Seating Depth Testing

[USAFRet]

So, question for you experts. When adjusting seating depth, let's say increasing the depth by .003 at a time, when do you know when to stop? When do you know the seating depth causes too much powder compression? Do you measure case capacity and do some math?

6.5 Creedmoor 140 Grain Nosler Custom Competition
Ruger American Predator
H4350 (37-39 grain node test range)
IMR4895 (33-35 grain node test range)
Listed COL 2.775 (Nosler Load Manual)

 

[DeltaBravo]

I let the target tell me what the rifle wants.

Edited to add that I start at "touch" and back away in 0.003 increments until the group tightens up and then gets worse. In my last 2 or 3 Shilen 6BR barrels this has proven out to be at 0.009 off the lands in each. HTH

 

[Sandstorm]

Some bullets and barrels like quite a bit of jump....your just going to need to put in the
time to test your combinations. A 308 I have shoots the best .088 out from touch.
Thats with a .175gr Berger LRBT and the lighter bullets are a bit different but not a lot.
There all going to be different and the target will tell you.

 

[Big Mc]

So, question for you experts. When adjusting seating depth, let's say increasing the depth by .003 at a time, when do you know when to stop? When do you know the seating depth causes too much powder compression? Do you measure case capacity and do some math?

6.5 Creedmoor 140 Grain Nosler Custom Competition
Ruger American Predator
H4350 (37-39 grain node test range)
IMR4895 (33-35 grain node test range)
Listed COL 2.775 (Nosler Load Manual)

Let the target tell you when to stop. I use .005 but thats just me. Tommy Mc

 

[jepp2]

Do you measure case capacity and do some math?

Both Quickload and GRT show powder fill, so I use that.

Listed COL 2.775 (Nosler Load Manual)

This may well be limited by magazine length. What is your touch or jam measurement?

I've used the Berger recommendation for their VLD bullets

Load 24 rounds at the following COAL if you are a hunter (pulling a bullet out of the case with your rifling while in the field can be a hunt ending event which must be avoided) or a competition shooter who worries about pulling a bullet during a match:
1. .010 off the lands (jump) 6 rounds
2. .050 off the lands (jump) 6 rounds
3. .090 off the lands (jump) 6 rounds
4. .130 off the lands (jump) 6 rounds

 

[USAFRet]

Thanks all. I have some chamber gauges on the way.

 

[K22]

Unless you only need to shoot one shot at a time, magazine size may limit your seating depth.

If you have a rifle with a lot of "freebore" such as the Remington 700, the longest seated depth is controlled by having enough bullet in the case to provide adequate neck tension (hold) on the bullet. Typically, the rule of thumb is one bullet diameter inside the neck of the case minus the boat tail portion of the bullet.

Because of variations in bullet ogive, I start my load development at .020" from the lands sometimes further away because of the factors listed above. I use the .020" starting point because a priority for me is not to have a bullet become lodged in the lands when I extract a live round in the field.

I've often found that the aforementioned seating depth produces excellent results. In the past I've experimented with different seating depths and while some showed a slight reduction of group size, it wasn't worth the effort or expenditure of components for my purposes.

However, I'm quite sure that at the benchrest level of precision needed to be competitive it can and probably does make a difference for them. Probably the same for the long-range F Class guys too.

 

[Ned Ludd]

As the bullet is seated deeper in the case, pressure will generally increase, even if it may not be obvious over small seating depth increments. The question of how far down the bullet can be seated before excessive pressure becomes an issue depends on a number of factors, including the case volume and how close to MAX pressure the load was initially. As has been noted above, reloading programs such as QuickLoad and GRT can be very useful for making pressure predictions in reloading scenarios such as this.
Alternatively, one can simply monitor velocity carefully during the seating depth test. A warning sign would be that velocity increases too much as the bullets are seated deeper.

In my hands, it is usually possible to move a jumped bullet by as much as .015" to .020" in either direction without causing a significant change in velocity. I often carry out charge weight testing with a bullet seated at .015" off the lands. Once a charge weight has been selected, I might test seating depth over a range of something like .003" off to .030" off or so. Because the initial charge weight testing was carried out with the bullet seated at [approximately] the middle of that test range (i.e. .015" off the lands), the seating depth test is effectively only moving the bullet (and thereby altering pressure) by half the total seating depth range that was covered.

If one were to optimize charge weight with a bullet seated fairly close to the lands, let's say something like .005" off, and then wanted to test seating depth with that bullet/powder/primer combination out to .075" or .100" off the lands, pressure might well become an issue if the load was anywhere close to MAX pressure initially. What it seems the OP is really asking is how do we monitor pressure during such a test to prevent possible safety issues that might be caused by a compressed load. I would suggest that not loading right up to MAX pressure is a good place to start. That way, there will be some head room built into the system, just in case. Using a predictive program such as QuickLoad or GRT can also be very helpful/informative. Finally, paying very close attention to pressure signs from the rifle setup is always a good practice. Unfortunately, pressure signs such as hard bolt lift, cratered primers, and ejector marks sometimes don't show up clearly until the pressure is already at an unsafe level. Nonetheless, maintaining vigilance is important so that testing can be halted if/when obvious pressure signs arise.