Fastest Way To Insert Range/sequence

I'd like to use a stored procedure to insert large amounts of records
into a table. My field A should be filled with a given range of
numbers. I do the following ... but I'm sure there is a better
(faster) way:

select @start = max(A) from tbl where B = 'test1' and C = 'test2'
while @start <= 500000
begin
insert into tbl (A, B, C)
values (@start, 'test1', test2')
set @start = @start +1
end

another question is, how to prevent that another user inserts the same
numbers into the field A?

Thanks a lot for any help!
ratu