Like with a gun drill? Done it many times.
Gravity my boy, its not just a good idea. Its the law.
Seriously, Gun drilling in a horizontal is affected by gravity. Just like on a gun drilling machine. Your exit hole will be lower than your entrance.
You will only need a good start hole. DME says you need a flat bottom hole but I have got by without it. You also need high-er pressure CTS. Again I have gotten by with 150 ~ 300psi.