For no good reason I want to load some fixed ammo for my Borchardt.  I normally shoot 14.2 grains of MP-300 and wondered how much I should reduce the charge for fixed, if any.  I'm not going to go through the whole process of load development for fixed as I normally breech seat the bullets.   

I also have other powders, 4225 and 4579 if I got the numbers right.   

Dave

Well, with 4227 and 4759 I shoot the same charges with fixed as I do with breech seated. However, the bullets themselves are not always the same, nor are the loads on the high end. 
My rifle has never been used with your powder, so can't tell ya anything about that, but fixed may surprise you if you are willing to spend some time working at what your piece likes.
Seat yer slugs way out, ought not make much difference in overall capacity- works on mine.

Be patient for him to weigh in, there is a shooter who has been experimenting with MP-300.  I have the Hoch 322200 208gr non-tapered I shoot 14.5gr IMR4227 fixed (1405fps).  I haven't found any MP-300, yet, and would appreciate learning of your experience.