View attachment 1550077its really simple.its this way for every cartridge all the way through every reloading manual.
When you use the slower burning powders you get more velocity.look at photo
You've failed to understand the relationships here. .300 Win Mag is a very large case high-case capacity 'magnum'. Its water capacity is c. 92gn. 308 Win holds c. 56gn, some 40% less. Their capacity to bore area ratios are 747 for the 308 Win and 1,227 for the 300 magnum.
The higher the capacity to bore area ratio, the slower burning the powder needed. All that Alliant's table tells you is that Re26 is better in that cartridge's capacity and bore-size relationship than Re23, and both are better than the very much faster burning Re16 which is pretty sub-optimal for this cartridge and bullet weight. Note how much lower the maximum charge weight is because it pressures out at a much smaller weight which in turn delivers less total energy and therefore gives a lower MV.
Every cartridge design allied to a particular bullet weight has some optimal powders, but there will also be many more whose burn speeds range from totally unsuitable through sub-optimal to in some cases dangerous. There are slower burners than Re26 available - load them into the 300 Win Mag + 180 bullet combination, and as the powder burn speed slows, pressures and hence MVs will drop. Likewise, load that trio into the 308 Win with a 180gn bullet and you find all are too slow burning for this application and give poorer MVs than optimal powder grades. If you stuff enough Re16 into the case with a massive degree of compression, you might just about get a half suitable result, but Re23 and 26 would be hopelessly low velocity and inefficient as their proper 'working pressures' simply won't be achieved with the 308's capacity to bore ratio. However going 'faster' than Re16, will improve cartridge efficiency and velocities here, until powders then become too fast-burning and MVs again drop off.
You say that loads tables always show higher MVs from the slowest burning powders. Not so. For a start, with the huge range of powders potentially available for most rifle cartridges these days, bullet companies are usually very selective indeed, omitting all marginal candidates of whatever burn speed. So, the slowest burning powder in a table is the slowest that gives good results, and as most purchasers in the US handloading market in particular are MV-obsessed, they're the powders tested and if found to deliver, included in the tables. Not every reloading manual does this exclusively though as some look for performance consistency too and may override sheer speed as long as the results aren't too low. Lyman is one such and returning to 308 Win, shows Winchester 760 ball powder in some its heavier bullet loads. With the 200gn SMK, 760 is the slowest burning powder shown, and whilst it wins the 'potentially most accurate load' award, it actually produces the lowest MV of the 11 powders in the table.
Moreover, burn rate is not the only factor affecting MVs. Specific energy levels are important and a very high-energy powder would normally be expected to give higher MVs than an equivalent burn-rate low-energy grade. Look at Vihtavuori's extremely comprehensive 308 Win tables
https://www.vihtavuori.com/reloading-data/rifle-reloading/?cartridge=30
and the highest MVs come from the N500 series added nitroglycerine grades. Now you're going to say these tables actually back up your contention as N550, the slowest burning grade listed nearly always gives the highest MV with mid to heavyweight bullets. (Not always as the faster burning N540 'beats' 550 with the 185gn Lapua Scenar.) BUT, bear in mind that Viht makes no fewer than 10 slower burners than N550, and not a single one is shown for any 308 Win bullet combination, and the reason is simply that their burn rate makes them sub-optimal or even totally unsuitable.
