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I'll add my $.02 for what it's worth. I've collected Tyranids for a couple of years now and really enjoy the hobby and game play aspect, despite the relative weakness of the current codex when compared to competitive lists/codices. Regardless, the 6th edition rules helped them out in a few ways, namely psyker abilities.
For your
HQ at 500 points, you want to keep it cheap. A Tyranid Prime comes to mind here. For a base 80 points, he's a bargain and brings a lot of utility at your points level. He's an independent character (our only one in the book) and can boost a squad when/where needed. I like a Deathspitter on him for a boost to shooting and it's cheap. I'd consider Toxin Sacs as they now allow rerolls on enemies T5 or below. All said, he's 95 points. Nice thing is you can pick up a "used" Tyranid warrior from Ebay or somewhere else cheap and just convert him.
For Troops, I'm leaning towards Hormagaunts for a 500 point list. They are cheap, fast and the
IB:Feed isn't all that bad if you get out of synapse range due to their speed. I'd consider Toxin Sacs on these guys as you will be able to wound
MEQ's on 4+ and reroll to wound against any T3 enemies. That's pretty good. For now, let's leave the Sacs off, the choice is yours. They may be too much for 500 points. Let's get 2 squads of 20 Hormagaunts totalling 240 points.
For heavies, I'm not seeing much that would be affordable at this points level unless you specifically tailored the list up to this point to take something like a Carnifex or Trygon. You may want something, but I'll pass for now.
For Elites, I'm looking at Zoanthropes and Hive Guard. Two of the best units in our Elites section and fairly affordable. With our remaining points I'd suggest 1 Zoanthrope for Psyker help and extra Synapse. Additionally, a brood of 2 Hive Guard will round out the list nicely with some ranged support.
HQ Tyranid Prime - Toxin Sacs, Deathspitter
Elites 2 Hive Guard
1 Zoanthrope
Troops 20 Hormagaunts
20 Hormagaunts
Tactics are simple- move toward the enemy and close the gap with the speed of the Hormagaunts and pop any transports with the Hive Guard/Zoanthrope. If you roll Biomancy you can beef up your Prime or stick with the book powers and lance whatever mech is on the table.
Total comes to 495. Not a bad list for 500 points. Should clock in at $200 or less, also. Additionally, you've got some units that you will more than likely keep as you move up in points and expand the list. The Prime will probably disappear once you hit 1000 point games but the others will still have a nice place.
You could switch out Hormagaunts with Termagaunts to free up some points and have a slower moving list altogether but with some ranged assault potential due to Fleshborers. You'd have 40 additional points to spend in that case. You could buy a biovore for exactly 45 points to get to 500 on the dot. Or something else, there are some opportunities there. A fun trick is to purchase a minimum unit of Termaguants (10) and let them camp an objective in cover. Leave them behind and let
IB:Lurk take over and hold the objective while the rest of the list does the business. With 45 points you can buy 9 and take 1 out of the other brood to make 10. Like I said, lots of options here.
It would look like this:
HQ Tyranid Prime - Toxin Sacs, Deathspitter
Elites 2 Hive Guard
1 Zoanthrope
Troops 19 Termagaunts
20 Termagaunts
10 Termagaunts
You have a lot of saturation in this list as you present several viable targets and can overwhlem an opponent with decisions on what to fire at first.
Additionally, the more I think about it, a Tervigon as your
HQ would be pretty fun at this points level. Chances are no one would expect to deal with an
MC like that at 500 points. Plus, the Termagaunts the Tervigon produces will be very valuable at 500 point level games. Even a brood of 10 will put you 10% over your opponent in points which at 500 points is a big deal in my opinion. The Tervigon buffs to gaunts are significant also (I'm looking at Catalyst for the
FNP buff). Something to definitely think about. Tervigons are definitely something you will run at higher point levels, so the investment now would be worthwhile.
So many possibilities. So much fun!