Wow this thread has gotten interesting. I'd like to think I have a pretty good grasp on both sides of the "usage" element here. I work full time in LE, teach patrol and defensive/tactical rifle and also complete in 3 gun. Obviously the best scenario is a dedicated rifle for each usage, the saying of the mission drives the gear is true and the best set up for each would be something specific. I know that's not the point of the OP but stay with me. A 3 gun rifle can be very reliable, accurate, and handle a variety of target distances well from close to far with ease. That sounds like great qualities of a home defense (HD) rifle as well. However, a true HD rifle should also have a white light and a sling, and it should not have an obnoxious comp/brake like most dedicated 3 gun rifles do.
The topic of trigger came up and yes some out of the box HD rifles have a generic mil spec type trigger. They are heavier, have more creep and take up but when used properly, especially for local MN matches its not a deal breaker, the best no but can you still go to a local match and have fun, absolutely. My first 3 gun match I shot with my patrol rifle with mil spec trigger and did fine. It was the shotgun part of my first match that really sucked. Now can you buy a nice quality HD rifle, put a good red dot on it and come out to a 3 gun match, yes you can and you will be welcome with open arms and if you can run the rifle you'll do fine. In fact there are more then a few who will be like you and some who do very well with similar type rifles. Some LE guys will use their patrol rifle in local matches and do fine. There is even a class for red dot/no magnification optics.
In regards to a 6920 as was mentioned placing in the top X of a match, I can't say I've ever seen it but its not because it s a bad rifle, its a great rifle and will run and run and run. Is it set up for 3 gun, no its not but even Colt knows this, its a battle type rifle. Most of the top X shooters in 3 gun are really into 3 gun. When you get into something you want to get whats best for the job. Most top X 3 gunners have the best specific gear for 3 gun, set up comp shotguns, pistol, and a rifle thats set up to run 3 gun. An ideal 3 gun rifle has a nice trigger, free float fore end, good barrel, compensator, and optic of your choice. Again all good qualities to have in a rifle and some of which you find on nice HD rifles with the exception being a competition muzzle device instead of a "tactical" muzzle break. I do have separate rifles for game and work, but there are overlapping areas of my patrol rifle and my comp guns. I don't think I would be at a huge disadvantage if I ran my patrol rifle at a local 3 gun match. On that same token, I really don't want to shoot my 3 gun rifle inside my house with no ear pro, its really loud and the concussion from the comp sucks, also no light.
So in short could you set up a nice HD rifle, have fun and practice at a local 3 gun match using the same rifle, yes you can. There were some rifles in this thread that are better then others, before buying take the time to look at specs, material type (steel), testing, and also customer support. Also shameless plug but no one has mentioned a Huldra or Korstog as an option so I will
.
To the OP, good luck and if you ever want to give 3 gun a try you won't regret it, its a very fun game.
Dave