All the machined breech plugs these days will be made of that lovely 12L14
You may need to soften it a bit before you spread it out with a hammer. Right now it is rather thoroughly cold worked & may not tolerate much more deformation.
One may soften this steel a bit by heating until it just starts to glow, indoors in mediocre lighting, then cool however suits you.
All you need do is remove the effects of cold work - the original 12L14 bar would have been cold drawn to make it nice & round to fit a Love Machine collet, and to further improve machineability.
I myself have never cold worked 12L14, someone around here must have that experience, which would be helpful to hear.
Personally I do not like this brittle steel & would not have a piece of it in my shop. Yes, I am entirely alone in this view. 's OK guys, I know you all love this stuff & that's the way it is.