For Giraffe I wouldn't advise anything less than a hand loaded 30-06 with a bear claw, accubond, a-frame or partition of at least 200gr.
I found Giraffe to be a very difficult animal to hunt. To put one into a giraffe off sticks your kill zone is about the size of a salad plate. (about one square in size on their hide) I found that to be sure I'd put it where it needed to go I needed to get within 70 yards. That great stalk had to be in a place where the giraffe was accessible by the safari cruiser after the hunt too. I believe I stalked about 12 old stink bulls before I could get one within A.) 70 yards, and B.) that was retrievable from that location. It was very easy to get bulls within 120 yards in places where a cruiser could not reach, however.
My shot was perfect with a Nosler Accubond 200gr bullet shot from a 300H&H rifle. The giraffe was able to walk at least 25 yards before he collapsed from a perfect shot that went through both lungs and the heart.
Lesson to be learned from this:
-I don't believe a rifle less than .30 caliber is advisable
-The 300H&H is the legal minimum caliber in Zimbabwe for a reason (minimum foot pounds energy)
-A perfect shot from a small bore could very easily end with a giraffe running far enough before expiring to make the animal very difficult to recover.
I'd use a .375H&H if I was to ever do it again but I don't intend to do it again. It took a full day of work to quarter, load, haul and dress that giraffe. The meat was just terrible and I usually love every kind of game. There are better things to hunt in Africa that provide better table fare but I'm glad I did it once.