Need help, Animation Idea

I think I have may came up with a new idea but I'm not for sure. I need some help with animations, not really the animations. Mainly changing the code around for the animations. Basicly what I want to do, is that I have a set of animations of the character in a stand position. In this position, the character will not have any weapons or items in his hands and his arms will be down to his sides. He will also run like a normal person would in real life and same with every other animation like the jump, etc.

That's the easy part, now comes for the fun part. I have a whole new set of animations with the character being in a rifle pose. This rifle pose will be in every animations. So now I have 2 different sets of animations (I bet you are asking yourself why do I need 2 sets of animations), Well for the last few days I have been animating Adam and came across some problems with the animations that I made so the only way that I can see fixing them would to be changing somethings in the code.

Now, here's the idea I have came up with or maybe someone else has but I can't find it. Well in my project you will first start in the first set of animations (the standing animations). Then when you equip you rifle, you will use the second set of animations (the rifle pose animations). Does this make any sence?

I need help with how to tell the engine to use the rifle animations when you equip the rifle. Any help would very much appreciated!

Thanks,
Stephen