Seating depth vs velocity - who is right?

[Titan700]

Following

 

[savageshooter86]

I just happened to test seating depths today and found little velocity difference. This was in a 308win using 200 hybrids

-10 off lands 2575
-13 off lands 2587
-16 off lands 2571
-19 off lands 2569
-22 off lands 2578
-25 off lands 2571
-28 off lands 2574
-31 off lands 2573

 

[savagedasher]

Alex -

If banging steel is equally challenging as trying to beat you Montana "good old boys" at your 1000yd Matches, guess I better change my tuning methods to the sanctimonious steel count method.
Funny though, just like you, I can keep lesser accuracy loads and components on my 12" plates shot after shot repeatedly at 1000, but find it a heck of a lot tougher to shoot sub-7" 50's and 100's like is demanded at actual 1000-BR Matches.
Like at this years Championship, I shot a 5.016" 50 that was only good for 3rd in that Relay, that demanded a 3.6" group or smaller to win.
Donovan

;D A 12'' steel plate has 113.09 Square inches of steel 8'' 50.26 or over 50% smaller 7'' that is the size of the 10 ring On IBS target. or 38.485 ''
8 '' =50.26 - 7''=38.48 or 11.78'' larger the your 10 ring . When we shoot 8'' steel it is the same as shooting 98 or better score on paper . Only we get a x or a miss instead of a score. Larry

 

[Jason Boersma]

You guys are funny! I chronograph Everything during my load development which I do at 600yds, because that's the farthest I can, and I always see a a velocity change during seating changes. I start at .020 jump then work in to .020 jam, from .020 jump until just touching I see velocity go down then when I start to jam the bullet the velocity goes up as does the pressure when the bullet goes into the lands.

 

[savagedasher]

When we shoot 8'' steel it is the same as shooting 98 or better score on paper . Only we get a x or a miss instead of a score. Larry

Larry
Better redo your math on that one......
If your shooting 10-shots at your 8" steel and all hit, it could be as low as a 90 score in comparison.
But you could say you shot a sub-8" group ;D 8)
Donovan The way I see it any bullet or a combination can not exceed I/2'' any place out side your 10 ring. or it is a X or a miss

We shoot 15 shots . We practice and have a 1/2 to 3/4 Hours before we get shoot with no more practice. 5 shot. The next 5 shot has the same amount of time between The third time we shoot is about two hours after our practice with no other practice. Also we have to wait for the gong to stop. ;D
Now add the condition we have in Florida. It is a Piece of Cake till you do it. ;D ;D ;D Larry

 

[lrgoodger]

The velocity change due to jamming is not from changing case volume. It is from building higher pressure against the lands before the bullet can start to move.

In the case of varying seating depth on a jump located BEHIND the lands so that this 'sticktion' does not come into play, if one does a simple volume calculation on a case, ignoring the bottleneck to get a rough approximation, and then compare that to the volume of a .050" jump times the area of the neck you will see that it is about 1% of total case volume. A .005" change in seating depth would be about one tenth of one percent, or one thousandth of case volume. Mathematically it has to make a difference in pressure and therefore velocity, but whether it is a detectable difference is questionable. I think it is likely that varying neck tension would greatly overshadow this small volume change. The small volume change would get lost in the noise of other variables unless they were held very, very consistent.

 

[savagedasher]

Using QL as a reference along with my findings 25' is a norm. With my Dasher load to get 25' faster it will take .03 more powder and will add 1715 more pressure. If your near peak pressure it will put you over the top.
Larry

 

[Mozella]

You guys are funny! I chronograph Everything during my load development which I do at 600yds, because that's the farthest I can, and I always see a a velocity change during seating changes. I start at .020 jump then work in to .020 jam, from .020 jump until just touching I see velocity go down then when I start to jam the bullet the velocity goes up as does the pressure when the bullet goes into the lands.

If you've got all that data, how about plotting a graph of Seating Depth vs Muzzle Velocity and Pressure and posting here it so we can see what you're talking about? I'm especially interested in your pressure data and how you collect it.

I'd also be interesting in how you separate the effects of wind from seating depth at 600 yards.

 

[Joe R]

Andrew said:
I just happened to test seating depths today and found little velocity difference. This was in a 308win using 200 hybrids

-10 off lands 2575
-13 off lands 2587
-16 off lands 2571
-19 off lands 2569
-22 off lands 2578
-25 off lands 2571
-28 off lands 2574
-31 off lands 2573

Andrew,
do you have target pictures to go with your data? How do you define "the lands"? Why are you starting at -.010?
IME, using the 200 hybrid, the best groups are between .005 and .010 off the lands.

Don't forget that the objective is to find a spot where small depth changes have little influence on the POI. It seems that you may have found the spot between .025-.031, but without seeing the targets no one is going to be able to answer your question.

Kindest regards,

Joe

 

[savageshooter86]

Seems -10 and -13 are good. I believe I caused the outside shots with maybe different shoulder pressure or something.

I redid seating test after I assisted my charge weight. Previous test results over in the 100 yd load development thread Erik C started