Have a bullet seating depth question

[smokin502]

I have been reloading for about 5yrs or so and started playing around with seating depth for my 300wm. I used a 230g berger vld seated 10 rounds to the saami specs. Then 10 rounds seated to .020 from the lands. Same bullet, same powder and weight, same everything. Tested both rounds and the .020 from the lands on average was 140fps faster. So my question is how? I've been told by extending it out further would lower pressures thus maybe being able to add more powder to increase fps with the same pressures. I'm I that far off here? Thanks for any help on this

 

[elkaholic]

Usually seating the bullet closer to the lands increases the pressure because it doesn't get as much "jump start" before getting resistance from the lands. I will add though that 140 fps is pretty extreme if you are still .020" off?....rich

 

[RAT807]

Possible, you had a lot less blow by with the shorter jump to the lands, keeping more PSI behind the bullet (pressure spike). But I haven't measured all your stuff to make a more educated guess... How far off the lands is the bullet in your rifle at Saami spec length?

 

[smokin502]

Usually seating the bullet closer to the lands increases the pressure because it doesn't get as much "jump start" before getting resistance from the lands. I will add though that 140 fps is pretty extreme if you are still .020" off?....rich

That definitely makes sense, and it's the conclusion that I have made. But I wanted to get some more experienced help from LRH. I've noticed that I can't even use the "max load" data as it has blown out the primer. I used 1g less and doesn't show any signs of issues.

 

[Mikecr]

I agree with others.
And add, you can assign less reference to 'specs' and 'data' for YOUR reloading.
Work up to YOUR max, test seating to YOUR best target results.

I also doubt a 140fps gain from seating. That's a lot

 

[smokin502]

I agree with others.
And add, you can assign less reference to 'specs' and 'data' for YOUR reloading.
Work up to YOUR max, test seating to YOUR best target results.

I also doubt a 140fps gain from seating. That's a lot

The smallest difference was 89fps, largest was 132fps. Tested with magnetospeed v3. Which blew the primer