is there a negative to loading compressed loads for ar's?

Depends if you have a long enough barrel to get a full burn. If not, you're just wasting powder.

What powder and bullet?

There was in the past the thought that a compressed load would burn more efficiently than a load that didn't fully fill the case. With the powders that we have available to us now that isn't necessarily true. And it usually has nothing to do with barrel length. It depends on the application.

As a general rule, a compressed load isn't dangerous IF it is inside a safe pressure curve. And that's a BIG "IF".

Once again ...... is this something that you are going to load / have loaded? Or are you just considering it?

One advantage of a compressed load for an AR or any semi-auto is that you won't have bullets pushed deeper into the case during feeding, which can raise pressure. There is no downside to a compressed load---unless you're trying to pack so much of a slow burning powder under the bullet that the bullet creeps back out of the neck. That can, and DOES happen.

Depending on the amount of compression (over 103%) it can and does cause the bullet to back out a bit as you just said. I loaded compressed loads for a 308 that had to be crimped to keep the bullets from backing out. Sometimes, it's best to avoid it entirely.

There are usually other powders that will give the same results without all of the problems. That's why I asked him what powder / bullet combination he was looking at using.

With the new selection of today's short cut powders I don't think its necessary to compress loads like you did in the old days. Not saying that you shouldn't do it but like stated above anything over 103% u run the risk of bullet moving in the case neck.

Powder and load?

Sometimes the best load you can cook up will necessitate a compressed load. One example would be the .35 Whelen with 225-grain Accubonds using Re15.

Nothing is wrong with a proper compressed load-It has the advantage of powder not shifting around because case is completely full. But watch for the creepage as stated above.

If you do go with a compressed load try this. Hold a engraver, handheld electric massage or electric shaver to the cartridge or to the cartridge loading tray. The vibration will make the powder settle down in the case. Be careful you will kick some powder out if not careful. Works better than a drop tube.