I know very little about weapons, but I can tell he means it is not a comfortable rifle to use. I assume that means inaccurate shooting due to poor positioning

I know very little about weapons, but I can tell he means it is not a comfortable rifle to use. I assume that means inaccurate shooting due to poor positioning, or perhaps he means they are difficult to carry. Ergonomics is ergonomics no matter whether you're talking a chair, a keyboard or a rifle.