Seating Depth Rule of Thumb

[Big Al]

3 hours ago, Laurie said:

As to the latter question, the Hybrid is actually a VLD, but with a 'fix' to try and overcome the VLD's main disadvantage. I'm not at all convinced the 'fix' always works ... or even works at all!

That would be where Im at Laurie.

Ive shot a lot of 6mm Berger 105 VLD and 105 Hybrids through my benchrest rifle these last 12 months. Ive done lots of load development at both 100 & 600yds and my experience is that the 105 Hybrid is affected just as much by seating depth changes as the 105 VLD and show no extra tolerance at all.

My feeling is that barrel harmonics and bullet timing is where its at although Im no scientist so can't explain that in any way above simple layman terms, it certainly works in my mind much better than any kind of self aligning of the bullet theory. I come to this finding simply from the fact that Ive had loads that have shot very well near the lands, say in the jam to 20 thou off range. Then the accuracy has came back again at a medium distance away from the lands, say in the 40-80thou off range and again I can find a node at what I consider a long jump to the lands of say a 100-160 thou jump. Of course I would prefer the longest OAL when shooting a single feed action but it shows there are times when being limited to magazine length doesn't mean we can't achieve great accuracy.

We accept that powder charge variations affect barrel harmonics and where exactly the muzzle is in relation to its travel determines group shape and size so, weak all pretty much agree on that. For me this issue of bullet timing, not OBT as I confused things earlier could be one of how long the bullets journey is from case neck to muzzle in relation to the harmonic waves travelling down the barrel and back to the action is.

Once we have established a harmonic sine wave over a fixed barrel length and powder charge then that wont ever change, our next variable then is how we slot that bullets journey down the barrel and time its exit in relation to the fixed sine wave. On that basis it would explain to me why Ive been able to fine multiple sweet spots on seating depth for the same barrel and powder charge.

I said this earlier and it repeats what you said in your last post though, its far more important to us as shooters to know seating depth testing is vital to accuracy and a comprehensive load development than to fully understand the science of why the changes matter. Of course I concede that I haven't tested as many bullets as you over the years so my findings on this are over a much smaller sample size and could be turned on its head anytime soon

 

[Laurie]

I've started a thread on the Bryan Litz / Applied Ballistics supported section of the Accurate Shooter Forum to see if Bryan has managed to find any answers in the eight years since he asked the question about what is going on here.

http://forum.accurateshooter.com/threads/bullet-nose-profile-and-seating-depth-sensitivity.3948062/

I'm not confident that we will get any new insights, but it's always worth asking.

[Alex Wheeler]

I do not subscribe to the idea of one caliber of bullet seating.  I think that may even stem from thread strength...   Bullet shape (pressure ring or not) and neck tension will have more effect.  Neck tension is huge.  You need to test it to see what you combo wants.  I would error on more than less.  I start at .003 and go up.

[VarmLR]

That's an interesting observation Alex and not one that many people seem to pick up on.  Neck tension changes can affect pressure sufficiently to alter velocity/barrel time by quite a margin, and by definition, Al's point regarding harmonics.  My simplistic take is that whatever the variable force acting to affect pressure and timing, once we understand this and can work with it, we can find combinations that work for a specific bullet in our barrels.

Like Al, I have experimented with seating depths and found at least two accuracy nodes by starting 10 thou off lands and working back to 120 thou off using the same charge.  I usually find at least two nodes by keeping seating depth the same and altering charge weight, or fine tuned by getting into the ballpark with one and varying the other.  I haven't yet tried altering neck tension other than using a Lee Factory crimp die to aid consistency but have noted when I did that I obtained a slight change in pressure and a lowering of ES, so neck tension is yet another variable to understand and play with.

I'm not convinced at all that some hybrid bullet designs address the shortcomings (as I view it) of secant designs.  The only ones tried have performed much as if they have been pure secant profiles (ie close to lands seems to work best).

Be interesting to see what Brian Litz comes back with.