Seating depth and velocity

[Viper26]

Does changing the seating depth have an effect on velocity?
I'm fine tuning my 260rem load. Lapua 139 over H4350 seated .010" off the lands at about 2750 fps. Just wondering if that speed will change much when I start changing the seating depth.
thanks

 

[vviking]

From what I've observed, varying seating depth by small increments has not changed vels. much, if at all. Pressures do spike if a bullet is seated very deap, or moved out from jump to jam. These pressure spikes do tend to increase vel. somewhat, but not in proportion to the pressure increase ... less additional speed than one would expect on seeing obviously higher pressure signs become apparent.
Very deep in the neck or a hard jam seems to alter the rate of ignition and cause a short lived pressure spike early in the burn that does not last long enough to yield a proportionate increase in vel..
Just my guess tho, am sure there are other theories and would like to hear them.

 

[H4831]

There is no end of unproven theories out there, and then there is what happens in real life situations. I have loaded a 30-06 with 180 grain bullets to a heavier load than shown in any modern reloading book, seated the bullets with normal clearance and seated others out to where they jammed into the rifling as the bolt closed. I could detect no difference in pressure signs on the brass, no mater what position they were loaded at. And I have been observing pressure signs on shot brass for a lot of years.
I do believe it can be proven in a lab that there is a pressure spike when bullets are seated out to the rifling. However, over all pressure is still low at this point of ignition and the pressure spike will have no effect on peak pressure, because peak pressure occurs when the bullet of a center fire rifle is several inches down the barrel.
Another theory is heavier weight brass cases will create more pressure than will lighter weight brass cases. If there is a great deal of weight difference, the heavy cases with smaller powder capacity will raise more pressure, with the same amount of powder.
But what about smaller differences in weight of the cases, like between two different, but similar, brand of brass cases, as one sees in normal reloading?
I was using two different brands of cases in my 243. I picked out five Winchester cases that weighed 168 grains each and five Federal cases that weighed 180 grains each.
I loaded all ten with 46 grains of H414 powder and seated 75 grain Sierra bullets in them, to the same depth and shot them over a chronograph.
The five with the lighter Winchester cases averaged 3491 fps.
The five with the heavier Federal cases should have registered more, according to the popular theory, but instead they were actually slower, at 3468 fps!
Again, for your normal shooting in your sporting rifles, don't get hung up on all the theories you hear.

 

[rral22]

My experiments showed me that velocity differences from different seating depths were less than the standard deviations of the specific loads I was using. In short, I couldn't detect any.

Changes in accuracy are sometimes easily detectable. Sometimes.

 

[vviking]

There is no end of unproven theories out there, and then there is what happens in real life situations.

True that ^.

Most often, seating depth variables ... within moderate load levels ... is a non-issue.
But not always.
A couple of 6.5x55's and a 6.5/06 come to mind. They did react strongly to seating out from kiss to .025" jam.
Yes, these ones were all loaded pretty warm, right on up to the highest(and best shooting) vel. accuracy node consistent with halfways decent brass life(lapua). An 'Old Hodgdon data' max. load using RL22. 8-12 firings, they would loosen the primer pocket and then get tossed. Half-flat primers with .0001" case web expansion and 1-2 thou. of case stretch being typical. Very low ES and very LR accurate they were.
With the only change being seating the bullets out by .025" to a firm jam, pressure changed a lot ... the vel's. tho, just a little.
Now the primers(GM215M's) were totally flattened to the point of extrusion into the pocket radius. Ejector marks. Hard bolt lift. Loose pockets. .0005 case head expansion. .007" stretch.
Way over the top they were.

Vel's went up by a measly 35fps after all this drama. ES's, they fell apart.
Maybe those long/heavy for cal., high BC projectiles exaggerated the pressure spike?. Dunno.

I don't seat out to jam without first dropping a grain these days. Three times was enough to impress this marginally intelligent punter.