More powder, less velocity - how is this possible?

[atl5029]

I have been continuing load development for my 260 AI, and I witnessed some weird behavior on my chronograph today. It's quite counter intuitive, so I could use some help figuring out what is going on.

I'm shooting a 260 AI, 26 inch barrel, 140 gr. AMAX. I loaded 5 rounds each at 48, 48.5, and 49 grains of H4831SC.

Velocity of my first 5 rounds at 48 grains were
1. 2821
2. 2824
3. 2862
4. 2863
5. 2854

This was expected, since at my last range session I loaded up to 47.5 grains and those 5 rounds averaged 2777 FPS.

Then things started to get weird. My next 5 rounds at 48.5 grains were:
1. 2873
2. 2853
3. 2852
4. 2847
5. 2847

and then at 49 grains:
1. 2798
2. 2791
3. 2773
4. 2800
5. 2808

How could this be possible. One possible explanation I can think of would be that my chronograph was too close to my muzzle. I use a Cladwell Ballistic Precision chrono, and I had it about 10 feet from my muzzle. I suspect it was too close because the shot string I shot was with 46 grains of IMR 4831, a load that I know from my notes should clock about 2950 FPS, and I was getting either low readings around 2780, or totally unrealistic values around 1500 FPS.

What does this sound like to you guys? Thanks!

 

[Gunpoor]

I would move to a propellant with a faster burn rate. I have had this diminishing velocity anomaly happen on a couple of occasions and I went to a different powder and achieved a better outcome. BTW, I tried to use H4831SC in a 308Win and it just wouldn't work, and I know the 260 isn't a 308 but they share the same case dna.

 

[MNbogboy]

You didn't mention time of day or weather conditions but diminishing light conditions often produce results like yours with "light differential" triggered chronos.

However I have witnessed on a couple of occasions a velocity drop at close to center of a powder "node" but these usually are within a few tenths of a grain and velocity again increases after the node is passed. These differences are often only a few fps different and may likely be caused by combination of other variables.

Just a thought

 

[atl5029]

You didn't mention time of day or weather conditions but diminishing light conditions often produce results like yours with "light differential" triggered chronos.

However I have witnessed on a couple of occasions a velocity drop at close to center of a powder "node" but these usually are within a few tenths of a grain and velocity again increases after the node is passed. These differences are often only a few fps different and may likely be caused by combination of other variables.

Just a thought

This was today. About 10-11 AM. Clear with some clouds. Low 50's (Southwest Ohio)

 

[atl5029]

I would move to a propellant with a faster burn rate. I have had this diminishing velocity anomaly happen on a couple of occasions and I went to a different powder and achieved a better outcome. BTW, I tried to use H4831SC in a 308Win and it just wouldn't work, and I know the 260 isn't a 308 but they share the same case dna.

When I was first researching appropriate powders, a lot of people on here shooting the 260 AI were using H4831SC with what seemed to be great success. I had an old canister of IMR 4831 and worked up a load with that, but seemed to get high pressure at the velocity I was trying to hit (between 2900-3000 FPS, which is what these other people also reported). So I've now tried H4831SC and it seemed to be working out til I saw this today.

However I have been thinking maybe a faster powder might work better. Was thinking maybe one of the Reloader series. Which do you think might work well?

 

[shaughn]

Move the Chrono out to 15 feet, I never have it any closer than that.

The other thing is did the light change during testing, position of the sun, angle, cloudy clear or combination, all of which can affect a photo sensor.

Fresh battery in Chrono? A used battery can be at the stage where it is giving inconsistent voltage and this may cause a delay in the Chrono.

Outside temperature? cooler temps can affect both load and electronics.

Is the Chrono level?


I would repeat the test, I have had something similar happen and repeat testing on another day, gave results that matched expectations or previous logged entries.

