Decreasing speeds with longer jump

[atkins08]

I have been in load development for my 6xc. Using Rl 16, norma brass on second firing. Fed 210m primers. When doing a seating depth test at 5,8,11 thousandths off the lands the speeds averaged 3070. At 14, 17 thousandths off it dropped to 3055 fps. This seems backwards to me because it would seem that seating deeper in the case, decreases capacity and increases pressures. What am I missing?

 

[Joe Wade]

It is not about compressing the powder column.
Pressure increases the closer the bullet is to the throat.
The more a bullet jumps, the lower the pressure. This is why many factory rifles have excessive free bore, for added safety margin.

One common reloading practice is to work up a new load with the bullet seated at the throat "at jam".
After finding the best powder charge, fine tune the load using bullet seating depth away from the throat
"jump". As you seat the bullet farther away from the throat, the pressure will drop since the peak pressure was already established by using the bullet at jam.

Good shooting

 

[M-61]

Think of factory Weatherby's. Loooong freebore.

 

[Ned Ludd]

Because the typical methods we use to find the distance to "touching" the lands can be imprecise, it is difficult to say exactly how close to the lands your longest rounds actually were. In general, .005" off as estimated by the usual methods is probably not going to be into the lands. It may actually be .003" off, for example, but it shouldn't be touching or .003" in. Once the bullet is fully disengaged from the lands, you shouldn't be getting pressure spikes due to the bullet being engaged in the lands. It's either touching/in, or it's not.

I typically start load development with bullets seated at .015" off the lands. This works for me because I'm not using bullets that normally need to be seated into the lands, which may not be the case for everyone. When I reach the point of doing seating depth testing, I typically test from .006" off through .024" off in .003" increments. I have yet to find a bullet that didn't "tune in" within that window.

The important point is that my .015" starting seating depth is exactly in the center of that seating depth window. So I'm only moving the bullets .009" in either direction from the initial charge weight testing seating depth. Under those conditions, I cannot reliably detect a difference in velocity. However, the velocity (ES/SD) values for a typical shot string are larger than the precision of the chronograph, which is about 3-4 fps with the MagnetoSpeed chronograph I use. For my typical loads with ES values in the 10-15 fps range, that means that the limiting factor of any velocity measurement I take is most likely the velocity variance of the shot string itself, not the chronograph.

I agree that you might think as you decreased the volume of the pressure cell by seating the bullet further off the lands, you'd expect that if anything, the pressure (and velocity) might increase. However, I would ask what the precision of your chronograph is, how many shots you fired in each string, and what the ES/SD values were for each string. In other words, are the two average velocities that differed by 15 fps really different? You really need to be able to know this to say with any confidence that the 15 fps average velocity difference is not within the variance of the samples. It may be real, but it also may be down in the noise. 15 fps is not "nothing", but it's not a huge difference either. If real, that small of a difference could come from a number of things including charge weight variation, primer variation, neck tension variation, even how much the barrel was fouled between the two strings if there were a large number of shots in between.

 

[jlow]

Exactly.

 

[Brians356]

Hmmm ... I'm not so sure.

If you start well off the lands (say .015") with a charge near max listed, and a high load density (but not necessarily compressed) then as you increase bullet jump (decrease net case capacity) pressure and velocity will increase.

Here's an example from my notes:

223 Rem, 29.1 grs CFE 223, 50-gr TNT

Jmp Vel (avg)
.014 3427
.017 3433
.020 3453
.023 3463
.026 3470
.029 3498
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[Twoboxer]

I think it depends on the caliber and your chamber. EG starting with the shortest tested COL in a factory 30-06, I've seen MVs decrease as COL increased up to a certain point. MVs increased from that point to the lands.

 

[Ned Ludd]

Is 29.1 gr CFE a compressed load (or close to compressed) in your setup at the longest seating depth (.014" off)?

I typically run .223 and .308 cases with a fill ratio between approximately 94% and 103%, depending on the cartridge and the powder. However, I generally try to avoid compressed loads for a number of reasons. What I can say is that I don't see velocity increases that are statistically significant when moving the bullets .009" in either direction. I can imagine it might become more noticeable (larger) the closer the load is to MAX pressure.

According to Quickload, my .223 F-TR load with 90 VLDs is predicted to increase/decrease velocity by ~3-4 fps if I move the bullet in/out by .009". That matches very closely with what I have observed on numerous occasions, and is well under the ES/SD values for that load, suggesting it is not statistically significant.

I also ran the numbers for a .223 load in a 24.0" barreled rifle, using 31.0 gr case volume and 2.260" COAL as "generic" inputs, together with the 50 gr TNT bullet and 29.1 CFE223, using the factory preset burn rate. With those inputs, 29.1 gr is only a 97.4% fill ratio, but I have no way of knowing how close the COAL, case volume, or barrel length is to yours. However, with those inputs QL predicts a change of only about 4.5 fps when moving the bullet .0075" in either direction (9 fps overall for a .015" total change in seating depth). The 70 fps change you listed above is what makes me guess that your load is on the warm side. Any time you approach the upper limits of velocity/pressure/fill ratio, etc., then of course that load is going to be much more sensitive to much smaller incremental changes in case volume than a load that is not so close to the upper limit. But those types of velocity changes are not necessarily the norm until you get close to the upper boundaries of pressure, volume, fill ratio, etc.

 

[Homerange]

I typically run .223 and .308 cases with a fill ratio between approximately 94% and 103%, depending on the cartridge and the powder. However, I generally try to avoid compressed loads for a number of reasons.

