Thank you yuppiejr. To go back on my word, I actually WOULD like to know about this Dillon that was totally set up for the 6.5 Grendel, but with different bullets.
1. Were any adjustments made to the powder charge thrown, or was it just left as it was for the owner's bullets?
2. What weight were the bullets that were used by the owner, and what weight were the bullets you used? You get one 800 pound reloading troll demerit if you don't know. You get two demerits if your bullet was heavier than the one the press owner was using.
As far as why your loads came out too long while the 550's owners ammo was supposedly okay is a bit puzzling, seeing as bullets with similar profiles should come out of the seating die at about the same length. The fact that they did not suggests that two different bullets with significantly different nose profiles were used, because rifle seating dies generally work off the bullet ogive rather than just mashing down on the point of the bullet like a 45 ACP seating die. Once again, do you know what bullets the other guy was using, and you obviously can tell us what bullets you were using.
And last but not least, there is the issue of the owner of the press checking your finished loads with a headpsace gauge, which as I mentioned has NOTHING to do with checking the OAL of the loaded round. In addition, let your mind replay the "Larry the Cable Guy" commercial for heartburn pills, where he says "that's like checking your burgers after they're burnt!" The purpose of the headpsace gauge is to make sure the shoulder of the case is neither too far up (won't fit in the chamber with the bolt closed) or too far down, which will give you a headspace problem which will cause the base of the case to stretch, and eventually may cause the case head to blow open where the brass has been thinned out from the stretching. You are supposed to check this WHEN you are resizing your brass, and once you have got it right you lock the ring on the die and never mess with it again. But here we have your "buddy" checking headspace AFTER the ammo has been loaded?" What was he planning to do if his gauge had showed that your loaded ammo DID have a headspace problem?? OOOPS!!

Seems to me that this guy wasn't totally up to speed on his own reloading practices, and then to have you show up with a significantly different bullet and use his press "as set up" is an invitation to a mishap of some sort.
So: Got answers to some or all of the above questions??
EDIT: And just for the record here, the purpose of this inquiry is not to flame the OP, but to find out what the heck exactly happened here. This is a real life OOOPS, and it would be interesting in finding out the details so everybody can see what went wrong and possibly why, so they can maybe see some gotchas they had never considered before.