 

[Gunpoor]

I would try some IMR4350 if I was loading for the 260ai but I don't load for that cartridge so my suggestion is just speculative. I was also thinking if you haven't tried already you might see if a Win. magnum primer might make a difference with the H4831SC. I say Win because I think they are a hotter primer than some others. In my ignorance concerning the cartridge I didn't realize the Nosler manual shows H4831SC to be the most accurate powder tested with a standard Rem 9 1/2 primer. I have never used a Rem large rifle primer and have no knowledge of their heat compared to other brands.

 

[fiftybmg]

.. a load that I know from my notes should clock about 2950 FPS, and I was getting either low readings around 2780, or totally unrealistic values around 1500 FPS ...

Chrony is most likely faulty.

Retest with another chrony.

I had a Shooting Chrony that gave me wierd numbers, drove me crazy till I figured out it was a lemon and sent it back to the factory with a letter on what they could do with it.

Never did hear back from that company.

 

[Frank in the Laurels]

I never heard of that being possible, must be your position of the chrony, move it out and retry..

 

[cape cove]

Is 49 grs of H 4831SC a compressed load with the 140 bullet.

 

[Clark]

I have never seen it in a rifle, but in handguns, with LONGSHOT in 38 Super, the velocity can go down.
In 357 mag with LIL'GUN, the velocity does go down, but the noise and recoil go up.

What causes it is shooting unburned powder out the muzzle making a bigger fireball. Quickload says it cannot predict this and just gives worst case.

 

[WildRose]

When I was first researching appropriate powders, a lot of people on here shooting the 260 AI were using H4831SC with what seemed to be great success. I had an old canister of IMR 4831 and worked up a load with that, but seemed to get high pressure at the velocity I was trying to hit (between 2900-3000 FPS, which is what these other people also reported). So I've now tried H4831SC and it seemed to be working out til I saw this today.

However I have been thinking maybe a faster powder might work better. Was thinking maybe one of the Reloader series. Which do you think might work well?

RL 26 is producing some really outstanding results for a lot of guys shooting .260's.

 

[WildRose]

I have been continuing load development for my 260 AI, and I witnessed some weird behavior on my chronograph today. It's quite counter intuitive, so I could use some help figuring out what is going on.

I'm shooting a 260 AI, 26 inch barrel, 140 gr. AMAX. I loaded 5 rounds each at 48, 48.5, and 49 grains of H4831SC.

Velocity of my first 5 rounds at 48 grains were
1. 2821
2. 2824
3. 2862
4. 2863
5. 2854

This was expected, since at my last range session I loaded up to 47.5 grains and those 5 rounds averaged 2777 FPS.

Then things started to get weird. My next 5 rounds at 48.5 grains were:
1. 2873
2. 2853
3. 2852
4. 2847
5. 2847

and then at 49 grains:
1. 2798
2. 2791
3. 2773
4. 2800
5. 2808

How could this be possible. One possible explanation I can think of would be that my chronograph was too close to my muzzle. I use a Cladwell Ballistic Precision chrono, and I had it about 10 feet from my muzzle. I suspect it was too close because the shot string I shot was with 46 grains of IMR 4831, a load that I know from my notes should clock about 2950 FPS, and I was getting either low readings around 2780, or totally unrealistic values around 1500 FPS.

What does this sound like to you guys? Thanks!

Either something squirrely with your chrony or a significant temp difference between the two outings.

 

[MagnumManiac]

It is common that once a load exceeds maximum pressure, velocity drops to no change or gets less, because pressure rises faster than the bullet can travel down the bore. As the pressure rises, so does the temp, this in turn causes pressure to rise further and faster, almost at an exponential rate.
This is why in some cartridges you see catastrophic gun blow-ups even though the previous load appeared fine.

If the chrony doesn't read a known load correctly, like factory .22 ammo, then I would suspect it was faulty.
I, too, never place the chrony closer than 15', have had erroneous results going closer than this. Another tip, even when it's cloudy and the sun is low, I use the diffusers, far less erroneous or error readings doing this.

Cheers.
gun)