Yep. You need a new rifle. In the arsenal tab there's a series of ascendingly powerful - and expensive - rifles for you to purchase. It's simple, the pricier it is, the better it'll be for you, with more power and stability than the one before it, making those later missions much easier. You'll know when you need a new rifle. Most missions will be giving you incredibly difficult shots, possibly even with bullet drop you can't even compensate for.