Alfred,
The first rifle was designated Rifle No3 MkI(T) and was introduced at the very end of WWI. it may have been possible that one or two trials weapons made it to the trenches but they where never on general issue in WWI. A lot of the 3(T) rifle saw service with the Commonwealth forces in WWII and there are a number of photo's in the Aussie War Memorial of the rifle in use in Timor etc. As a matter of fact I'm in the process of restoring a 3(T) with Aussie property marks which has been in the hands of Bubba.
The second rifle was designated Rifle No3 MkI(T)A. These rifles where introduced at the very beginning of WWII and saw limited use as a stop gap until the No4(T) came along.
Take a look at the Knowledge Librarythere's some great info there including some detailed photo's.
As regards Ammunition use, bear with me and I'll post an extract from a letter sent to me by Harry Furness in which he talks about ammunition selection, although Harry carried a 4(T) as the tool of his trade.
I've just put the info up as a seperate post "Sniper Ammo Selection"
Cheers,
Simon.