Ok, we arrived at 11 grains of either IMR4227 or Accurate 1680 for my friends 300 Blackout with a 7.5" barrel (subsonic). Both were producing 2" groups at 50 yards.

Now he wants a supersonic round.

But all load data I've looked at is for a 16" barrel. How can I determine a min-max for a 7" barrel? Do I just load to the max for a 16" barrel? If I do that then the max velocity will around 300 fps less due to the short barrel. Using the two above powders.

Or, does any know where I can find load data for short barrel 300 Blackout?

I'm kinda tempted to try Titegroup. And consider it a 30 caliber. pistol round.

The maximum load data for a certain bullet is the maximum.......
weather fired from a 2" or a 30" barrel.

FPS will be gained by a longer barrel, up to a point, then drag and friction take over.

The smaller barrel might loose 15% in fps but the powders used
will also cause highs and lows of the bullets speed.

You will just have to load the max and see what happens.

But, is it not true that a slow powder in a short barrel would not have time to develope as much pressure?

False, peak pressure happens before the bullet moves a half inch or sooner.

If that were true..................

then why do I get top fps out of my 38 snub nose with Unique and Blue Dot, rather than Bullseye or w231 powder?

Because the pressure drops from the peak more slowly with slower powders.

Unless the case is so small or the power so slow you can't fit enough, the max load is still hitting max pressure almost immediately with any powder.

Powders that makes the highest velocities in long barrels are also the powders that make the highest velocities in short barrels. Muzzle blast will be atrocious in the latter.

Because each powder has a different set of characteristics which influence internal ballistics and external ballistics. Burn rate is just one of many and is not a indication of suitability or performance to an application.

be safe
Ruggy