This is what I was thinking about when I started this thread.
SEAT DEPTH ???
- Thread starter Bill Norris
- Start date
Steve Donlon
Gold $$ Contributor
That is one thing I look at when the bullet is to far away from the lands to have enough bearing surface, for me, in the neck. I have just placed the bullet where I want it to be with all the bearing surface in the neck and no air space and it has worked out. Mostly with Berger bullets.
I'm not among those that think there is a significant change in pressure from varying seating depth. Pressure can not build inside the case without the resistance from pushing the bullet into the bore and down the barrel. There's a video out there of some firefighters burning cases of ammo, the bullets have very little energy because it takes very little pressure to push the bullet out of the case. Also, a squib load will demonstrate that the primer has enough power to push the bullet into the barrel. From these two observations I surmise that the bullet is moving forward and is potentially already getting squeezed into the bore before the powder starts to burn and even if the powder has started to burn before the bullet hits the leade there is no appreciable rise in pressure until then.
As an aside, the assertion above that the primer is primarily responsible for getting the bullet moving forward into the bore is consistent with the frequent observation that primer variation is a significant contributor to accuracy.
As an aside, the assertion above that the primer is primarily responsible for getting the bullet moving forward into the bore is consistent with the frequent observation that primer variation is a significant contributor to accuracy.
I agree, but I am EE, not an ME. And also not a serious competitive shooter. That said, I have a feeling it affects the front edge of the pressure curve, and hence the impulse function (shock, if you will) to exciting the barrel harmonics.
Similar to why some powders work and some don't, some primers are better, some bullets (diameters, jacket hardness, ogive shape, bearing surface) better, and some barrels (dimensions, lead angles, metallurgy) shoot.
I have dealt with (electrically) exciting tuned circuits throughout my career, so my take might be tainted by drawing those parallels.
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Bill, I went back a few years to some of my old notes to a test Berger did
They tested 4 seat depths for their VLD Bullets
+.010" in
- .040" out
-.080" out
-.120" out
Noting this test as speedier method to sneak up on optimum seating
Asserting that one of those seat depths will outperform the other 3
(also note, I dunno if they made a type, but the pressures listed for their second bullet list
-.020" out instead of the above noted -.040" out
Which I believe is irrelevant since the test still gives us an idea of whats happening ...
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Once you find that 1 depth, you then adjust .002" - .005" at a time from there to hone it in
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Here is the most interesting part I find with their test Done
The pressures
I won't take up a lot of throwing out my speculations about whats happening in the bore...
such as - is the bullet starting off with a higher initial velocity with a higher pressure, thereby exiting the bore sooner?
maybe it doesn't and pressure goes up because it is resisting movement and harmonics amplitude goes up instead?
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I will allow you to make your own conslusions based on the data they provide
But I believe it is helpful data to allow you to make some of your own calculations
regarding Jam vs Jump vs Large Jump
These were calculated with Quickload
+.010" in Max pressure ----60,887 PSI
-.020" jump Max Pressure ---59,645 PSI
-.080 jump Max Pressure ----62,413 PSI
-.120 Jump Max Pressure ----64,420 PSI
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It is very interesting that with every JUMPED load, they reduced initial pressure
(likely the primer pressure) down to 3624 vs the higher 5500 PSi for the Jammed inital pressure
and every JUMPED load other than .020 off, went considerably up
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which shows me a trend that the further the jump, the higher the presure
and one reason why -.010" - .030" off the lands is "USUALLY"
where most rifles like a Bullet
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Also maybe why some factory rifles we feel a different (sharper) recoil impulse
with the same load as a custom
since most factory rifles have more jump, (leade, distance to the lands)
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I took pictures of my notes of Bergers test so you may read it if you wish.
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Say it isn't so!!! :O
its my favorite test!!!
Since once finding your optimum seating depth, it clusters suddenly like you want it to
and you can pretty much change anything else
I like seating depth because it allows me to get there quick
Both of these were within 50 rounds or less from a fresh chambering
the group on the right is 3 different seating depths, -
-.005" edge of window, middle of window, +.005" edge of window
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The 2 pics below are of a couple seating depth tests where I have found where the window is
being +/- .005 of the window
from there I can change bullet
start increasing powder charge
change powder (which may require a small fine tune of depth again, but this depth is often your window)
if it doesnt shoot like this including a factory gun, most likely you need to simply try a different bullet
This is how I get even factory rifles to shoot like this. (provided you can get close to the Lands)
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Also there is usually one certain powder that performs lights out above the rest
1)Seat depth
2)then try every powder in that burn rate area.
