Seating Depth Pressure Change

[Nomad47]

Sounds to me like the 30-06 graph is backing up what Walt Berger said.... The combination of seating change and "jump" change interact. But obviously it is not linear.

 

[Hedge]

"Sounds to me like the 30-06 graph is backing up what Walt Berger said.... The combination of seating change and "jump" change interact. But obviously it is not linear."

I believe that is correct. Weatherby worked out a high velocity/pressure solution by using a long freebore. Thereby allowing more seating adjustment without attendant pressure problems.

 

[gilream]

Park you car/truck 5 feet away and perpendicular to a 6 inch curb. Give it a little gas to get it moving and notice how it easily steps over the curb and keeps going.

Now, pull you car/truck up to the curb with both wheels touching the curb and come to a dead stop. When you give it the gas a lot of pressure and energy will have to build up before those wheels can jump that curb and move on. I'm no scientist, and not a competitive shooter but it makes sense to me that a bullet with a bit of jump, irregardless of a small degree of decreased capacity in that flexible brass case, has some momentum to easily engage the rifling and keep going.

Mike G.

 

[quartermaster]

So now all of these posts have got me wondering. I am having a 25-06 Ackley made and should get it in about 2-3 months. I have been reloading for 20 some years now and always been told also, that jamming heads would lead to pressure spikes. I have many times seated the heads very very close to the lands, but never touching. On the other hand, I have to fire form 25-06 brass for this rifle. The chamber on the Ack is slightly shorter than the chamber on the 25-06, but I have been told by many knowledgable people who have been through this process, that I should jam the bullets to be sure to completely form the case with a rather stiff load too boot. I was just getting over worrying about it until reading these comments. I realize that doing this will keep the case head against the bolt face and form the case fully with enough pressure, but I guess I'll have to work my way up with powder charges and check for signs of pressure. I have been told to use a hefty charge from the parent cartridge to start the fire forming. Maybe I'll have to strap down the gun and tie a string to the trigger. LOL

 

[SDDasher]

Quatermaster: "So now all of these posts have got me wondering".

Honestly, before all this started I was just blindly doing what I have done for the last twenty years following the sage ole advice of my mentor what taught me to reload....."son, jump the SMK's and jam the Bergers...." end of advice on jump/jam......I am so confused now I may take up bowling or checkers or sumtin else....hahaha.

 

[amamnn]

One thing to keep in mind when experimenting with seating into the lands, is that folks who do this routinely also have experience in dealing with neck tension and neckwall thickness, which are also factors involved in bullet release and start pressure. They also shoot actions which are built to tolerances that factory rifles are not. Until and unless you understand how these things affect pressure and accuracy, you may be at best, disappointed with long seating and at worst injured or sued by the guy at the next bench you injured.........If done properly, seating in the lands has some advantages in some applications, one of which is less variation in accuracy when throat erosion occurs.

I like the analogy of the bullet jump likened to driving your lil pickup truck over a curb. Consider your factory rifle to be a pickup truck and a custom built BR rifle to be an M1A1 Abrams tank. The tank backs up for nothing and no one.

 

[gilream]

It would seem to me that loads worked up with bullets jammed into the lands are perfectly fine as long as the load is worked up carefully. Berger 52 grain bullets work great jammed +.010 into the rifling of my Cooper 22PPC and that's the way I developed the load. With the 50 and 55 grain VMax bullets, I developed the load starting .020 off the rifling hoping to get all of the velocity possible and tweaking the seating depth as the final step in load development. I would NEVER move the bullet into the lands with this hunting load without reducing the powder charge first and working back up.

Mike

 

[mikecr]

Mike when you say the 52gr bullets 'work great jammed', and 'that's the way I developed the load', does that mean you tested for optimum seating before adjusting the load?

That's what I do.
I think that we lose a fantastic tuning aid with seating at any extremes.

 

[gilream]

Mike when you say the 52gr bullets 'work great jammed', and 'that's the way I developed the load', does that mean you tested for optimum seating before adjusting the load?

That's what I do.
I think that we lose a fantastic tuning aid with seating at any extremes.

No. I worked up the target bullet load starting with square rifling marks on the bullet jacket then adjusted the seating depth a bit both ways but all the while staying into the lands. With hunting bullets i usually start at .015 to .020 off the kiss, work up to the most accurate powder charge, then tweak the seating depth without going too close to the lands. As i stated earlier, I'm no expert and seem to learn new stuff all the time that is pretty much conventional wisdom to most of the informed folks on these threads.

Please explain what you mean by "optimum seating before adjusting the load" Sounds like I am probably missing something that could be of value...Thanks

Mike G.

 

[mikecr]

I see seating adjustments, and powder tune, as seperate and different things.
Sure they affect each other, but usually to detriment, and best case gain amounts only to minor shaping of
groups. The final tweak..
I can find powder tune regardless of chosen seating, and optimum seating regardless of charge, -provided
I'm not changing both at the same time.

If you choose to seat 125thou off(maybe due to magazine), and then powder tune with this, then going
backward to seating adjustments from there would cause two changes at once. You'll probably hit other
seating nodes, but by then you've taken yourself too far out of tune to recognize another seating as
better.
you'll likely find the 125thou seated tune as best at that point.
If you took the same barrel & then powder tune at 10thou off(chosen), well further seating changes from
10thou will just mess up your tune, and now 10thou off seems best..
This can not represent the truth. 'Best' is one or the other,, or neither.

My approach is to first separate tune from seating. On 2nd fireforming of new brass, I load half ~1gr under
my max pressure, and half ~2gr under, with bullet seating incrementally adjusted. One set is away from any
tune for sure(maybe both). Then I 3sht group each seating to identify the best of whatever I got.
Optimum seating will end up as the same depth in both sets as tighter for the charge, regardless of actual
group sizes at that point.
In other words, the -1gr charge grouping might reduce to 1/2moa at 5thou off, AND the -2gr charge grouping
might reduce to 1/4moa again at 5thou off. If that's the best I can do there, then that's the seating I'm
going with for further load development.
After powder tuning, I'll tweak seating +/- a few thou, and then powder in smaller increments.

Hopefully I'm good, but my best there might mean staring over with other bullets or powder..
I don't stick to bad loads for long.
Now this is my approach, and it won't be universally effective to every barrel or cartridge.
A 'competitive' 6PPC is pushed harder than possible with longer range or hunting cartridges. So
jamming may be the only and best way to get there.
With other systems, I would not assume so. No reason to really.

 

[spclark]

A 'competitive' 6PPC is pushed harder than possible with longer range or hunting cartridges. So
jamming may be the only and best way to get there.
With other systems, I would not assume so. No reason to really.

Not to mention the chance you'll face a "cease-fire! Make your rifle SAFE!" after you've just chambered a round, then end up spending five minutes trying to get all the powder granules out of your locking lugs - maybe your trigger too - once you can get back to the business you came for.

Bullets stuck in the lands when you clear your rifle will raise your blood pressure too. Not too many ranges are left where you can raise your muzzle past horizontal with a live round in the chamber.