Seating Depth Question in .308

[Crusader]

I posted this over on 24 HR Campfire but figured I'd do it here as well to check with you gurus. I wanted to run something by you, sort of a sanity double-check. I am switching bullets for my Savage Lightweight .308; I'd been loading and shooting the Barnes 168 gr TTSX, for which I'd established a good seating depth/COAL using the Stoney Point bullet comparator tool (measuring case head to ogive). I am now going to the Barnes 150 gr TTSX, which is a fair bit shorter bullet than the 168. So I have a couple of questions related to this, sort of conceptual but also, applicable.

First, is it reasonable to expect that the 150 bullet would perform similarly (as far as accuracy) if it is seated such that the COAL is the same as how I had loaded the 168? It would therefore be the same distance from the lands which I presume is the biggest factor in performance. My other thought was that in that scenario (same COAL) the shorter bullet would have more empty space/volume in the case so then would the combustion and other physics result in different performance?

Second, to produce the same COAL in the 150, I would just use the existing setting in the seating die that I used for the 168, correct?

Maybe I'm overthinking this, I just want to get to a load I'll be happy with minimal time and component usage. Anyway, thanks in advance!

 

[A382DWDZQ]

is it reasonable to expect that the 150 bullet would perform similarly (as far as accuracy) if it is seated such that the COAL is the same as how I had loaded the 168?

I would not expect this, but maybe it will turn out that way. You have to take into account the barrel harmonics, the bearing surface, and velocity, just name a few. There are a lot of variables that change, and any small change is magnified at the POI.

My other thought was that in that scenario (same COAL) the shorter bullet would have more empty space/volume in the case so then would the combustion and other physics result in different performance?

I'd agree with this. I believe that you'll get different performance.

For example, for the same charge:
168TTSX:

Cartridge          : .308 Win. (SAAMI)
Bullet             : .308, 168, Barnes 'TTSX'BT 30370
Useable Case Capaci: 44.351 grain H2O = 2.880 cm³
Cartridge O.A.L. L6: 2.800 inch = 71.12 mm
Barrel Length      : 24.0 inch = 609.6 mm
Powder             : Hodgdon VARGET *T

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
 %       %    Grains   fps   ft.lbs    psi    psi      %        ms
+00.0  101    40.00   2534    2395   49175   6560     96.5    1.250

vs
150TTSX:

Cartridge          : .308 Win. (SAAMI)
Bullet             : .308, 150, Barnes 'TTSX'BT 30366
Useable Case Capaci: 46.577 grain H2O = 3.024 cm³
Cartridge O.A.L. L6: 2.800 inch = 71.12 mm
Barrel Length      : 24.0 inch = 609.6 mm
Powder             : Hodgdon VARGET *T

Step    Fill. Charge   Vel.  Energy   Pmax   Pmuz  Prop.Burnt B_Time
 %       %    Grains   fps   ft.lbs    psi    psi      %        ms
+00.0   96    40.00   2538    2145   40811   6438     92.9    1.284

And then if you match the pressure, the 150TTSX looks like this:

Powder type          Filling/Loading Ratio  Charge    Charge   Vel. Prop.Burnt P max  P muzz  B_Time
                                      %     Grains    Gramm   fps     %       psi     psi    ms
Hodgdon VARGET *T                  102.3     42.8     2.77    2700    95.5    49175    6911   1.182

Second, to produce the same COAL in the 150, I would just use the existing setting in the seating die that I used for the 168, correct?

This would depend on the profile of the bullet. I am guessing that using the same setting will result in a shorter COAL for the 150 because the profile of the 168 is a more gradual slope to the tip, but I am not sure how much of a difference it would be. It will be the same distance from the bottom of the case to where the bullet engages the seating die, but the angle of the profile before and after that point are probably different.

edit: p.s. I am curious to see how it turns out though, so hope you can post your findings when you have them.

 

[N2TRKYS]

I don’t fool with measuring to the lands. I find it to be a waste of time. I seat all of mine to the tested COAL in the load data. This has proven to work in over a dozen different rifles and different cartridges.

 

[SDHNTR]

I wouldn’t expect anything. You will never know until you see the results of the change on paper. And no, the same setting on your seating die will likely not work because the ogive is probably in different places between the two bullets.

 

[MattB]

So I have a couple of questions related to this, sort My other thought was that in that scenario (same COAL) the shorter bullet would have more empty space/volume in the case so then would the combustion and other physics result in different performance?

Your max charge for the 150 gr. bullet will be more than for the 168, which will consume some of that space.

 

[EdP]

You are starting from scratch any time you change bullets. Different ogive profiles, different hardness, different length, different velocity, different powder charge, etc. Start from the beginning and enjoy the process.

 

[N2TRKYS]

You won’t necessarily have to adjust your seating die. I find that I don’t have to adjust mine to get the depth that I want when I go down in bullet weights. It works out very well that way. No adjustments needed and it produces accurate loads. You’ll just have to try it and see if it’s where you want them.

 

[49ereric]

play with the seating length-depth or you’ll never know what you might be missing.
some rifle cartridges the factory length work very well and others don’t.
My K98 Mauser shot horribly at factory length but slowly got better as I lengthen 3 thousands at a time and it ended up with bullet inside the case just 18 thousands. Shoots great for Mauser sights and old eyes.
that I figure is a worse case scenario.

 

[Crusader]

Thanks for the replies fellas. The 168 load I used had a COAL of 2.840" and jump of 0.100" and that was very accurate in my rifle. I'll likely start with the 0.100 jump for the 150 and go from there. Will post again once I'm finished.