Does a particular guns preferred seating depth node with a particular bullet change with different charge weights of powder or even different powders. Been reading a lot on seating depth testing before powder and I’m having a hard time believing that the seating depth node worked up at a minimum charge of powder would stay the same at a maximum charge
Is seating depth a constant?
- Thread starter Markg308
- Start date
I believe it does. I can tune a load with a set coal with powder charge or I can tune with a set charge by adjusting seating depth. If one or the other changes, groups will spread. I believe there are bullet designs that are less sensitive and have a larger node, both powder and depth than others but at distance, they will show the changes
Seating depth work in concert with the powder to produce a certain pressure curve that causes the rifle to resonate at a particular frequency for a particular gun. So, keeping the same seating depth with different powders will almost always produce a different pressure curve and change the harmonics. Guns and their barrel prefer a certain pressure curve for the harmonics produced where the barrel time of the bullet matches (when bullet exits) the amplitude of the harmonic at the null point/node. To find this, one must play with various powder charges and seating depths to find it for the particular gun/barrel.
While the harmonics are pretty much the same for either the minimum charge of powder or a maximum charge, the bullets barrel time will vary, either charge could find a node . . . but it's not automatic. This is where the Optimal Barrel Time vs. Barrel Length comes into play (you can do a search on that).
. . . .hmmm, I hope I'm not confusing you.
I have found that in general, a bullet will like a seating certain depth even when changing powders. But that comes with qualifications.
For example, the 300 PRC usually likes around 79 grains of H-1000 or around 83 grains of N570 with 210-220 grain bullets. Usually, I start the bullet at .020" off the lands and most of the time that's what it likes. So if I start with N570 and it isn't happy, I'll change to H-1000 but keep the .020" seating depth.. That usually makes it happy. There might be a slight improvement at .005" either side .020, assuming it's shooting pretty well at .020, but I am not sure it's worth it in a hunting rifle.
Now for my LR BR rifles, .5 MOA is not good enough and I will tweak seating in .002" to .003" increments, depending on the cartridge. I also tune the rifle before every match. However, these rifles are capable of showing very fine levels of tuning.
For example, the 300 PRC usually likes around 79 grains of H-1000 or around 83 grains of N570 with 210-220 grain bullets. Usually, I start the bullet at .020" off the lands and most of the time that's what it likes. So if I start with N570 and it isn't happy, I'll change to H-1000 but keep the .020" seating depth.. That usually makes it happy. There might be a slight improvement at .005" either side .020, assuming it's shooting pretty well at .020, but I am not sure it's worth it in a hunting rifle.
Now for my LR BR rifles, .5 MOA is not good enough and I will tweak seating in .002" to .003" increments, depending on the cartridge. I also tune the rifle before every match. However, these rifles are capable of showing very fine levels of tuning.
They work in concert with one another. I will always test coarse powder, coarse seating, fine powder, and then finalize with another seating test.
BoydAllen
Gold $$ Contributor
Generally I start with a seating depth that has worked in the past in a number of situations and work up a powder charge that works best with it, then I may explore other depth options. Many times this will be .006 longer than touch, but given how I establish touch, it is probably farther in than that from a very accurate touch. From there I start low and test charge weights in increments that are appropriate for the case size. ranging from .2 to .5 gr. If the rifle is well built, given that the day's conditions are conducive, and I am shooting off of a good bench over flags with a good rest setup, I may initially work with two shots per charge, look at groups and then test the best with more shots.
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I noticed a drastically different seating depth node when I switched from RE26 to Retumbo when shooting the Berger 156EOL in a 6.5PRC Proof carbon barrel. Just a single example but it caught my attention.
That’s one of the reasons I quit looking for short cuts.
JEFFPPC
Gold $$ Contributor
Nnealm66, don't you understand. We live in a world of 'shortcuts' and 'do less work'. Extremely popular.
I'm in the process of tuning for 2 different bullets for my 6br. 105 gr Berger vld and 95 gr Berger vld. I found, to my surprise, the both shoot best at touch -.006. 2 different powders, n540 for the 105s and Shooters World Precision rifle for the 95s. Both bullets started to group at 2850 fps. I checked my notes on the 90 gr Berger boat tails, the like to be jambed +.015.
Morale of the story.....Test
PopCharlie
Morale of the story.....Test
PopCharlie
In my experience yes it changes (edit misread the initial quote). If you change bullets with the same powder primer and brass the preferred depth will change. If you change powder primer or brass but use the same bullet the preferred depth will change.
Seating depth is done to accomplish two things. Find a place where the bullet will align itself in the bore consistently when fired. And make micro adjustments to muzzle exit timing for better harmonics. For 7 caliber tangent ogive bullets and 1.5 degree leades this is easy to do at multiple depths and will often exhibit periodic accuracy responses to seating longer or shorter. And typically the further out you go the physically longer the node becomes. A node at .010" off might only be .003" long. A node at .050" off will typically be wider than .003".
Secant ogive bullets are harder to align in the bore. That is why if they can be jumped at all they typically like being very close to the lands. So often times with secant bullets there will be only one or two seating depths right off the lands that works well at all. There is a reverse condition that sometimes works as well. If you give a secant bullet a long running start sometimes this will force a consistent alignment and can work as well. Middle distances typically have a hard time.
Jam removes alignment as an issue (if your bullet runout isn't too bad). This is why in my experience the average jam load is more accurate than the average jump load regardless of bullet.
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Do you remember the opening of comedy spoof soap-opera episodes of years back:
"Confused? You will be after this episode of SOAP!"
I must admit to being dubious about Berger's advice to initially stick to the starting charge while changing seating depths having always done it the other way round. I have finally loaded up some 7-08 rounds with a new bullet (162gn ELD-M) using the Berger method and will see it's superior sometime in the new year.
Interesting, I never looked at it that way but makes sense.
Of course it does because both seating depth and powder charge control the time it take for the bullet to exit the muzzle. And considering the speed of sound steel is almost 10 times the speed of sound in air, you can imagine how a slight change in the timing of the bullets exit can make a dramatic change in the impact. You want the bullet to exit the barrel at the top of the vibration profile. Changing timing changes where the bullet exits and negatively effects accuracy.
Dave M.
The more I learn, the more details I find giving me room for improvement. Although this post has some time on it, it is a question that I found an answer recently. To me it makes all the difference when playing with seating depths, This well respected individual (Satterlee) explained it very well. He is up there with Erik Cortina. Enjoy.
BoydAllen
Gold $$ Contributor
I don't work up a seating depth at minimum charge. Over time I have found a particular starting seating depth that has given pretty good accuracy over a number of calibers and bullets. Essentially it is slightly into the lands so that there are marks that are very short, shorter than wide by about half. Starting there I use a manual to come up with a safe starting load that is lower than I would want and do a one shot per charge test up to the point where pressure signs tell me to stop. I shoot all shots at the same target, holding center, shooting on a day when the wind is easy to handle with a couple of flags out between me and the target which is at 100 yards. I don't bother working up loads for rifles that have not been properly bedded. What I end up with is usually some clusters and it is in the middle of the most likely looking of those clusters that I try my first three shot group. I do all of this loading at the range, and if I care for a particular application, I shoot all shots over a chronograph. This has worked very well for me over the years. Some shooters seem to be afraid to seat bullets into the rifling, but of course when you start low, you are not going to have a problem, unless you are totally clueless about pressure signs. Always use a published reference for your starting load because some powder case combinations can be dangerous with loads that are below a safe minimum. After I am satisfied with my charge weight I start experimenting with seating depth, making moves that are no more than .002 when in the rifling, or close.
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