. Do I start load development over or just drop charges until I get back around 3130fps?

 

[Falfan2017]

The lands can expand quickly with the first couple hundred shots. Might want to recheck seating depth too.

 

[spclark]

The lands can expand quickly....

Expand?

(You mean by this that the interior diameter between the lands' faces decreases? The bore gets tighter? Never heard of that happening....)

Or erode?

(As in the beginning of the lands that you started out with has moved closer to the muzzle as you've been sending rounds down the bore. Pretty much normal, accepted barrel behavior.)

I'm curious what bullet you're pushing to 3,100+ in that 6Creed myself.

Whidden pushes 105's to that velocity with his 243WIN. That Creedmoor case has almost the same volume so I'd expect pretty similar ( = fast ) throat erosion if you're playing on the hot side of the load schedule.

 

[Falfan2017]

Expand?

(You mean by this that the interior diameter between the lands' faces decreases? The bore gets tighter? Never heard of that happening....)

Or erode?

(As in the beginning of the lands that you started out with has moved closer to the muzzle as you've been sending rounds down the bore. Pretty much normal, accepted barrel behavior.)

I'm curious what bullet you're pushing to 3,100+ in that 6Creed myself.

Whidden pushes 105's to that velocity with his 243WIN. That Creedmoor case has almost the same volume so I'd expect pretty similar ( = fast ) throat erosion if you're playing on the hot side of the load schedule.

Distance to land expands.

 

[LA50SHOOTER]

I now that I'd be looking at it with a borescope and seeing if a carbon ring was starting to develop. (Depending on cleaning habits & rate of fire). - I'm not a "creed" shooter but after 308 rounds I'd say your barrel should have sped up all its going to and if you went though a good break-in regime should be "settled" to where speed should be consistent with good hand-loads.

Do I start load development over or just drop charges until I get back around 3130fps? - After I looked at the throat area with a bore-scope and if needed addressed any carbon build up, then yes I'd drop the charge a few tenths back to where it was shooting accurately as a first step and see how accuracy is.

- Ron -

 

[mikecr]

Why would a barrel speed up with shot count?

 

[JFrank]

Less friction

 

[ppaul]

Hate to open this can of worms but, isn't less friction what we got with moly,etc. That gave me less velocity having to do with less pressure.

 

[mikecr]

Moly doesn't affect velocity due to lower friction. It lowers velocity by cooling the powder burn.
Bullets and bores coated with WS2 (way slipperier than moly) do not change velocity at all.
So I'm sure that the subject in discussion isn't due to friction in itself.

Maybe the difference is cleanliness, restrictions?
I break-in to knock out chambering interfacings with the bore. This is 10shots w/Tubb's FF.
I was also taught to put guns away cleaner than pulled, and every bore in my house is cleaner than brand new, while dry prefouled with fresh WS2.

I've never seen velocity change as a trend with further shot count.

 

[LA50SHOOTER]

Why would a barrel speed up with shot count?

They do speed up, I've witnessed it on several barrels, in several calibers, some more than others, button rifled barrels seem to do so more than cut-rifled IME. I believe that the actual surface & final lapping have much to do with it. - I also "treat" a barrel with a very fine moly sub-micron solution that is suspended in alcohol sold by Gre-Tan Rifles when I break-in a barrel and use the Graf-oil after the break-in period.
The "speeding up" process is not an exact replication from barrel to barrel that I've seen. When a barrel stops or minimizes copper fouling it seems that the speeding up process minimizes or stops.

- Ron -

 

[LA50SHOOTER]

Moly doesn't affect velocity due to lower friction. It lowers velocity by cooling the powder burn.

I don't agree at all with this statement, the powder burn temperature is purely a chemical "trait" of the individual propellant that is engineered into the formula. - You may want to reference some articles on moly & BHN in regards to friction in some articles. - when friction decreases so does pressure and velocity.

- Ron -

 

[mikecr]

when friction decreases so does pressure and velocity.

No it doesn't. You can prove it yourself with tungsten disulfide, which has the lowest friction coefficient of any dry lubricant. It does not change muzzle velocity.

Molybdenum disulfide absorbs charge energy in phase change(from solid to gas).
This, referred to as latent heat of vaporization, and moly is the only dry lubricant I'm aware of that affects velocity. Moly also gives the energy back, but too late, as it condenses into an eventual constriction later down the bore..

