Seating depth

[pj1]

After some advice please. I’m having to change the seating depth of my new homeloads to improve the accuracy.
Not had to do this before as always had good groups with standard for calibre coal.
By how much do you guys change the seating depth in increments.
Eg.
Seated at 71.6 mm I get 5 inch groups
Seated at 71.1 mm I get 1.5 inch groups.
Do I go in .1 mm increments or more or less.
Cheers Pete

 

[jcampbellsmith]

Not sure what these mm things are. I move bullet seating 5 thou (0.005 inches) at a time. Reloading hasn't gone metric.
Regards
JCS

 

[yellow belly]

Try 3 thou at a time.

Are you chasing the lands? If so re measure the CBTO and you'll probably find its grown since you last measured it

 

[pj1]

Not sure what these mm things are. I move bullet seating 5 thou (0.005 inches) at a time. Reloading hasn't gone metric.
Regards
JCS

sorry I’m normally imperial but just could remember the metric measurements without looking up the conversion.
Would folks generally think a group could be shrunk from 1.1/2 inch to 1/2 or 1/4 inch by changing a seating depth.
Not chasing the lands at all. Went with a coal that was recommended to me but finding further of the lands works better in my rifle.

 

[User00040]

For stalking ammo, I'd load with a decent jump.

Nothing wrong with loading to the COAL specified by reloading manuals.

Trying to tweak more than that is for the anal retentive type...

 

[Bowland blades]

Not sure what these mm things are. I move bullet seating 5 thou (0.005 inches) at a time. Reloading hasn't gone metric.
Regards
JCS

1mm is approx 30 tho'

 

[Sakoodin1]

Not sure what these mm things are. I move bullet seating 5 thou (0.005 inches) at a time. Reloading hasn't gone metric.
Regards
JCS

This

 

[malmick]

Do I go in .1 mm increments or more or less.
Cheers Pete

With 0.1mm you would have a 4 thou increment.

 

[DavyG]

For stalking ammo, I'd load with a decent jump.

Nothing wrong with loading to the COAL specified by reloading manuals.

Trying to tweak more than that is for the anal retentive type...

It’s hard to beat “a decent jump”.
Ok…he’s not chasing the lands, but I’d still like to know how far I can stick it in and pull it back out without the end falling off and emptying my load in the throat.
Sort of doesn’t sound right that…
DG

 

[tim@tcs]

I found a problem with seating far enough out to reduce the jump meant that it was a tight fit in the magazine so I stuck to magazine fit as more important

 

[Alantoo]

Do I go in .1 mm increments or more or less.
Cheers Pete

I have also always used the bullet manufacturer's recommended COAL, or 5 thou over in the case of TTSX in 308 which brought the case mouth away from the cannelure for use with a FCD.

I have just experimented with the Yew Tree bullet's seating depth...I did my initial pressure load test string with the bullets set with 0.308" of bearing surface in the case neck...the results were around the 1" between the two shots at each load...and they came out a bit slow on the Magnetospeed. Not a sufficient sample to show maximum group size but fine to show minimum.

Following discussion and advice from Richard at Yew Tree I increased the load a bit and seated them at 50 thou off the lands which gave me 0.367" and then subsequently some more with the same load at 10thou increments. 48thou gave 0.543" 50 thou off gave me 0.214" 60 thou off gave 0.284" 70 thou gave 0.594" 80 thou gave 0.792"

I will do a few more confirmation groups at 50 thou off and maybe at 55 but I was really surprised at the differences and would think in your case increments at ten thou or 0.254mm would be worth exploring...

Alan

 

[MGD]

There will be a accuracy node every 6 thou so I go 3thou and start 30 thou of the lands .

 

[User00040]

@MarinePMI already did us a favour by getting in an authority on the subject.... Satterlee Method - Practical Reloading

 

[Bowland blades]

remember the position of your rifling will change with round count. Personally i like to be off the rifling hunting because wet ammo or chamber will rise pressures further. Always ensure you have seated at least the caliber deep into the case or your rounds can be too easily knocked out of concentric and having a run out of no more than 2 tho' is imo far more important than jump . Like i mention your jump will increase so you end up chasing the rifling .
Hunting bullets are made to be jump tolerant because they will be fired in a large range of rifles both brand new custom to grandads old deer rifle

 

[pj1]

Thank you all for your comments and thank you Alan for the explanation and picture showing how the seating depth can change the group sizes.

 

[ChesterP]

I'll buck the trend and say DON'T chase the lands as the throat erodes. There's no need. I don't find that need even with secant ogive bullets. Find the seating depth that works, as said with no more than 2-3 thou jumps back. I start at least 15 thou off the lands and work back from there. You find the seating depth for that specific load which gives best group AND lowest SD/ES and STICK with it, Don't chase the lands as they wear. Find that seating depth and if accuracy goes off (as with a different batch of powder whch can be more or less "hot") alter the powder charge in steps of 0.2 increments either way and you'll find that node again. It's about keeping the same barrel time that matters from a harmonics POV.

 

[NullMac]

Seated at 71.6 mm I get 5 inch groups
Seated at 71.1 mm I get 1.5 inch groups.

5" really ?

That does not sound like a seating issue. Are you sure you have consistent ignition ?? Is your ES/SD ok? Shooting? Scope? Whatever. Seating depth tuning will tighten up the grouping a bit but is tuning. A load that is giving 5" or 1.5" I would not consider worth tuning.

I have never tuned seating depth and 308 was always ~sub 0.5moa, 243 may be 0.25moa (it's small, don't really care how small), hence it feels like a more fundamental problem to me

You have to get ignition right first (Saterlee method as above), then seating depth to get a good group at 100m, then if groups are rubbish at long range change the bullet.

 

[pj1]

I kid you not
Five shot group shots 1 and 3 nearly through the same hole the others all over. And for comparison three shots near through the same hole with factory ammo

 

[Muir]

After some advice please. I’m having to change the seating depth of my new homeloads to improve the accuracy.
Not had to do this before as always had good groups with standard for calibre coal.
By how much do you guys change the seating depth in increments.
Eg.
Seated at 71.6 mm I get 5 inch groups
Seated at 71.1 mm I get 1.5 inch groups.
Do I go in .1 mm increments or more or less.
Cheers Pete

How did you come up with the original seating depth? And yes. Bag the metric for reloading. ~Muir

 

[09mutley05]

I kid you not
Five shot group shots 1 and 3 nearly through the same hole the others all over. And for comparison three shots near through the same hole with factory ammo

Interested in what rifle you are shooting also could you tell me what bullet weight, powder and barrel length I have a friend that is having a similar experience great groups for 3 shots then open up on shots 4 and 5.

Many thanks