Question about harmonics and OCW tests

[M1Amen]

Hi Guys,

Question here. I am wondering about OCW tests conducted using different seating depths of a projectile.

Let's say I want to do an OCW test, so I load up my rounds and set the ogive to base length to a certain distance (we'll call it X) using my micrometer die. Then I go shooting and find an optimum charge.

Now let's say I go home and remake all the same charges, but change the seating depth of the bullets to a different length (we'll call it Y) using the micrometer die.

Is it likely that I will find the tightest group again occurs at the same powder charge, or could the two different seating depths effect where the rifle finds a nice harmonic node?

The way I have done this in the past is to pick a seating depth and stick to it for all the rounds in the OCW. Then once I find the best charge, I experiment with seating depth. I am wondering if I could pick a charge near the top of the velocity range and then tune the seating depth to tighten the group. I am sure the group size might vary, but is it possible to bring the rifle into a harmonic node based on seating depth alone?

M1Amen

 

[Ol' RD]

M1Amen, what the first OCW test is going to give you is your node ranges for the barrel. Every barrel I've seen this test done for has at least 2 distinct nodes. It gives you the range of the node, say between 43.0 Grs an 44 Grs. You still need to run it a second time to refine it. Now instead of .5 gr intervals i'd use .2 Grs. Now we will know the center of the bell curve is around 43.4 Grs. If you want to refine it still is when you play with seating depth. Just my observation others may have their own method. If your using a node that is on the high end of the cartridges pressure scale, yes, changing the seating depth alot could be detrimental to say the least.

 

[McCrazy]

I start my OCW with the bullet touching the lands. That way after I find a node to use I vary my seating depth away from the lands. This prevents having a pressure spike like if I chose a high node while away from the lands and then tried a touching/jammed seating depth. I don't jam by the way.

Once I find a good node, I vary seating depth for accuracy and Load development is complete.

Could you pick a node and then be unable to find an accurate seating depth? I suppose you could but I haven't heard of that being the case for anyone yet.

 

[Greg Langelius *]

Barrel harmonics result in muzzle displacements in two dimensions, not just one, and can simultaneously reflect different frequencies along different axes. This is why dispersion can, but only sometimes, results in stringing.

The relationships between seating depths and rifling engagement depths can alter pressure curves, resulting in entirely different bore transit times. This is precisely why I deliberately choose not to seat my bullets into contact with the lands. Charge weight variations introduce more than enough variable into the accuracy challenge, without adding altered transit times as well.

I do not experiment with seating depths, except to ensure that all the bullets jump, and sometimes that they will feed from the magazine, as with my Mosin-Nagant 91/30's. With three rifles, I don't have the time and patience to load different seating depths for each, so I optimize seating depths on the basis of feeding from the strippers/magazines.

For some rifles, ultimate accuracy will be the key priority; for others, that takes a back seat to overall functionality.

Greg

 

[Jedi]

I determine seating depth by case OAL to neck shoulder junction, that plus bullet OAL from boat tail junction will correctly position bullet in case.
VlD type bullets i seat .010 crush, others i jump .010
i vary the charge weight, then OAL. Anything that effects PSI of shot will affect barrel time
Keep the variables to min, less crap to track
Depending your comfort level/skill will determine your load routine
Nothing wrong with setting OAL and just varying charge weight

 

[Sniper Uncle]

I "set" my "standard" seating depth based on measurements taken from my chamber with the guaging tools. I do not touch the lands, rather have a few thousandths jump. I then found my nodes, (using OBT calculations and then verifying in the field) and use seating depth to refine them, as a difference of even .001" seating depth can change barrel time by .03 milliseconds.

I only change one variable at a time. I have not done any touching or jamming in the lands, as I am only loading for one bolt rifle in each of two calibers. Maybe I am missing something in not testing the pressures jammed, but I haven't seen any real problems with my groupings that I can't blame on myself and lack of shooting skills. I personally think for myself that doing pressure tests with touching or jammed bullets is not beneficial to my status in shooting now.

 

[AZ.noob]

I start my OCW with the bullet touching the lands. That way after I find a node to use I vary my seating depth away from the lands. This prevents having a pressure spike like if I chose a high node while away from the lands and then tried a touching/jammed seating depth. I don't jam by the way.

Once I find a good node, I vary seating depth for accuracy and Load development is complete.

Could you pick a node and then be unable to find an accurate seating depth? I suppose you could but I haven't heard of that being the case for anyone yet.

McCrazy is 100% spot on. With bullets jammed, I find & fine tune my node, and then I leave that alone (assuming case capacity remains constant). Only then will start in with deeper seating depths in .010 increments.

 

[sandwarrior]

First, As Greg noted you have to decide what the round is for. If shooting super tight groups, I'll change anything I think isn't working to get me tighter accuracy. Seating depth can be a big factor in pressure and 'run at the lands' to get extra speed. But when considering match or 'field precision ammo', you have to work within the parameters you are given. The bullet must be off the lands. It's pretty much got to work across a number of rifles. (Although I only use dedicated brass to each rifle). I used to obsess over accuracy until I realized that that kind of accuracy really only applies at short ranges. Long range shooting is going to require more skill in determining the environmental factors than the .1 or .2 moa you can tighten up a group. You will want to get it as tight as you can. But, at some point you have to stop tweaking and go with the best load you can for the whole lot.
In short if this is your tactical load then set the parameters you need. Work up a powder load that will get you the required accuracy. There isn't a whole lot of seating room to mess with.

In short you can tighten up groups with seating changes, but it's not going to happen without an optimal charge weight in the case to begin with.