Velocity increase

[Monterey21]

Morning all, I'm reaching out to the experience of the group. Have a new project that I have been working on for a few months. 6.5X284, SR3 long action, AG Composites, B&A trigger, and Preferred Barrel Blanks pre-fit with Zermatt /Savage nut. Their #4 contour. The issue is from cold bore the velocity increases for 3 or 4 shots until heat gets in the barrel then the velocity stabilizes at about 10FPS ES or less. point of impact changes, gets higher@ 100yds, with the first two rounds then is 1/2 MOA or so. Depending on me. The load is 53.5 4831SC with 140 Berger Elite hunter and a CCI match primer .010 off the lands. Same everything with 57.0 N565 is a little faster.
the question is why does it get faster with heat? I understand the bore can get tighter, bullet gets tighter, better seal in theory.

 

[DJ Fergus]

I can't tell you the textbook answer to this problem but I can tell you tell you what I have done to remedy such situations. If the barrel has been broke in with around 50 to 100 rounds or so and is still doing this, well it shouldn't. Next, make sure you aren't over cleaning it. 10-12 rounds down the barrel and it at least should settle down by then. A change to a different powder or find a seating depth tune farther from the lands:. .040", .050", .060" etc.... Sometimes I've saw this happen & sometimes it doesn't when seating fairly close to the lands.

 

[DJ Fergus]

Something else that you will hear from 6.5-284 owners when their particular rifle seemed finicky, a switch to retumbo solved their issues. Also, any powder that you could run that will generate less heat will help.

 

[nmbarta]

I can't tell you the textbook answer to this problem but I can tell you tell you what I have done to remedy such situations. If the barrel has been broke in with around 50 to 100 rounds or so and is still doing this, well it shouldn't. Next, make sure you aren't over cleaning it. 10-12 rounds down the barrel and it at least should settle down by then. A change to a different powder or find a seating depth tune farther from the lands:. .040", .050", .060" etc.... Sometimes I've saw this happen & sometimes it doesn't when seating fairly close to the lands.

For sure if your starting with a freshly cleaned barrel, this can cause some of those problems. I have a 280ai that doesn't shoot until you get about a dozen down the barrel after I clean it. Then it runs great for the next 100 or so, she just likes it dirty I guess.
How much is the velocity increasing? You mentioned it settles down, but didn't say what the spread is for those first few shots?
As mentioned above, I've found those loads that are that close to lands to be a bit finicky at times, I always prefer to find a load that's .030+ off if I can....doesn't always work, but usually there is a sweet spot somewhere between .030 and .080. I've found that this "node" is typically more forgiving. I can't back that up with any reasoning, it's just my observations.
Have you done a ladder test? If your close to the edge of your node, a slight increase in velocity can push you out, if you pick the middle of the node, it will allow for a little more variation and still give good results ( depending on how far your shooting)

 

[Monterey21]

I can't tell you the textbook answer to this problem but I can tell you tell you what I have done to remedy such situations. If the barrel has been broke in with around 50 to 100 rounds or so and is still doing this, well it shouldn't. Next, make sure you aren't over cleaning it. 10-12 rounds down the barrel and it at least should settle down by then. A change to a different powder or find a seating depth tune farther from the lands:. .040", .050", .060" etc.... Sometimes I've saw this happen & sometimes it doesn't when seating fairly close to the lands.

I thought it was a fouling thing and kept shooting. Letting it cool for an hour back to ambient. It's got 30 rounds since clean. Florida Morning in the summer, 80ish. Other rifle to shoot. the barrel is broken in. A snap to clean. 150 rounds or so now. The seating depth is a possibility but I got it close by moving out to .050.

 

[Monterey21]

Something else that you will hear from 6.5-284 owners when their particular rifle seemed finicky, a switch to retumbo solved their issues. Also, any powder that you could run that will generate less heat will help.

If I could find Retumbo I would be on it. It is cooler burning and is really efficient.

