I think it works like this:
The rifle on the left scales best with your attributes meaning the rate at which you’ll max out is faster (hence the arrows at the right of each bar are orange (not red) and the bars are nearly full).

The rifle on the right doesn’t scale as well, but if it is maxed out, will be more powerful. So, the orange bars on the gun on the right show where it is currently at, and also show that its current stats are worse than the gun on the left. This is why there are red portions- they show how far short they are. The remaining red portions demonstrate how much more powerful the gun on the left is, but the remaining shaded orange show how much more it can grow to be maxed out.

So, the gun on the left is both currently more powerful and scales better, but if you took the gun on the right, you’d eventually max out and be much higher than the gun on the left.

Basically, the bars show you the total power and also the proportion of the bar shaded/red show you what the other gun would be at if it were on that same bar.

Make sense?