Optimizing jump first vs. charge weight in load development.

[seymour fish]

I'd vote for 1) tangent ogive .010-.015 off, crude powder ladder, play with seating to optimize, then fine-tune powder to achieve desired ES in the dead center of the node 2) Chase the lands while tipping the can as needed,. Experience says must do this, or you are leaving something on the table. Seymour

 

[mikecr]

Mike, Are you saying you don't chase the lands?

That's correct. Land chasing is only needed while relying on starting pressures jam provides. Where jam is not relied on, tested best seating holds.

Most shooters are not jammed, and they don't need to chase anything.
But your combination, when fully tested, will tell you what you need to do.

 

[seymour fish]

Mike, In my admittedly limited experience with long-range I found one very clear instance where it was necessary to maintain seating depth at .005 off the lands to maintain best grouping, and to add a bit of powder to compensate for the slight increase in usable powder capacity. As the throat moved, groups went from best of 2-1/2"/worst of 6", to 8-10" at 1K, in nice steady conditions. Groups came right back with above titration. Just one example, but repeatable in this particular rifle. Speculation as to why ? Seymour

 

[mikecr]

I can't speculate why, hell I have no idea why one setting is better or worse.
I'm just saying that I haven't run into a need to chase anything yet. And I've read the same from others. But your observation is as good as mine I'm sure. I'll remember it.

 

[lurcher]

I know there is a lot of voodoo surrounding the physical effects of adjusting seating depths, as very minor adjustments, which you would think would cause infinitesimal changes in pressure seem to have definite effects. I've no idea whether chasing the lands or not is necessary but I have read that seating depth changes of 3 thou can have measurable effects on group size and so like everyone I make every effort to maintain a tight tolerance in seating depth. What I don't get is that if you tune as Mike says which is fine, surely throat erosion will affect group size as the throat and hence relative seating depth changes as a throat wears. Mike might be right, I have not burned through a pile of barrels as many have on here have so I just don't know. But if what he says is true (ie no need to chase the lands) then it just doesn't compute?? essentially initial seating depth is not affected by changes in actual seating depth as a barrel wears. How can this be???? I appreciate that just because it doesn't compute doesn't mean it isn't correct.

 

[seymour fish]

Lurcher, Rank speculation, but perhaps if a U-shaped distribution could be assumed for pressure, with higher pressures to be found at hard jam on the one hand and deep-seating of bullet into case on the other hand, then throat erosion could be seen to decrease pressure if best seating depth was initially set into or just off the lands. Perhaps some idiosyncratic differential in powder burn-rate regards initial pressure spike. Chrono ought to show it. Seymour

 

[AlanPF]

...Most shooters are not jammed, and they don't need to chase anything...

Fair enough. Its rare that I jump, because I have more success jamming. Maybe the reason for poor success with jumping is that I chase the lands! ;D

 

[barefooter56]

seymour fish & mikecr,
Load density changes as the bullet is seated out as the throat moves and may affect velocity and therefore bullet BC. A slight increase in powder can / may get you back to your accurate load. There are many variables to the tune of a rifle and when things change the best we can hope to do is to get them close to back to what they were before.
Take care,
Phil Hoham
Berger Bullet Tech.

 

[mikecr]

Maybe the reason for poor success with jumping is that I chase the lands!

Over at the [Dumb question about Seating Depth] thread
« Reply #14 on: Yesterday at 01:16 AM »

Joe I've got a buddy that throated his out, because he didn't like the bullet in the case as far as it was, can't remember how far he went, but he took the bullet forward after he throated it. The groups opened up, but by putting the bullet back to the same seating as before, the groups tightened back up. This was at 1000 yards so you will see a lot easier any changes. So Barrel Harmonics are a big part of it.

Joe Salt
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I read similar out there now & then, and I haven't ever needed to change seating for a bullet & barrel.
It's a clue about what's going on with seating. I'm just not intelligent enough to understand it. But I know while off the lands, seating is not about pressure, and it's in no way a 'fine adjustment'. You could exactly rematch pressure with powder while mis-seated, and accuracy will not be regained. You would have to restore best seating to get accuracy back.
But while at best seating(as originally determined), you can chase pressure with powder if needed to keep your accuracy.
It has always seemed to me that seating and powder are somehow completely independent (where not relying on jammed for pressure also).

 

[jlow]

So here is a question for those who jump and not jam – I understand that throat erosion will eventually happen and this will automatically lengthen the jump if not adjusted. I also understand that the degree and rate of erosion is dependent on bullet weight and charge, but on the average, I understand that a slower caliber like 308 will do this slower than something like a 6.5 Creedmoor.

My question is at what jump distance would have a negligible effect over the life of a 308 barrel? I understand that this of course would not be the primary parameter to look at when choosing a jump distance but precision/accuracy, but given the choice, what would that distance be? For example, I have heard the number 15 thousands thrown around as a good distance because “one would not have to worry about throat erosion”. Anybody have a real life experience to back this up?