 

[Monterey21]

For sure if your starting with a freshly cleaned barrel, this can cause some of those problems. I have a 280ai that doesn't shoot until you get about a dozen down the barrel after I clean it. Then it runs great for the next 100 or so, she just likes it dirty I guess.
How much is the velocity increasing? You mentioned it settles down, but didn't say what the spread is for those first few shots?
As mentioned above, I've found those loads that are that close to lands to be a bit finicky at times, I always prefer to find a load that's .030+ off if I can....doesn't always work, but usually there is a sweet spot somewhere between .030 and .080. I've found that this "node" is typically more forgiving. I can't back that up with any reasoning, it's just my observations.
Have you done a ladder test? If your close to the edge of your node, a slight increase in velocity can push you out, if you pick the middle of the node, it will allow for a little more variation and still give good results ( depending on how far your shooting)

I did a ladder after the first few rounds With 4831SC. There was a 4 grain spread with .3 intervals and there were two spots that the velocity was flat. The test was at .025 off. The whole thing was about 1.5" @ 100yds. I did not do the ladder test at different seating depths. I do know the further off the lands the slower the velocity. But I really didn't notice the cold bore difference while doing the ladder. I never run the barrel real hot. I always fire two then wait 5 minutes between shots. I'm going to keep on until I get 60 or so down the pipe. The velocity increase from shot one thru four can be 35 to 50 FPS. But the point of impact change on the first two shots is what's killing me. I need to work on me as well. Thanks for all the input.

 

[DJ Fergus]

If I could find Retumbo I would be on it. It is cooler burning and is really efficient.

I know that retumbo, RL26 and several others are hard to find right now.. I've heard really good things about ramshot LRT where people have used it when they couldn't find retumbo.

 

[DJ Fergus]

One more thing to add and I don't know if this will apply to your situation or not. I've got a friend who bought one of the big savage F-class 30" barrel 6.5-284 rifles. He was a pretty good hand loader. He couldn't get it to shoot with h4350 & 140 Berger's. He decided to turn the necks on his lapua cases. After that it would shoot bug holes. I'm assuming he didn't have enough neck clearance. Measure your fired case neck diameter and add .001 that number. That will probably be what your chamber's neck diameter is. Then measure your loaded rounds neck and subtract that from what your chamber neck diameter is. I'm thinking .004" difference would be good.

 

[Mikecr]

It is normal for MV to be different cold bore to hot.
So from load development the next stage is cold bore development.
Most people never bother with this and I'm sure many assume their cold bore accuracy rather than testing it.

You can dry pre-foul a bore so that the first shot is always on the mark, but there is nothing you can do about velocities with changing barrel temps.
For competition it doesn't matter because there is always a sighter period to warm everything up.
For hunting it doesn't matter because you kill, or should be, with single cold bore shots.

 

[rammac]

Another confusing conversation from my perspective.

All firearms will demonstrate an increase in velocity as the barrel heats up.

So why are people saying that they can prevent it or that it is surprising that it happens?

 

[rammac]

The explanation is that a bullet is pushed down the bore by expanding hot gasses from the burned powder.
The powder has X amount of energy available.
The energy is converted to heat and pressure.
Some of the heat energy is absorbed by the barrel and the rest is used to push the bullet.
As the barrel heats up it absorbs less of the heat energy and more heat energy is left to push the bullet.
More heat energy pushing on the bullet results in faster bullet speed.

As the barrel heats up a balance is reached where less heat energy will transfer to the barrel because the total energy available (based on your rate of fire, ambient air conditions, and barrel characteristics) doesn't produce enough energy to increase the barrel's temperature any further. When that balance point is reached the bullet's velocity will level out.

 

[DJ Fergus]

I can go for three shots from a cold bore and have consistent es. Below 20fps spread most all of the time & single digit in most of my rifles. Any more than three shots and It really doesn't apply to me because I don't need to go put 50 rounds through a bolt action in one sitting. So a 4th shot within 20 fps es shouldn't be unachievable.

 

[Monterey21]

I can go for three shots from a cold bore and have consistent es. Below 20fps spread most all of the time & single digit in most of my rifles. Any more than three shots and It really doesn't apply to me because I don't need to go put 50 rounds through a bolt action in one sitting. So a 4th shot within 20 fps es shouldn't be unachievable.

I have a 6.5 WSM built on a defiance XM with CF barrel. Cold bore has no effect on it. I have a Mauser with Lilja Barrel in 7mm Blaser. Cold bore has no effect on it. I have a 28nosler proof CF Cold bore has no effect. I've loaded the N565, N560, Retumbo, 4831sc, H1000, reloader 26 and N570. All these are proposed Hunting Rifles. So everything is a load development process. And that is where the fun is for me and a matter of personal satisfaction. I'll keep working on it. I have an AMP annealer to eliminate the neck tension variable. I'll get it but the experience on this forum has huge value. I take every suggestion with acceptance and I don't toss anything out.
Again, thank you everyone.