OP
OP
Andrew Leigh
Member
Let’s model this in my 30.06. Now QL is unable to model the effect of the increase in inertia due to distance from the lands, all it is able to do it to model the pressure change from seating depths. From the work that I have done I have observed that the pressure change due to inertia when seated 1mm (0.040”) and more away from the lands is not significant, here it is all about seating depth. However as start getting closer to the lands the pressure spikes.
My go to hunting load is 51.6gr. of a local powder. This produces a MV of 2 632fps at a modest pressure of 46 148psi. This being seated 0.5mm (0.020”) off the lands.
So let’s seat that same load 2.5mm (0.100”) off the lands and see what she does. The speed lifts to 2 661fps (29fps change) and pressure lifts to 48 406psi (2 258psi change). This is not a significant change for a 2.0mm (0.080”) change in seating depth.
Ok now let’s take the other extreme. Let move the bullet 0.5mm (0.020”) which now has the bullet in the lands. The same load now produces a velocity of 2 710fps (78fps change) and pressure lifts to 57 593psi (11 445psi change).
Now clearly one can see that when deviating from the original load, in the first instance moving back by a sizeable 0.080” brings about a modest change in pressure yet moving a mere 0.020” forward to touch the lands brings about a large change in pressure.
On all the large magnum calibres my friends and I own none of us are able to get close to the lands and I believe that this is the manufacturers way of building in safety.
Back to the example at hand, now what if my go to load was at or near to maximum pressure and I then jammed it into the lands?
QL predicts a fairly linear response as one approaches the lands with a dramatic spike as one meets the lands (the red trace) while in reality the green curve is what happens. The bottom scale is mm away from the lands and the other scale was one I used just to approximate the pressure effect by way of example.
My go to hunting load is 51.6gr. of a local powder. This produces a MV of 2 632fps at a modest pressure of 46 148psi. This being seated 0.5mm (0.020”) off the lands.
So let’s seat that same load 2.5mm (0.100”) off the lands and see what she does. The speed lifts to 2 661fps (29fps change) and pressure lifts to 48 406psi (2 258psi change). This is not a significant change for a 2.0mm (0.080”) change in seating depth.
Ok now let’s take the other extreme. Let move the bullet 0.5mm (0.020”) which now has the bullet in the lands. The same load now produces a velocity of 2 710fps (78fps change) and pressure lifts to 57 593psi (11 445psi change).
Now clearly one can see that when deviating from the original load, in the first instance moving back by a sizeable 0.080” brings about a modest change in pressure yet moving a mere 0.020” forward to touch the lands brings about a large change in pressure.
On all the large magnum calibres my friends and I own none of us are able to get close to the lands and I believe that this is the manufacturers way of building in safety.
Back to the example at hand, now what if my go to load was at or near to maximum pressure and I then jammed it into the lands?
QL predicts a fairly linear response as one approaches the lands with a dramatic spike as one meets the lands (the red trace) while in reality the green curve is what happens. The bottom scale is mm away from the lands and the other scale was one I used just to approximate the pressure effect by way of example.
