What is the smallest caliber for deer hunting?

Here in uk it’s .222 for muntjac & water deer then .243 for roe and goats and soay  sheep then .270 for fallow sika and red + boar 

 Although I have nicer guns I find the Surplus Enfield .303 with the sporting stock to be more than effective.

 This gun was still in the factory packing when my Dad got a case of 5 of them (never fired). At the time he paid $50.00 !!! each for them !!!

 He sold 2 and modified the one I use today. 2 are still stored in their factory packing.

Hello Angel,

  I have you narrowed down a planet called 'Earth'.  You must be a new visitor.  See, Earth has almost 400 different countries, with over 5,000 different states and more than 150,00 cities that all have different laws for hunting.  You can never ask a ' what is' question about hunting without telling the country, state, and what part of the state you want to know about."  There is no 'one right answer'.

Your state hunting regulations will be online at the official state website.  The answer you seek will be in the first 10-20 pages for 'general information'.

Keep in mind three things.  First - what is ok in one state will not ok in another.  Second there can be exceptions. Alaska, where I live, clearly states the firearm be 'center fire' however in another second of those regulations is says a 22 rimfire is ok for shooting swimming caribou. 

Third and last.  Using the smallest also means less powerful and less accurate at distances. Gravity will pull a smaller slower bullet down more than a heavier faster moving one.  Wind will push a small bullet around.  With a small cartridge you need to be a much more skilled shot and stalker - because you need to get closer to the animal and shot placement is critical.  The reason hunters use high power rifles is to get a one shot clean kill.  Moving to a lower power rifle could mean you just wound it and it runs off.  Legally, once you wound an animal - it is 'yours' and if you were foolish enough to use too small a cartridge - that is your problem - because you are legally required to track down that hurt animal and finish the job.  If it takes you two days of tracking - so be it. 

Bottom line:  Smallest is never going to be best.  You need to be both a top shot and skilled at knowing exactly where to hit an animal to be successful. If your shot is a few inches off - that is the difference between a happy hunt, or, spending the next few days in hell trying to finish the job.      

.3030 calibre , the require shooting over a distance .

I use a 270.   

My state only specifies that it be centerfire.  Which means someone could take a 17 Hornet into the woods for deer.  Not that they should, but they could.

That is determined by the state you live in. Some states require 25 caliber or larger with a certain amount of footpounds of energy. Some states require 30 caliber with ft/lbs requirements. Where I live we must use nothing smaller than 22 caliber centerfire rifle.