The outputs will only be as good as the inputs. All inputs need to be as precise as you can measure them, including actual bullet OAL, COAL, case trim length, barrel length, case volume, and ambient temperature.
You also need to "calibrate" the burn rate for a given powder to your specific setup. To do this, obtain an average muzzle velocity using a known charge weight in your setup. After setting the correct ambient temperature, adjust the burn rate (Ba) until
predicted velocity exactly matches
actual velocity for that charge weight. At that point, everything usually matches up pretty well in my hands.
My optimal loads almost always tend to be just a tick slower than the predicted OBT. For example, Chris Long's predicted OBT for a 30" barrel is 1.3684 ms. For my best loads (with adjusted Ba so that predicted and average [actual] MVs match) usually end up with a barrel in the 1.3670 to 1.3680 ms range. Also, just be aware that even small changes in MV will change barrel time. So it's important to adjust the temp setting (if necessary) for range trips with different ambient temps and/or the Ba if necessary.
Finally, I'm not sure where you're getting your OBT values, but they don't match what's in Chris' OBT Table for a 26" barrel (link below). You should also be aware that there are
more accuracy nodes than just OBT nodes. For example, if you do a ladder test over a very wide charge weight range, you will find accuracy nodes that simply don't match up to any OBT node. In other words, OBT nodes represent a
subset of possible accuracy nodes. In addition, they will usually come in pairs for a given caliber/barrel length/bullet weight; one fast (high node) and one slow (low node). In some cases with specific bullet weights and barrel lengths, it is simply not possible to hit the high node without going way over pressure. A perfect example of this is Berger 200 gr Hybrid bullets in the .308 Win. In order to hit OBT Node 4 in a 30" barrel, you would need to be moving them about 2725 fps, which requires extremely high pressure (>65K psi). Even with a 32" barrel, that is not a great idea. For that reason, most people are shooting these at around 2650 fps or so. Although there is very clearly an accuracy node in that region, it does
not correspond to an OBT node.
The barrel time value you listed above (1.245 ms) is a little less than halfway between Node 4 (1.1905 ms) and Node 5 (1.3295 ms) and is likely
not on an OBT node. In order to hit an OBT Node, your choices are to either
A) speed up your load to decrease your barrel time to the area of 1.1905 ms, or
B) slow down your load significantly until the barrel time increases to the area of 1.3295 ms. Honestly, it may be that neither of those is feasible. Because OBT Nodes are relatively far apart in terms of their respective velocities, the load corresponding to the low node may be so slow as to be of little use (i.e. you're giving up way too much potential bullet performance). Conversely, the high node may not be realistically attainable due to over-pressure issues as I mentioned above for the 200 Hybrids. If you think you can safely speed up your load a bit, you
might be able to reach Node 4. Node 5 is going to be much, much slower than where you currently are; I wouldn't even think about trying to hit it.
If the pressure will be too high for Node 4, simply leave the load where it is. It sounds as though you have already hit on an accuracy node for your setup. If it does not correspond to an OBT Node, so what? It will still work for you. I refer again to the large number of people using 200 Hybrids in F-TR competitions. They are for the most part not hitting an OBT Node, but they are shooting excellent scores and winning matches nonetheless. So it's not absolutely essential to be on an OBT Node. If you can hit one with your setup, great. If not, I wouldn't lose sleep over it.
http://the-long-family.com/OBT Table.pdf
