Well, this can be a topic that may require more explaining than I have time or room to type but I’ll try to condense it to where it’s understandable.
When most of us began building smokeless mzls we built 45’s because of the bullet & barrel choices that were available. Years of trial and error have all but perfected those 45 caliber guns. It’s relatively easy to build a gun , buy components and get a load recipe from a buddy that shoots remarkably well ( less than half a moa ) out to 500yds.
Then came the 40 cal. There are now great barrels and bullet choices available. So what’s the difference u ask ?
In smokeless mzls most guys smooth form their bullets. These rifles are land riders. That means ( for anyone that isn’t familiar with the term ) the bullet is pushed down the bore only touching the lands. When the powder ignites, the bullet must swell or obturate to cut the grooves in the bullet. After the bullet exits the barrel there is powder fouling left behind inside the bore. The next bullet that is pushed down the bore can be difficult to load because of the powder fouling.
The secret to any good shooting smokeless mzl is finding the right powder that gives the velocity and accuracy without making too much pressure plus burns clean enough so that it’s easy to reload multiple times without the bullet getting tight during loading or stuck in the barrel.
Many including myself are working on the 40 caliber but it is a completely different animal than a 45.
Sorry for the long winded post. I hope it clears things up a little.