would seating depth affect the OCW?

[darth_ritis]

I'm trying an OCW test to find the best recipe for my rifle. I've got the powder charge test done, and now I'm ready to test seating depths. I was wondering if changes in seating depths, and the related changes in pressure, might have any affect on the OCW? I don't want to do another charge ladder if I can help it.

 

[Bender]

Shorter Oal will have higher pressure. Start at factory length and move to mag length. If you bench single feed move to touching the lands. If you have a good bug hole ocw now, just shoot it.

 

[Luky]

Yes, after my ocw test to find the most consistent load, I did another round of groups starting from touching the lands to .030" jump in .005" increments. Shot 5 round groups at 100yds and 200m. Found my gun likes .020" jump the best and also that my 200m groups were lower moa than the 100yrd groups of the same load. Wound up with .698 moa at 200m in my Frankin-SASS. Have not had a chance to get back to the range with a box full to confirm them though.

 

[SnkBit]

Shorter Oal will have higher pressure. Start at factory length and move to mag length. If you bench single feed move to touching the lands. If you have a good bug hole ocw now, just shoot it.

Wrong. Jammed will have the highest pressure. Always start load development touching the lands or slightly jammed. Once you have found your max charge weight this way it is safe to seat the bullet deeper to find optimum seating depths. If you start off at max pressure jumped and seat out longer towards jam you may experience dangerously high pressure spikes.

 

[Bender]

Wrong. Jammed will have the highest pressure. Always start load development touching the lands or slightly jammed. Once you have found your max charge weight this way it is safe to seat the bullet deeper to find optimum seating depths. If you start off at max pressure jumped and seat out longer towards jam you may experience dangerously high pressure spikes.

Yes you are correct. I didn't explain that as well as I should. I was meaning if the bullet is set back further into the case than the shortest OAL listed. That will cause pressure problems as well. As the bullet is moved out in the case, you create more case volume thus reducing pressure, until you jamb in the lands.

 

[darth_ritis]

Thanks guys! But we're a bit off topic.

Changing seating depths affects the chamber pressure, similar to how different charge weights affect chamber pressure. Those are two variables which will affect finding the "accuracy node" vs "scatter node" sweet spot for your rifle. So, if you decide on your optimal recipe based on charge weights alone, wouldn't the pressure changes from testing different seating depths have an effect your "optimal" recipe?

Perhaps it would be good to run a small follow-up OCW test with +/- 0.1,0.2 charge weights just to see if the variation in seating depth made a difference.

 

[Twoboxer]

Thanks guys! But we're a bit off topic.

Changing seating depths affects the chamber pressure, similar to how different charge weights affect chamber pressure. Those are two variables which will affect finding the "accuracy node" vs "scatter node" sweet spot for your rifle. So, if you decide on your optimal recipe based on charge weights alone, wouldn't the pressure changes from testing different seating depths have an effect your "optimal" recipe?

Perhaps it would be good to run a small follow-up OCW test with +/- 0.1,0.2 charge weights just to see if the variation in seating depth made a difference.

Practically speaking, if you've found a true OCW node pressure variations of that size shouldn't affect POI very much. I think that's part of the reason why once at the node, you just test to see if OAL variations at that charge weight tighten the groups up. The other part is testing multiple OALs at several charge levels opens up a very large number of new combinations.

I suppose you could have fun testing them all - but at the end of the day you can't test all combinations of everything (including ambient weather conditions), so . . .

 

[CharlieNC]

While seating depth is important in terms of how the bullet best interacts as it engages the lands, it's effect in terms of pressure/velocity/barrel time is relatively small compared to powder charge. For example for a 308 using 175-178gr bullets and IMR 4064, a change of .010" depth is roughly equivalent to a 0.1gr change in powder based on Quick Load. Of course this does not apply to the the pressure spike associated with jambing into the lands.

 

[gstaylorg]

If you're using a tangent ogive (non-VLD) bullet, chances are very good it will perform well somewhere between about .010" and .025" off the lands. Start the OCW workup with the bullet seated close to the middle of the seating depth range you think will work. That way, when you do the seating depth testing, you won't be moving the bullet more than half the total seating depth range in either direction. Unless the bullet is jammed either into the rifling, or way down into your powder load (compressed) changing your seating depth by .010-.015 in either direction is not likely negate the results of your OCW test such that you need to do it over.

 

[darth_ritis]

Sweet! Thanks everyone, I get it now.

 

[Fred_C_Dobbs]

Newberry directly answers your question in his OCW instructions.


Also, changes to shot start pressure do not necessarily directly correlate to changes in Pmax or MV. Jam seating produces the highest shot start pressure but not the highest Pmax or MV. To the contrary, Roy Weatherby built a firearms company on the principle that a long jump produces the highest MV.