First of all, congrats on your new rifle! What kind of scope are you using?
Next, don't know much about that specific sub model rifle, but I know the 700. If it's a wood stock, and it's not already, float the barrel. I don't see the point with a synthetic stock, but I guess its better to do so anyhow, if it makes you feel warm and fuzzy inside. It's too easy not to... Next, if you don't like the trigger, try adjusting it. Nothing to worry about, just do it. If you don't like the end result, adjust it back or get a new trigger assembly.
So, you shot a .4 group one day and a .7 group another. That's not terrible, but you can get it better at 100 meters since you're hand loading. If you're loads are consistent to the tenth of a grain, your inconsitency in your group was more than likely you, however, you already noted that to be the probable cause. Another cause could be if you didn't let your rifle cool down completely between shots. Many rifles start tossing rounds on a hot barrel, I know mine does. You just simply need to shoot your rifle more. Don't compare it to an AR, because it's not an AR. It's a different rifle, with more recoil, and you're going to have to get use to it.
Once you float that barrel, if not already, and get the trigger where you want it, it's an ammo game at that point. But, you probably already know that too, since you load your own. If you can't get the groups you want with the round you're using, go to a little heavier or longer boat tail round, I wouldn't go lighter. See what you get then. I'm about to order a box of 150 grn Hornady SST's for my .270 and see how it likes them. They've got a great BC, and my rifle likes heavier lead vs. the lighter stuff.
Seriously though, don't get too caught up in it. If you're getting half inch groups at 100, that's good for anything out to 500 meters for hunting. I wouldn't have a second thought, especially with a .308 rifle. That's such a stable round. I went on a mission of perfection for my .270 not long ago, and I finally have it real close. I'm still playing with it though. If precision is your mission, it's addicting...... And can be costly.