Welcome to the forum PeteinGermany!
I think youre going to need a 20 barrel to get enough energy. A 123 grain SST at 2500 FPS is doable and that produces 1487 foot pounds of energy at 100 yards. I dont know of any powder that will get you 2500 FPS with 120-125 grain bullet out of an 18 barrel. Here is the info from Hornadys ballistic calculator.
![[Image: 53395596842_654b764612_z.jpg]](https://live.staticflickr.com/65535/53395596842_654b764612_z.jpg)
I think youre going to need a 20 barrel to get enough energy. A 123 grain SST at 2500 FPS is doable and that produces 1487 foot pounds of energy at 100 yards. I dont know of any powder that will get you 2500 FPS with 120-125 grain bullet out of an 18 barrel. Here is the info from Hornadys ballistic calculator.