Originally Posted by
TurnipEaterDown
Paul,
Having never used a 338 WM (though I badly wanted one decades ago) myself, I took a look into Ken Waters Pet Loads.
IMR 4831 and IMR4350 consistently pop into accuracy lead on loads he had.
Whether that pans out in another particular gun would need to be evaluated, but often good loads do translate well, though occasionally needing minor adjustment.
Due to lot changes of components and dimensioning of a particular firearm parts, I would never start at a particular published load. It should be worked up.
I wouldn't expect obvious correlation on powder for accuracy to barrel length.
For instance, I found for use in a 30-06 Encore 15" & Springfield 03 23" sporter, the exact same loads worked really well. From 220s w/ I4831, 180s w/ Surplus 4895, 165s w/ H414, etc. Just a mention to note that some published guidance on using faster powders in shorter barrels never was proven true to me (jab at Lyman's guidance on rifle cartridges in pistols).
Look at any of the free ballistic models like GRT, and you will find that as long as the pressure is in design capability range, peak burn is actually fairly close to the breach. It isn't the case where 4895 is burnt up at 2" travel, and 4831 at 12". After peak pressure point, combustion nearly stops. So, you can figure out distance to ~99% combusted, if not directly provided. Mostly expansion from that point onward.
Muzzle flash is secondary combustion: high temperature residuals reacting w/oxygen in air, Not primary combustion of the self oxidizing nitro compounds.
Sometimes powders that seem to fast or too slow work really well for accuracy. For instance, I had personal experience w/ two 'huh, what do you know, look at that' loads:
Friend had a factory 300 WM years ago, we found it shot 180s really well w/ AA8700. Like, 5 shots ~ 3/4" at 100 yd.
A 35 Whelen Imp I have shoots 250s well w/ XMR4350.
Both of these also gave speeds comparable to max expected, just had to deal w/ considerable compression during loading.
Never know until you know.