Check out the beginning engravers kit at NgraveR. It has a set of sencils for some basic curves, scrolls, leaves, etc. that I found useful. I do like the french curves, have a set of 3 or 4, but those above were a good helpful addition.
You may want to trace a piece or two from a pic of an original as it will give you a feel for it better than an eyeballed sketch. (just remember you cannot transfer this to a finished rifle...flat pictures do not result in good patterns for round surfaces!)
If the design is too large try putting a light grid over it. This is used by pencil artists and it will bkeak down the "elepant" into more do-able pieces.
I like to draw and find that a good wood #2 pencil and a tablet of good sketch paper will do wonders. I have to fight to get over not doing it perfect as seen and allow myself the luxury of developing my own style as another is used as a practice piece. (And please belive me I could not draw a card from a pile of aces! Practice will build your confidence!)
Keep your practice sheets in the tablet. See how you improve with just a bit of practice. Your goal is to develop a style rather than copy a master?
I do not know how much you have practiced, but do not doodle. trace or draw out a design you like (read a small part of the design) and do it again and again until you can do it with ease. Doodling I think is without a goal and best saved for boring staff meetings!
The most important part of your drawing tools will be a good eraser. I recomend a pink pearl
as a personal fav.
Please be encoraged as the style will come with practice and focus. Allow your practice time to be relaxed time...this will help with the final carving and the finishing and after all is that not the real root for most of us in this wonderous realm?