I've been doing some chronographing lately and getting slower than expected results from some loads which have same components, but of course different lots. But my thoughts are barrel wear in the case of the .257 Ackley which has too many rounds down range at this time. Hot and about 3,000 in total of all bullet weights. The other is a relatively new tube in .30-06 Ackley which also has shown about 55 feet per second loss to a powder and primer lot change as the bullets are the same lot as previous. In the case of the .257 AI the loss is in the 150 ft/sec range and more of a concern. Comments anyone?

I'd guess 3000 hot loads in an AI might result in some throat errosion. I don't know, how does throat errosion effect velocity? Somebody here will probably have an answer.

I haven't shot any one rifle enough to see the effect, but I have shot a number of .38 Super pistols to see a velocity drop followed by loss of accuracy after many rounds.

Barrels used to last 18-32,000 rds before they have to be replaced.

I would expect rifles to go through a similar loss of velocity and then accuracy but in a much smaller number of rounds.

DCM shooters say 7000 rounds is about barrel life time. 223/308/30-06 are the cartridges we are refering to in that case

It would be no surprise to me that after 3000 rounds through a 257 Roberts AI. A pretty hot number, it has a magnum case characteristic. I'd be surprised if some of your throat hasn't found it's way down the barrel.

The question is not just if it's slower, but if it is losing accuracy as well. Accuracy is the gage most competition shooters use to decide when abarrel is shot out. Make no mistake, using jacketed bullets at full pressures, you will eventually shoot the barrel out on anything.

As to the '06, try a swith back to your original components and see what it that tells you.

I doubt that a barrel will slow down, if you keep the pressure the same.

However, if the throat has moved forward, the peak pressure will be lower, and it would stand to reason that velocity will be lower. In my limited experimentation, lengthening the throat has the same effect as increasing case capacity by approximately the same amount.

It always surprises me how much difference one or two grains of extra case capacity changes peak pressure. FWIW, Dutch.

Pretty common, as the bore (throat) wears, it's just like lengthening the throat, takes more powder to keep the velocity up. You can probably load a couple grains more powder than when it was new if acuracy is still there.
some of the prairie dog shooters will shoot 50's in a .223, when the bore starts to look like snake skin, switch to 55-60 grain bullets loaded as far out as possible and get another couple thousand accurate rounds out of the barrel before trashing it.

quote:

Originally posted by 308Sako:
I've been doing some chronographing lately and getting slower than expected results ... Hot and about 3,000 in total of all bullet weights. ... In the case of the .257 AI the loss is in the 150 ft/sec range and more of a concern. Comments anyone?

Hey 308Sako, That is about the best use of a Chronograph that I've seen posted on here in quite awhile - noticing a Velocity decrease.

Your reference to "Hot" is interesting. Do you mean Loads too hot, shooting with the barrel hot to the touch, both, or something else?

Are you Neck Sizing or Partial-Full Length Resizing?

I don't suppose there is any chance at all you Recorded the following:

1. Original Kiss-the-Lands distance for a Set-Up bullet, and you still have that exact same Bullet in your Reloading Die box?

2. Measured and Recorded both CHE and PRE of your Development Loads?

3. Since it is an "AI", what information did you use for knowing when to STOP adding Powder while you were Developing Loads?

4. Have you Annealed the Necks and if so, how often?
4a. How many shots through those cases?

5. Do you Fine Tune your Final Loads by altering the Seating Depth?

6. How do the group sizes compare today with the original Developed Load groups?

7. When you sit down at the Bench to shoot, how many shots do you take in succession before stopping to let the barrel cool?
7a. How long between shots?

8. How many shots between "Cleanings" of your Bore at the Range?
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It might just be the Throat moving forward as the others have suggested, but that should have changed the Barrel Harmonics resulting in the group size changing. Won't necessarily get Larger "if you did not Fine Tune the Seating Depth to begin with", because it could just as easily make the groups smaller.

I did...

Steve, you're the man... I knew I could count on your thoughtful in depth analysis. Many thanks.

