It (flat primers) can also be a sign of over sizing.
I would load five as normal and shoot them over a chronograph to see where they are.
Since you don’t have one, you’ll need to either load them and “run what you brung” or make a set of loads very near to your charge weight and see what shoots.
The powder may be a little different, but I doubt you’ll blow your eyebrows off or squib a bullet.
It’s all about the hole distance for me...
All that flies in the face of proper and safe practices, so if you do come back without eyebrows,... well, honestly we are gonna want pictures of that!
Good luck!