(VS Stubbornly saying you are going to use one certain powder and try to make it shoot)
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{RabbitSlayer said:
All i know is seating depth is my least favorite test to do!}
Here are the same 2 guns respectively showing what they shoot like
"NOT in the window"
the one on the right is much more pronounced since it is a much larger caliber
Notice how much difference there is in the second group posted above for the same rifle.
pretty easy to tell when you have found your seat depth window
it seems to WANT to cluster without you really even trying or putting much effort into "driving the gun"
(the problem I see when people don't find their optimum seating depth is...
...if it is shooting like the below right picture, and they think they simply need to practice more
they will never know if they are improving because the load is not accurate enough to reveal improvement)
All i know is seating depth is my least favorite test to do!}
Here are the same 2 guns respectively showing what they shoot like
"NOT in the window"
the one on the right is much more pronounced since it is a much larger caliber
Notice how much difference there is in the second group posted above for the same rifle.
pretty easy to tell when you have found your seat depth window
it seems to WANT to cluster without you really even trying or putting much effort into "driving the gun"
(the problem I see when people don't find their optimum seating depth is...
...if it is shooting like the below right picture, and they think they simply need to practice more
they will never know if they are improving because the load is not accurate enough to reveal improvement)
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Yea, I tried Berger's test a couple of times with their bullets and simply got nowhere fast. Maybe it's an isolated case for me but I have never been able to get any rifle factory or custom build to shoot that far away from the lands. Every single time I have been successful with a seat depth tune it has always been within .020 in the lands from just touch or .020 off the lands from just touch and most of the time in the lands @ .006 to .010 is where I find joy. As you might expect I have sold quite a few factory rifles that had way too much free bore for my use.
Yeah, I have never tried more than .040" out myself
which is what surprised me about Bergers test going out that far
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With VLD's like Sierra 107, I have went IN, jamming the lands as much as +.060"
and the gun liking it, (likely the Ogive seems to match the Leade angle with long Ogive types so doesnt build dangerous presure like a more blunt nosed bullet would)
Also, this allows a lot of room for wear before having to "Chase" if ever needed
One of my current rifles like a +.030" Jam
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Quite honestly, when I see people using jam with VLD's I dont believe they have gone in far enough
for a complete depth test, most people only jamming up to .010"
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Last I heard , sierra 107 were tangent ogive, regardless of that I typically shoot them with a bit of a jump in a br whereas a vld I’ll test from touch to .025 or so in the lands and just let the target do the telling.
I really don’t see much speed variations from jamming or jumping, indicating that while seating is likely a relationship affair it’s certainly and to coin a phrase “a small but important knob to turn” I don’t get too technical, I just load the small one for a second look.
I really don’t see much speed variations from jamming or jumping, indicating that while seating is likely a relationship affair it’s certainly and to coin a phrase “a small but important knob to turn” I don’t get too technical, I just load the small one for a second look.
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I wonder if it makes a big difference, Boat tail or flat base? I've never tried boat tail bullets so don't know.
I will offer this, I changed from .040" freebore to zero last year and found that the zero freebore chamber produced a noticeable amount of extra pressure, like nearly 1 grain difference in my rifle. Heavier bolt lift, that is and the load was right at where I wanted to be speed wise with one less grain.
I will offer this, I changed from .040" freebore to zero last year and found that the zero freebore chamber produced a noticeable amount of extra pressure, like nearly 1 grain difference in my rifle. Heavier bolt lift, that is and the load was right at where I wanted to be speed wise with one less grain.
CharlieNC
Gold $$ Contributor
Bill I recall looking at the RSI Pressure Trace web site several years ago where they showed the MEASURED pressure increased as the bullet was seated deeper. Can't find anything now as they have gone out of business. In addition to affecting group size, it also changed the point of impact for a number of my tests. Pressure can affect everything.
Absolutely agree. Don't see how it could not.
Steve Donlon
Gold $$ Contributor
That's the way it was with my old barrel. !44 Bergers needed .115 jump but then I got a new barrel at the same time Lapua came out with their 144 gr. bullet and everything came together with the bullet being .015" off the lands and all the bearing surface in the neck and no air space and never shot better with a 6.5 Creedmoor.
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