Hot Core, Yup warm means by pressure and then backing down to a sustainable load where the case lasts about 7 firing. Not proper for the scientists, but years of experience and patience. Barrel length by the way is 25" of hart's best. Well, my memory has just been jogged a bit, so I can say that maximum velocity was always a goal with this rifle. So powder choice was by this parameter more than any other. With IMR 4064 75 gr Hornady HP's went to 3750 ft/sec's., 85 gr Nosler BT's went to 3550 with IMR 4350, and Nosler 100 gr Partitions at 3300 aslo IMR 4350 or IMR 4831, with the 115's and 120's coming in at 3050/3100 IMR 4831 or IMR 7828. I have used R-P, WW, and Lapua 7 Mauser. Guess which is best. Also an assortment of primers and powders. I've tried turned necks, and partial sizing (mostly this method) and just now beginning to use a Redding bushing neck only die. For the most part I shoot 120 and 115 grain bullets with IMR 4831 and IMR 7828. I am now trying 4831SC and am amazed at how much it takes! Since this is foremost a hunting rifle, I seat to a maximum overall length of 2.810 to function through the action. Bullets don't reach the lands anymore, I seem to remember that there was about .1 jump from that length to the lands when new. Most accurate bullet was some that Jack Carter made for me back in 1990. Lucky to still have a few boxes of those. I am close to rebarreling this rifle, and have always felt that the .257 AI is the most you can do on a short action. Accuracy however is not totally lost just yet... I am surprised actually. Last, the groups aren't quite what they were, but they haven't gone to Hades yet either. I clean after every 10 rounds at the bench, occasionally let it ride to 20, never more.

quote:

Originally posted by 308Sako:
...Yup warm means by pressure ...the case lasts about 7 firing. Not proper for the scientists, but years of experience and patience. ... maximum velocity was always a goal with this rifle.

Hey 308Sako, I also like SAFE MAX Loads and it looks like you are running yours just a bit hotter than I do(only 7 firings/case), but that doesn't mean there is anything wrong with that.

And I see you lean toward the Slower Powders to accomplish your goals - so do I.

quote:

Since this is foremost a hunting rifle, I seat to a maximum overall length of 2.810 to function through the action. Bullets don't reach the lands anymore, I seem to remember that there was about .1 jump from that length to the lands when new. Most accurate bullet was some that Jack Carter made for me back in 1990. Lucky to still have a few boxes of those.

Have you considered trying the 117gr Hornady Round Nose #2550? The RN Profile will get "closer" to the Lands than the sleek bullets and might even reach them depending on your specific rifle. I've used them in a 25-06 many years ago(still a Wildcat back then) and have a buddy that still uses nothing but them for Deer. I'm sure I made a few kills with them beyond 300yds, but not many because I normally carried a different rifle when I knew I'd have those shot opportunities.

quote:

I am close to rebarreling this rifle, and have always felt that the .257 AI is the most you can do on a short action. Accuracy however is not totally lost just yet... I am surprised actually. Last, the groups aren't quite what they were, but they haven't gone to Hades yet either. ...

That being the situation, you probably want to just keep on with what you have and not want to learn a new trajectory.
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You skipped over "how many shots taken at a time". I sometimes only take 2-3 depending on how hot the barrel is. Really prefer cumulative 1-shot groups with the barrels cleaned and lightly lubed between each shot. It takes a good bit of time, but it provides me with what I want to know and I don't get the barrel way too hot.

The Cleaning Fluid really helps cool a barrel quicker than air and the fouling is much easier to remove while still warm.
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You have probably moved the beginning of the Lead forward a good bit, but there is really no way to know how much without having an original Set-Up bullet to use for comparison. That is just the way it works with "Hot" Loads. When you do eventually re-barrel, try to remember to retain a Set-Up bullet in your Reloading Die box and a note about the Kiss-the-lands distance.

And some of the velocity loss could still be due to changing components as well. But, 150fps shouldn't matter at all to the results "On-Game".

How far are you taking shots?

I have a 40 year-old .264 Winchester which I have owned since new that has had several thousand rounds through it. It now shows significant throat erosion. While its accuracy is unaffected, it does now take about 1-2 grains more of the same powder to achieve the same velocities as when it was newer.

So my experience would be "yes" barrel wear can result in lower velocities. HOWEVER, I agree with Dutch, that velocities will remain the same IF pressures remain the same, ie. by boosting the powder charge appropriately.