Would you mind sharing those reasons ?

 

[Ned Ludd]

In F-Class, we typically shoot strings of 25+ shots, so velocity changes during the course of a string of fire are already a major consideration. Depending on your location, the effects on velocity due to seasonal variation in ambient temperature, as well as heating of the action and barrel during the long course of fire can be more than enough to worry about without adding compressed loads on top of them. As I mentioned, any time a load approaches the upper limits of pressure and velocity for a given cartridge, there is a strong tendency for even small incremental changes to have a much larger effect on pressure/velocity than they might in a load at a reduced [relative] pressure/velocity. That effect can become magnified even further with long strings of fire when the barrel and action heat up.

Compressed loads have a much better chance of falling into this region than non-compressed, although I must clarify by stating that I'm talking about really obvious compression, you know, powder crunching when you seat the bullet compression. Relative "compression" can differ quite a bit depending on the powder. For example, none of my .308 Varget loads ever "crunch" when seating bullets, even though they're listed in QL as having fill ratios in the 103-104% range, which is technically compressed. However, I estimate I would need to have a fill ratio of at least 105-106% (or more) with Varget before it started to crunch when the bullets were seated.

In any event, the other issue you can run into with compressed loads besides being that much closer to the edge of the cliff as I mentioned above, has to do with seating bullets. If you have every run into this issue in virgin brass (which has less capacity than fired brass) using certain powders, you know it can be maddening when you're bullets don't want to seat all the way down because there is too much powder in the case. It is not always easy to know for sure whether this is the case by simple visual inspection of a charged case neck. Even Quickload won't always give a clear answer because most people don't know the exact capacity of their virgin brass.

I generally try to avoid these potential compression issues altogether by not knowingly running highly compressed loads. It's really just my personal choice, not any kind of reloading rule written in stone. It is certainly possible to run compressed loads with some powders (working up safely, of course) and there are plenty of people that compete with compressed loads in F-TR, no doubt about it. Personally, if I need a little more velocity and case capacity with a certain powder is limiting, I can usually find a slightly faster powder that will do what I want, and/or a powder with slightly smaller kernels that pack better (i.e. better load density). There are usually several different ways to get to a specific result if you play around with it enough.

 

[Homerange]

Thanks, not being a competitive shooter action heating affecting loads is something I've not experienced and it makes perfect sense. I have only one truly compressed load, 308 130gr and case neck half filled with ADI 2208 (Varget). It's not particularly hot but shoots well with low ES.
I'm well aware of new vs once fired case volume and pressure issues and never expect miracles from brand new brass.....and I neck size only.
In a load I worked up for a 222 I had the opposite to the OP when substituting a FB bullet with a BT one with all other things the same. From knowledge gained since and what I've read in this thread, this 222 load was close to max and both bullet bearing length (friction) and volumetric efficiency changes resulted in a HOT load that I smartly dropped.
Thanks, we learn a little each day.

 

[Brians356]

Is 29.1 gr CFE a compressed load (or close to compressed) in your setup at the longest seating depth (.014" off)?

I typically run .223 and .308 cases with a fill ratio between approximately 94% and 103%, depending on the cartridge and the powder. However, I generally try to avoid compressed loads for a number of reasons. What I can say is that I don't see velocity increases that are statistically significant when moving the bullets .009" in either direction. I can imagine it might become more noticeable (larger) the closer the load is to MAX pressure.

According to Quickload, my .223 F-TR load with 90 VLDs is predicted to increase/decrease velocity by ~3-4 fps if I move the bullet in/out by .009". That matches very closely with what I have observed on numerous occasions, and is well under the ES/SD values for that load, suggesting it is not statistically significant.

I also ran the numbers for a .223 load in a 24.0" barreled rifle, using 31.0 gr case volume and 2.260" COAL as "generic" inputs, together with the 50 gr TNT bullet and 29.1 CFE223, using the factory preset burn rate. With those inputs, 29.1 gr is only a 97.4% fill ratio, but I have no way of knowing how close the COAL, case volume, or barrel length is to yours.

Thanks for taking the time to explore this, I'm impressed. But I can solve the mystery - the 50 TNT in my rifle affords little neck engagement when seated even .014" off the lands. To seat it touching would not be an advisable option, but I prefer to jump bullets these days anyway.

PS
My COAL with 50 TNT seated to the lands is 2.367"! If arithmetic serves, my "as tested" initial COAL is .093" longer than your generic QL figure.
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[Ned Ludd]

I figured that might be the case. I haven't been loading anything less than the 80.5s in .223 and don't have a good feel for load parameters with the lighter pills. I think is probably wise to make the general assumption that the closer a load is to the upper end, the more concerned one needs to be about changing the pressure/velocity with small changes in seating depth. I just haven't ever seen changes with my .223 loads during seating depth testing that were outside of the noise.

 

[Brians356]

I figured that might be the case. I haven't been loading anything less than the 80.5s in .223 and don't have a good feel for load parameters with the lighter pills. I think is probably wise to make the general assumption that the closer a load is to the upper end, the more concerned one needs to be about changing the pressure/velocity with small changes in seating depth. I just haven't ever seen changes with my .223 loads during seating depth testing that were outside of the noise.

It could be that CFE 223 is more sensitive to volume changes in that load density range than other powders, it certainly behaves erratically in certain cartridges for reasons Hodgdon can't explain. For example they do not publish CFE data for 6mm Rem for that very reason, even though CFE works great in both 243 Win and 243 WSSM. So it's not unreasonable that it could exhibit other quirks.
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