Probably just a glitch. Contact Bungie. You will have to be insistent with them, as they will probably skip over most of your messages.
I did some of the 'new' achievements on the legendary map pack. Does that have anything to do with it?
Not necessarily true. The security helmet along with the katana are pretty glitched items. I have around 49 achievements, but I haven't done all of the original achievements and I still have security/katana. It all depends on your disk I suppose.
My disk? When did you get yours? I got mine from one of my cousins who got it when it came out. Lol...
I don't know about security, but I can't get the helmet for the Leuitenant achievement because I can't/don't have it and I'm a Major Gr 2
Ive have just recently had the same problem. I should have had security and katana for about two months ago. Yesterday i checked and i suddenly have them. I'm still missing an elite body piece that i should have though. I suggest just waiting or trying to do some more achievements. Thats sometimes works.
That's why I did the "You're In For Some Chop" achievement some time after, because I thought it might work. I'll try some other achievements.
Try getting 49 achievements then if not contact bungie and yadda yadda yadda but the weird thing is my friend has only like 39 of the achievements and he has the security and everything and he only has 990 gamer score from halo 3.
I had 48 of the original achievements and a few of the new ones, and I got it... Well, now I have almost all of them, but... My friend had to get all the new achievements too to get the helmet and katana, and I don't know why.
Just what does that mean? It has been pretty well proven, though not in definite terms, that you don't need all original achievements. Why does this rumor persist? Anywho, check your disc. Clean it, spit on it, wash it. Do whatever to it to get it clean and then see if its there. That may help, its what helped me when I had most of the achievements needed and didn't have the katana piece....