Pretty sure all this was known(by shooters) in the ~70s,, it's in one of my books around here somewhere.
Tungsten disulfide (WS2), merchandised as Danzac back then, Possibly TUBBDUST today, specifically cured all the woes of Moly, including the benefit of cleaning it all out (which you can't do with moly). The purpose of either was not to reduce friction, change velocities, or increase barrel life, but to reduce copper fouling. However, due to moly charge cooling (in the throat), barrel life was assumed to be improved. Possible,, if moly constriction is constantly managed with application of JBs. Otherwise, accurate barrel life with moly, would be shortened. The killer of accuracy in barrels is not wear, nor throat erosion. It's a groove constriction.

 

[LA50SHOOTER]

I can see we are at impasse Mike so I'll leave it where it is. - nothing gained in an argument and I don't want to foster any ill will here.
- I will say that I am NOT a fan of moly or any other coatings for bullets & leave it at that.

- Ron -

 

[338 dude]

Hate to open this can of worms but, isn't less friction what we got with moly,etc. That gave me less velocity having to do with less pressure.

When I started Molly coating the same load dropped on average of about 50 ft./s

 

[Ccrider]

So what exactly does Tubbdust do?

 

[swd]

So what exactly does Tubbdust do?

Tubb Dust is an anti coppering agent. You mix it with your powder. I was talking to a guy in the pits during a match that tried it and he said it seemed to work quite well.

 

[mikecr]

When I started Molly coating the same load dropped on average of about 50 ft./s

Moly does this very consistently (only moly). And it can be an advantage, in that you might bump up the powder & get a better case fill. The throat will also erode slower due to it's cooling of the charge.
Moly fills crevices that copper would otherwise get into, and the reduced jacket temps mean less expansion and jacket tearing. Some bullets can be spun up a bit faster and still survive, if coated.
Sounds great, but there are issues with moly..

Issue #1
Moly is fouling. A particular fouling.
Moly fouling in barrels has to be managed like any other, and if you do so, you'll never have an issue.
The trick is that initial moly prefouling takes 30-50shts to settle. Then you can shoot another 100+ shots easily without copper fouling out.
Then you chemical clean only to get fouling back to near the 50sht settle. Take too much out and it will take more shots to settle again.
Take too little, and moly builds up to issue #2.

Issue #2
Moly has an affinity for itself. It sticks & layers, smearing down a bore like shingles. The first 6-8" is flashed away with firing and then a bullet meets a building constriction of layering moly. If allowed to continually build up, this constriction will kill accuarcy.
Now it's time to deal with it, and eliminating a moly constriction -without ruining a barrel, is a nightmare that many failed with.
This is the strongest basis for moly falling out of favor (a long time ago).

 

[Dgd6mm]

I broke in a barrel on my new 6mm creed and after 80 rounds of factory I started developing a load. I found 42.5 and 42.8grs of h-4350 both preformed pretty good and settled with 42.8grs at 3137fps. I had a shoot on Sunday and went in with 180 rounds down the tube and left with 260 rounds and noticed towards the end of my shoot my rounds felt higher pressured, heavier bolt lift. I took the remaining rounds from the shoot to the range yesterday and ran them over the chrony again and they came out at 3198fps and groups opened up. I’m now at 308 rounds down the barrel and need to get my accuracy back. Do I start load development over or just drop charges until I get back around 3130fps?

Matt, your barrel should be done speeding up by now. I have seen it go to 300 rounds from new with speed increases. Seen it on my buddy's 6br. I just shot mine today, 100 rounds down the barrel. Shooting good enough for now and my real load development will start at 200 or so.

 

[Matt_3479]

Awesome thank you guys. I did what my gunsmiths recommended for barrel break in and clean in when I feel it needs too. Rifle was shooting incredibly well but has opened up with the speed increasing. I cleaned it and was going to put a few foulers down and reshoot with same charge weight to see what happens. But hoping if I have to drop charge weights a bit I will begin to preform again around 3130 still!

Bullet is 105 hybrids

 

[Evan]

This seems a bit alarming. None of my rifles have had dramatic accuracy falloff during speed up, and they all settled in within 100 rounds.

When you say your groups are opening up, how far are we talking about? Easiest experiment is to see if you drop your charge back down to restore your speed, does the accuracy return, suggesting it drifted off the node? If so, then no big deal, just load down and get back to it.