Any solid hit in a vital with a .303 diameter bullet moving over 1600 FPS will kill.
Any solid hit in a vital with any type of .303 diameter bullet moving over 1600 FPS will kill. Even if it is a solid spitzer with nickel jacket.
The question is, can you put the bullet where it needs to go, or are you the type of hunter that merely puts the front sign on the center of the elk, and pulls the trigger and hopes for luck?
Elk are brought down with black powder guns every year in one-shot kills.
Elk are missed every year by many shooters.
Elk are wounded every year by many shooters.
Most of your shots will be at less than 200 yards in timber country, and the vast majority of shots might be from 50 to 80 yards.
The moral of the story is dont take shots at elk that you are not proficient at taking, regardless of what you are carrying for a rifle.
The .303 will serve you well as a hunting rifle. You must serve the rifle well. Keep the rifle in good repair, and use only non-corrosive ammo. Practice with your hunting ammo even though it costs more, and do it just before the season starts. Vary your target distance. Work out on large targets and small targes in dim light. Dont wear sun glasses unless you wear them hunting, and then use the same pair on the range.
I like heavy bullets for heavy game. You hit the elk in a vital with a heavy bullet, and you will score an elk. You hit the elk in the ass, and you might not find him.
Learn to hit where the head and the neck join, or learn to hit him in the heart. If you cant be sure of a hit in one of those two areas, then you cant be sure of taking your elk humanely and efficiently.
Elk have been killed many times with a low powered .30-30 rifle from a hit to a vital. They have been killed with shotgun slugs. Elk have been killed many times with black powder guns, and with large centerfire rifles capable of killing an animal twice the size.
If you like the .303 and you can hit with it, then be responsible with your shooting and enjoy your elk hunt. Just dont consider taking "gamble shots" and only shoot when you know you can hit and kill quickly and efficiently and humanely by causing a solid hit to a vital.
Find yourself a 180 grain soft point or heavier; consider a spire point if you intend to work your bolt action rifle rapidly for followup shots; and understand that the round nose soft point is a great bullet, but it may not feed too well under speed in an Enfield action. Always practice with those hunting bullets before you hunt, and tune that rifle until you and the rifle are good to go. If you hand load, tune those loads to you and to your rifle and practice with them extensively at varied distances.
You will get your elk if you prepare accordingly and shoot accordingly.