In your shoes with this barrel length, I'd settle on something that has an accuracy node on or just below the 2,800 fps MV level. All three powders quoted are either double-based or high-energy so contain nitroglycerin. In the Alliant range those manufactured by Bofors at Karlskoga in Sweden, (also makes Norma propellants there), most grades have low nitroglycerin values, 5% by weight or less. Two though, Re15 and Re22, are up in double figures which is what gives them their reputation for high MVs, and also likely their reputation for being badly temperature affected.
RS70 rarely seems to work outstandingly at modest pressures in smaller cartridges despite what QL might suggest. I've had reasonable, but not the best results, with it in such applications in my very limited use, but I've seen others find the same thing. It seems to need a good size magnum cartridge and stout load to excel. Maybe others have had outstanding results in Creedmoor etc size 6.5s or 7s.
My plays with the 6.5 Creedmoor in a factory rifle (Savage 26-inch barrel LRPV) came up with RS62 as the outstanding powder of those I tried. It is still the best replacement for H4350 that I've yet found and H4350 is still regarded as the outstanding powder for this cartridge with bullets of 130gn and heavier. I'd go further and say RS62 is a better powder than H4350 in suitable applications with the possible (likely?) matter of temperature sensitivity where H4350 is still an outstanding performer showing up better even than other ADI / Hodgdon 'Extreme' grades such as H. VarGet whenever people do side by side tests.
Your getting increased MVs after some initial use from new (and almost certainly accompanying increased pressures) is a common feature as the barrel runs-in a bit and has become very marked in some makes of stainless custom match barrels although it usually takes a lot more than 40 rounds to show through fully in them. I know several people who got fantastic results from their first run-in and load development session with a new barrel, then 100 or 200 rounds down the line find precision has gone to hell and worse they're blowing primers. It's a strong argument for settling on a modest load in MV terms with a new rifle or barrel and then revisiting it after a bit of use. A bit frustrating in that modern barrels run in so easily and you want to get the definitive load straight away. (It does seem to be much more of an issue with 6mm and 6.5mm than 7mm or 30 calibres.)
The nub of this wee rant is this.:- BEWARE of the interpretations one makes and uses of loads one is attempting to work up and analyse using software like this QuickLoad... These really clever tools - like QuickLoad - are only as good as the information one gives them, and the skill of the artisan using this investigatory and informational program. It is the old computer adage of G.I.G.O. (Garbage In, Garbage Out) that rings true here.
I'd agree 100% with this, but in fact go much further. Even with all of the key factors measured and fine-tuned, results are often well out. I love QL, but to me it has three uses. 1) The ability to model alternatives (eg how does long-throat 7mm-08 compare in match use against long-throat 6.5 Creedmoor or 260 Rem?) 2) A rough guide to powder selection and starting / work-up loads, and 3) the ability to back-check on likely pressures. (For example if expecting 2,800 fps from a load combination but actually getting 2,900 on the Labradar, what is the approximate pressure being generated with that powder? This is done simply / crudely by reducing / increasing charges input into the program until the actual MV being obtained is predicted.)
Having got through these three stages, all that matters is actual range performance which is why a chronograph, and arguably a known reliable model such as Labradar or MagnetoSpeed is essential. QL results without actual results correlation is at best potentially unreliable, at worst potentially dangerous. There are certain powders and cartridges I simply don't trust with QL. IME it massively underestimates MVs (and hence pressures) for Viht N160 and Reload Swiss RS52. So, for the former, I set my top initial load development charge weight to that which gives whatever pressure ceiling I want minus 5,000 psi. For the latter, I never use QL but as it gives very close results to H. VarGet in real life and QL is pretty accurate usually with this grade, I'll select VarGet to get starting loads and play it cautiously too. In every case - where available - published / tested loads MUST be consulted too as a sanity-check.
There is a near industry out there in people adjusting QL default values by playing around with factors in its database, most often the factor that represents how quick or slow burning a powder is. I don't do this for two reasons. First, having got my starting point, actual range results are much more valuable (and fun in getting them); second, burning speed is NOT a fixed value but can vary considerably depending on the cartridge's internal ballistics characteristics and relationships, and even through changes in a single cartridge such as bullet weight and pressures being generated. So changing the burning speed value in the program is maybe OK as long as it's remembered that having done so with experience from cartridge A, the 'new' value may be even more inaccurate in cartridge B if left as a new default and it's probably better and safer to return to the manufacturer quoted value that QL uses as a default.
