There are a lot of caveats in your reply. However, let me point out that high velocity IS a sign of excess pressure, actually probably the most reliable one. People navigate under the misapprehension that if the case doesn't explode, the primer doesn't come out of the pocket and the bolt only requires two hits to open, that there is no excess pressure. When these "signs" occur, you're way deep in excess pressure territory.
Also, I have yet to meet someone who doesn't claim that a gross overload is safe in HIS rifle; seems everybody has super strong rifles.
If the OP is at 2350 now, getting to your 2800 or 2900 is going to be, in a word, epic.
Take pictures.
Don't need pictures. You can always recognize someone who has done this at the range: it's the person wearing an eye patch and pulling the trigger with their hook. Pick a firing lane on the other side of the range.
In all seriousness, people carry out these kind of reloading practices all the time (i.e. - running very hot loads) and may never get bitten. Then again, it only takes once.
Cam - I took the liberty of running some numbers through the QuickLoad reloading program for you. I'll tell you at the outset that these numbers are crude estimates ONLY, don't race out and load up a bunch of rounds at the highest charge weight listed!!! If you want to try any of this, ALWAYS work up to any charge weight in small increments. Also, this is going to be fairly long and image intensive, so bear with me.
I use a reloading approach known as "Optimum Barrel Time" in combination with Quickload, which gives barrel time as an output and allows the user to make a number of useful predictions regarding load performance. Quickload is not a replacement for actual testing of any load, it is simply a tool that helps streamline and expedite the process. The final verdict on any load should always be the chronograph and the target. I mention all this because not everyone buys into the OBT approach. That's fine, I'm not interested in starting a debate here about whether the explanation of OBT theory is correct or not. I use it and have had good success, which is enough for me. Here is a link to the OBT website if you're interested in exploring it further:
http://the-long-family.com/optimal barrel time.htm
I input your barrel length, bullet choice, and charge weight into QuickLoad (24", 168 SMK, 40.0 gr). Not knowing the specifics of your setup, I used some "generic" inputs for case length (2.005"), case volume (56.0 gr), COAL (mag length - 2.800"), and temperature (70 degrees F). Note that your
actual output values may differ significantly depending on how to close these "generic" values are to your "true" values. That is why it is important not to try to directly reproduce any of this without starting low and working up slowly and carefully.
The following series of images are the outputs from Quickload. You can look through them as desired. The input data is generally in the upper left and right panels, the output data in the bottom left (graphical) and bottom right (numerical) panels. Some key things to look at are the input variables on the left such as barrel length, case length, COAL, case volume, etc., especially look at some of those I had to fudge with generic numbers. On the right, look at the fill ratio, pressure, barrel time, and muzzle velocity values. The following outputs are shown below:
1) 40.0 gr N-140, preset powder burn rate (Ba) for N-140
>>>illustrates how close the program is to predicting your actual numbers without any calibration
2) 40.0 gr N-140, Ba adjusted to give
your actual MV of 2350 fps
>>>illustrates how it looks after calibration so the program returns your actual measured velocity as an output
3) 42.3 gr N-140, adjusted Ba, your max load tested
4) 42.9 gr N-140, adjusted Ba, predicted OBT Node 6 load
5) 44.9 gr N-140, adjusted Ba, predicted OBT Node 5 load
>>>illustrates outputs for two higher pressure loads above your MAX tested charge weight, that should still be well below MAX safe operating pressure*** ***Caveat - ALWAYS work up slowly in small increments
6) 43.2 gr Varget, preset Ba for Varget, predicted OBT Node 5 load
7) 42.3 gr H4895, preset Ba for H4895, predicted OBT Node 5 load
>>>illustrates OBT Node 5 loads for your setup, NOT calibrated, using Varget and H4895
8) 43.0 gr Varget, preset Ba, predicted Berger 168 Hybrid OBT Node 5 load
9) 42.0 gr H4895, preset Ba, predicted Berger 168 Hybrid OBT Node 5 load
>>>illustrates OBT Node 5 loads for your setup with Berger 168 Hybrids, NOT calibrated, using Varget and H4895
What you can see from these outputs is that you are not running anywhere near a MAX pressure load with N-140, using the ASSUMPTION that your load has a COAL of close to 2.800" (mag length). Not surprisingly, the initial prediction was off (velocity predicted = 2463 fps
versus 2350 fps actual). This is common due to variance in burn rates between different lots of powder, variance in primer brisance, or differences in case volume/capacity and COAL, among other things. After calibrating the burn rate (Ba) to give 2350 fps velocity for 40.0 gr N-140, you can see that both the fill ratio (90.5%) and predicted pressure (35,727 psi) are quite low, so your low velocity really shouldn't be much of a surprise. Even at your MAX tested load (42.3 gr), you're still about 20,000psi under MAX pressure for .308 WIN SAAMI specs.
FWIW - with a barrel time of 1.431 ms, your 40.0 gr N-140 load falls very close to OBT Node 7, which has a optimal barrel time in a 24.0" barrel of 1.4343 ms. That is likely one of the reasons it shoots so well. To hit the next OBT Node (6), your setup will require approximately 42.9 gr N-140, giving a predicted barrel time of 1.308 ms (as compared to the OBT table value of 1.3084 ms). So you were headed in the right direction, but stopped short at 42.3 gr. Note that with 42.9 gr, or even an extreme value of 44.9 gr N-140, the predicted pressures are 43,814 psi, and 50,435 psi, respectively, both well under the 62,000 psi SAAMI MAX. FWIW - I'm not at all suggesting you go out and load 44.9 gr, but it appears with N-140 as though you're going to run out of room in the case before you go over pressure.
Here is a summary of the pressure and velocity outputs for varying charge weights, powders, and bullets in your setup:
Loads corresponding to OBT Node 5 with either N-140, Varget, or H4895, are all predicted to be well under MAX SAAMI pressure. All are relatively slow compared to the velocity numbers some have been throwing out in this thread. However, the value with 42.3 gr H4895 is comparable to the box value for typical .308 ammo with 168 SMKs of 2650 fps, which is from a 24" test barrel. So even that is not a particularly "hot" load, being comparable to commercial ammo. I think you can see pretty well from this data about where you are in terms of your load and pressure/velocity. In other words, you still have plenty of safe margin to work up a bit higher. You should hit Node 5 with N-140 somewhere in the neighborhood of 42.9 gr. At a predicted 2618 fps, it's still very slow, but it should be a lot better than your 2350 fps load in terms of wind deflection. Just remember, the value shown for Varget and H4895 have not been "calibrated" with an actual measured velocity from your setup. Thus, the charge weights for either of those powders are only crude estimates at best.
The last point I'll make has to do with BCs and performance. Even at the relatively slow velocities predicted in the loads above, just take a look at the predicted effect of a full-value 10 mph wind at 600 yd on the 168 SMK and 168 Berger Hybrid bullets, both at 2650 fps muzzle velocity. According to JBM Ballistics, the SMK 168 (G7 BC - 0.218) would experience wind drift of 5.5 MOA at 600 yd (34.4"). The Berger 168 Hybrid (G7 BC -0.266) launched at the same muzzle velocity would experience wind drift of only 4.3 MOA at 600 yd (26.8"). That's a difference of 7.6" wind drift at 600 yd, simply by using a much higher BC bullet at the same velocity.
In any event, I hope you at least find this information entertaining, if not educational
. Having a copy of QuickLoad can be a very useful tool and worth every penny, IMO. Good luck with your load development, I'm sure you'll get it all worked out.
