I use these loads primarily for fire forming the .303 brass and then headspacing on the shoulder and not the rim of the case to extend case life. A small thin rubber o-ring is slipped over the case, when the bolt is closed the compressed o-ring centers the case in the rear of the chamber and holds the case against the bolt face.
Once the case is fire formed it headspaces on the shoulder of the case and then the shoulder of the case will hold the case against the bolt face and thus preventing case stretch in the web area
A .303 case forming and file trim die can be used as a shoulder bump die when the neck sized cases start to close hard in the chamber.
You can use SR 4759 or Trail Boss for your reduced loads, the load data came from the Number Nine Speer manual using a .308 100 grain plinker bullet and 4759 powder. The load with the 100 grain .308 plinker was 16 to 20 grains of SR 4759. NOTE: With a 150 grain .303 bullet the load was 21 to 25 grains of SR4759 so the lighter load is very safe with .312 pistol bullets.
For the Trail Boss load just go to the Hodgdons web site and look at the Trail Boss reduced loads and use the 70% rule as your starting load.
You can shoot hundreds of reduced rounds and the brass butt plate or rubber butt plate of the Enfield rifle will be as soft as warm butter. I ruptured a disk in my neck in 1992 and even after disk surgery I became a little recoil sensitive but found ways to sill enjoy shooting.
And a mouse gun has very little recoil.

(but sadly throws perfectly good brass away)