Have just run your load through QL which has a ‘default’ 6.5x54 H2O capacity of 57 gr; I’ve just measured a couple of federal and they averaged 57 gr
Way down on pressure and velocity at 2466 fps, 44 gr as your original post gives 2588 fps and 45975 PSI Pmax so way down on pressure. I'm sorry but something is definitely wrong with your readings, the load data and QL plus other people's experiences suggest you are getting 2-400 fps more than you should be this is not possible just because you are shooting a blaser. I really would try and get hold of some N160 from another batch and retest!
I simply don't trust QuickLOAD with Viht N160 (and one or two other powders, notably Reload Swiss RS52), at least in my v3.6 program. (I've read that the latest v3.9 has some major changes to some default powder values, but don't know if that applies to N160.)
My usual practice when using QL to assist in deciding on initial charge weight batch sizes and steps is that once having selected what looks like good powders, to select maximum charge weights that according to QL produce pressures maybe three to four thousand psi below where my final load / pressure / MV is expected to end up. Having done that, select another four or five sets of lower charges, usual step sizes being around 1% of likely maximum, so 0.3gn for 223 sized cartridges, 0.4gn for Creedmoor, 260, 7-08, etc and 0.5gn for larger cartridges with ~50gn weight top charges. The very lowest charge is usually up to a full grain weight below the second lowest and is used as a barrel fouler and (if needed) to sight in, also as an indicator to see how actual MVs compare to computed / expected ones at what should be a non-risky pressure level. A Labradar chronograph is used which is likely as accurate / consistent as you'll get without buying professional kit (at professional kit prices which are way beyond my means), and unlike the otherwise excellent Magnetospeed doesn't change barrel harmonics with possible effects on grouping and POI.
Loading manuals and manufacturer's data are also consulted to ensure there isn't a vast difference between their maximum loads and what QL says, although this is often problematic thanks to various factors, especially when throating is very different from SAAMI/CIP.
Over time, I've found that some bullet-powder combinations can vary significantly either way. Take 260 Rem, not too dissimilar from 6.5X55mm. Starting from scratch with a new barrel in 2018 - 28-inch Bartlein, 'minimum SAAMI' spec chamber dimensions and 'throated out' to suit 130-142gn bullets with considerably more freebore than SAAMI.
Load: Remington case (55.6gn fireformed 'overflow water capacity'), Fiocchi LR primer, 140gn Berger BT Target, 2.940" COAL) and these values input into QuickLOAD. The 2.94" COAL is 15-20 thou' off the rifling in this custom throated chamber and will therefore reduce pressures compared to SAAMI / CIP standard specced barrels.
Six sets of 5 rounds N160 loaded this time around in 0.5gn steps, the range being similar to loads I've used for many years in 260 with 140s.
Viht's data gives maximum charges ranging from 43.1gn - 44.3gn N160 with similar lead core 139-140gn bullets. (NB considerably less for all-copper and Swift A-Frame bullets) However, these loads are for heavy Lapua brass and what presumably is a CIP chamber, quoted COALs being around the standard 2.800", usually marginally less. The higher capacity Remington brass and longer freebore would normally require an extra 1-1.5gn powder to achieve Viht's MVs / pressures, so 'adjusted' maximum loads for my set-up rise to the 44.5-45gn N160 bracket.
Unfortunately recent editions of US manuals are distinctly light on Viht combinations, in fact often without any at all in popular cartridges. Lyman #50 quotes 45.0gn max N160 with Rem brass and the 142gn Sierra MK at 2.800" COAL, the nearest equivalent bullet to my Berger 140 BT. The sole Viht load in the new Sierra Edition VI for 140/142s is N560 and its maximum is a shade lower than Viht's in a different make of case (Hornady v Lapua).
Because I don't trust QL with N160 my usual practice when trying this powder is to adopt an initial max level that is estimated to produce
at least 5,000 psi less than that which is my ultimate aim. As I'd be looking for combinations that perform well around 55,000-58,000 psi in the 260, that means that if I were acting solely on QL predicted outcomes, my initial maximum charge would produce not much over 50,000 psi according to the program. 45.0gn N160 was predicted to produce 50,658 psi and 2,734 fps from my 28-inch barrel with my brass and freebore values. So, that's getting close to Lyman's max (shown as 58,000 psi) and well above Viht's.
However, if I had trusted QL here with this powder, its results say I'd be OK up to around 47.3gn for 59,386 psi / 2,869 fps. With many years of loading the 260, I knew this was a 'not a chance job'.
Actual results in terms of MVs actual v QL predicted worked out at:
42.0gn ......... + 165 fps
42.5gn ......... + 139 fps
43.0gn ......... + 124 fps
43.5gn ......... + 114 fps
44.0gn ......... + 103 fps
44.5gn ......... + 118 fps
The 44.5gn actual was 2,823 fps. To achieve this in QL, it needed 46.5gn N160 estimated to produce 56,181 fps, a charge that if tried would I'm quite sure have been significantly over-pressure and seen new cases thrown into the junk case bin with expanded primer pockets.
These results were also in a nearly new barrel, barely run-in. Both pressures and MVs usually rise after around 150 rounds in stainless match barrels, so 44.5gn would likely end up as the practical maximum.
