COAL versus Velocity

[Muddyboots]

A recent thread got me thinking " has anyone taken a load and adjusted the CBTO/COAL by an increment and chronographed each load"? Yeah I know thats reloading but once we find a node we never then adjust COAL to see the velocity changes. Just curious if anyone has done specifically this.

 

[jasonco]

On purpose, kinda. I do OCW method. When I'v finalized the particular powder load, I do seating depth testing, up and down from the original starting depth of -.020 off the lands, in +/- .005 increments, and verify my findings at long range on steel at 1000 yds. The velocity does in fact change up & down. So kinda.

 

[scope-eye]

It would be safe to say that the longer the coal the more velocity or FPS, due to when the bullet is set into the lands it is meeting resistance from the get go which builds pressure which translates into velocity. When I load rounds that are to hot even for me IE: "sticky bolt" what I do reseat the bullets deeper in remaining loaded ammo fifty to a hundred thou which allows the bullet to jump the lands and is bleeding pressure due to the bullet already moving forward. I can tell if it is working if the signs of pressure have gone away.

Dean

 

[Muddyboots]

Here's the question again. We all know the pressure spike from seating to lands but a lot of rifles cannot do so from mag limitations. So in this case has anyone actually taken a load, adjust CBTO/COAL by 0.005 and chronographed each adjustment in CBTO/COAL change? Pressure is not necessarily the only aspect of velocity changes. Sometimes load density versus case capacity can affect velocity due to powder burn efficiency. So sometime in next week or so I plan on taking a load and do this just for hoots.

 

[scope-eye]

I think that there are too many factors to get a fair reading, every round would have to be exacty the same IE: neck tension, barrel temp to name a few. Even at just 3% difference between all the ammo "which is not alot", that is suppose to be the same at 3000fps that is still close to 100fps without changing anything.

Dean

 

[Gwine]

I did a seating depth test with my 22 creed over .080" with no real velocity change. I will have to look up the data to get exact numbers.

 

[Mikecr]

Normally MV would go down as you back away from a very close land relationship, and then level off, and then go back upward with rising load density(unless density is too low to begin).
I don't see how this could be useful.

From best TESTED CBTO, you can just adjust powder charge upward to dial in anything achieved with your wondering.

 

[Muddyboots]

Good stuff, sometimes random thoughts brings out good comments. One of things came to mind are those rifles with mag limitations that you might get better velocities closer to SAAMI which some may not consider and max the COAL instead.

 

[Gwine]

I posted this last spring in a thread about Bullet jump.

Looks like you got the answer, but to your original question. I just did a seating depth test this weekend from .030" off to .100" off in .010" increments. Chrono'd all shots. SD of 9, ES of 37 with 24 shots. The average for each seating depth was within 10fps fastest to slowest.

 

[Muddyboots]

Perfect! What was the bullet, powder combo?

 

[QuietTexan]

This isn't a large enough group to be statistically relevant, but it was so odd I knew where to find it in my notes.

300 RUM, Nosler ABLR 210gr, WLRM primer, 81.0gr IMR 7828SSC.

COL / FPS (avg 5 shots):
3.543 / 2800 (.125" off lands)
3.596 / 2805 (.072" off lands)
3.628 / 2814 (.050" off lands)
3.658 / 2823 (.010" off lands)

The hidden detail here is that each of the ES ranges overlapped with at least the seating depth above/below it. The averages just make it look like it's moving up with the bullet out further. The whole string was ES 39, SD 13, which is not too good.

 

[jasonco]

Is this what you might be looking for (images). Seating depth/CBTO finalization test for 25 Creedmoor and 131 Black Jack Ace.

 

[Gwine]

Perfect! What was the bullet, powder combo?

80eldm, 40ish grn H4350. Looked back at my notes, the larger jump was slower. Didn't notice before, but the ES of the shorter coal had significantly better ES. The ES of the last 12 shots was 13.

 

[LongBomber]

I was running this more for the groups size to see if I could tune in a load for the 144 hybrids in my 6.5cm. Barrel doesnt seem to like them very much, but I have a couple boxes of them. Good thing I have a few 6.5's..
I didn't fire enough shots to have any kind of statistical relavance, maybe with 20 shots at each length there would be something provable, but my results from 3 shots at each length was a drop in speed coming in closer to the lands. Thats said the es of each load cover basically covers the spread between loads. The longest bto is 0.010" off touching the lands.