Powder increase velocity drop?

[VD2005]

So I'm hoping I'm not just crazy and someone could help answer this question or if their is a scientific answer.
Recently I was shooting over a chronograph as I was working up some load data I had 4 different loads ready and 4shots each. this is in a 6.5-284 with H4831sc and fed215 primer
As I worked up in powder grains going up .5gr each load I seen steady increase in velocity until I hit my last string. it was at 53gr. I believe this was around max I seen slight ejector markings on the brass. This load velocity drop by 46fps from my last one with .5gr less powder. Where I seen on average 70fps increase in velocity between the last 3 loads. Is their an answer for this? I have seen it befor in other cartridges but never could find a answer.

 

[davkrat]

Regardless of the reason it sounds like you're past the point that your barrel can burn the powder before the bullet leaves the muzzle. I've had barrels plateau with slow powders. No increase in velocity a grain or two below pressure signs.

How many shots did you measure to get your average? Of all armchair scientists as reloaders we are probably the worst. Often using questionable at best measurement eequipment then putting stock in 3-5 statistically insignificant readings

 

[Ingwe]

Maybe you are going crazy, but if you are I am as well.

When working up my load here is what I found:

7mmRemMag 26" Benchmark Barrel 1:9 twist with 168gr Accubond Lr's

With 79.5gr of RL-33 I got a 10 shot average of 3,114fps

With 79.3gr of RL-33 I got a 10 shot average of 3,128fps

The accuracy was pretty much the same and with the lower charge the high-low spread was within 50fps so I decided to go with it.

I hope that someone here comes up with a good explanation for it as well since this always bugged me!

 

[Ridgerunner665]

Inconsistent ignition...

 

[Ingwe]

Inconsistent ignition...

Can you explain that a bit??

 

[Deleted member 46119]

Maybe you are going crazy, but if you are I am as well.

...

I hope that someone here comes up with a good explanation for it as well since this always bugged me!

Well, I'm in the same looney bin with Y`all.

I see this a great deal. When working up loads or strings of pressure tests, velocity will rise predictably then suddenly fall off 50fps or more. If not at max, the velocity will often "regain footing" until the next "discordant harmonic".

and that's all I can put on it just now "discordant harmonics".

I have no empirical data, just a "feeling" and the "feel" I get through the rifle.

By "discordant harmonics" I mean a load that is sooooo far off node that it actaully slows down the projectile. Sometimes I can even "call" it before looking down at the chronograph read out.

I have no way to "predict" it. It does happen. Why? Someone with more engineering and physics than me got any ideas?

 

[Ridgerunner665]

Can you explain that a bit??

I experienced this with a 30-06 using H4350 and 168 grain bullets...others have experienced it too with the same components.

30-06, 168 grain Ballistic Tips, H4350, CCI BR2
55 grains = 2,699 fps
55.5 ....2,743 fps
56....2,737 fps
56.5....2,795 fps
57...2,789 fps
57.5......2,850 fps

EDITED TO ADD: Every charge thrown by hand then trickled by hand and weighed on an RCBS 505 beam scale...not thrown on an automated scale setup....so its not that.

This isn't chrono error...I repeated this with 3 different chrono's on 3 separate days and got essentially the same results, one of which was an Oehler 35P.

Below was done on a friends Oehler 35P...its 57.5 grains of H4350 in Lapua brass
2,862 fps average for 7 shots. ES was 18, SD was 7...EXCELLENT accuracy (range verified to 600 yards)...57 grains was complete garbage.

2,873
2,861
2,861
2,873
2,855
2,861
2,855

As to explaining it, I wish I was qualified to do that...but I'm not...inconsistent ignition is my as yet unproven hypothesis.

 

[g0rd0]

it is physics, case capacity + type and weight of bullet + barrel length = expansion ratio.
Once you have passed your true expansion ratio you are just blowing powder in the wind

 

[Deleted member 46119]

it is physics, case capacity + type and weight of bullet + barrel length = expansion ratio.
Once you have passed your true expansion ratio you are just blowing powder in the wind

So your sayin, "the answer my friend is blowin in the wind, the answer is blowin in the wind".

 

[4xforfun]

Did you try and repeat the test? You shouldn't need to do the whole test...just the last two powder charges. Maybe shoot them round robin......heavy, light, heavy, light, etc.....

I have taken EVERY GUN that I have ever loaded for to the end. I have seen some guns that show normal increases in velocity, I have seen some where the increase was very minamal, but I have never seen a DECREASE in FPS.

 

[Canadian Bushman]

Shoot three to five shots of the two powder loads and then review. The averages will more than likely show an increase in mv as powder increases.

In my experience high E.S. of shots between nodes can easily look as if youre going backwards when in fact they are both just results of slowly increasing pressure of a wild load.

 

[Ingwe]

Mine is shooting good as is so I am leaving it alone

 

[Deleted member 46119]

Did you try and repeat the test? You shouldn't need to do the whole test...just the last two powder charges. Maybe shoot them round robin......heavy, light, heavy, light, etc.....

I have taken EVERY GUN that I have ever loaded for to the end. I have seen some guns that show normal increases in velocity, I have seen some where the increase was very minamal, but I have never seen a DECREASE in FPS.

This is genuine, not snarky. Good on ya mate.

I however have experienced this numerous times. Through the course of testing the 26-Nosler "wildcats"/variants with 7 loads for each bullet and 6 or more bullets per caliber with 4 calibers that's 7*6*4 = 168 minimum different loads. I see it often.

Maybe its a "magnum" thing?

 

[Deleted member 46119]

Or an "overbore" thing.

I don't remember seeing it in a 30-30 or 30-08 or 308 or 260 or 223.