For smaller batches, I really like this recipe. It's so simple and quick.

For larger batches, like your six pounds, I've done this recipe several times from the TD cookbook:

Tavolatim’s Barbecued Shrimp
2 pounds butter (Yes, you read right. Two pounds. Eight sticks. Don't whine. DO NOT use margarine! Real butter only.)
2 tablespoons Creole seasoning, to taste; OR
2 - 3 teaspoons cayenne pepper and 3 - 4 teaspoons black pepper, to taste
2 tablespoons chopped rosemary leaves
1/2 cup Worcestershire sauce
6 ounces good beer (microbrewery is preferable to mainstream swill)
5 - 10 cloves garlic, finely minced (or as much as you like)
1 medium onion, very finely minced
3 ribs celery, very finely minced
3 - 4 tablespoons chopped parsley
2 teaspoons fresh-squeezed lemon juice
5 pounds shrimp, heads and shells on
[This feeds 8-10 people, BTW ... cut it in half if you need to.]
Melt a stick of the butter in a skillet. Sauté the garlic, onions, celery, parsley, rosemary and seasoning blend for about 2 - 3 minutes.

Melt the rest of the butter. Add the beer (drink the rest of the bottle). Add the sautéed stuff, Worcestershire and lemon juice.

Drown the shrimp in the seasoned butter, using as many baking dishes as you need. Make sure the shrimp are more or less submerged. If they're not ... melt more butter and add to the sauce. (Aah, what the hell ... what's another stick or two when you're already up to these butterfat levels?) Bake in a 350 degree oven until the shrimp turn pink, about 15 minutes.

Serve in big bowls. Put in a handful of shrimp and ladle lots of the spicy butter sauce over it. Roll up your sleeves and wear a bib (DO NOT wear nice clothes when eating this!) Serve with plenty of French bread to sop up da sauce!

Try to avoid going to have your cholesterol and triglycerides taken for a few weeks afterward. Remember that this is a special treat. Don't eat this all the time if you want to live. But hey, every now and again ... LIVE!