I have taken both deer and hog with the 303. I use BLC2 and Reloader 15 powders with the 150 & 174 Hornady. The 174 round nose went from the brest to the butt on a deer with a close shot 50-75 yards and all the way though a 130 Lb pig. The stock sights were set with 175 grainers zeroed at 200 yards with about 2400-2500 fps, if you max out with Reloader 15 you can get pretty close. 150 grainers might shoot low in Enfeilds with stock sights. You must try different bullets and weights to find what your rifle likes. These rifles seem to be ammo sensitive more so than others. One I have that likes Rem. 180 factory loads and 174 hand loads and HATES all 150's. Also some, do not like to feed the last round do to wear on the mag lips. Buy some powder bullets and have fun, the old SMLE will get the job done.
