My first question is what groove dia. did the bore slug out at,a .309" dia. bullets may be to small in the old Eddy it may prefer a .310" or even a .311" dia. bullet for best results.
You can't just assume that a .309" dia.cast bullet will always work in a 30 cal. rifle especially an old Mil Surplus rifle. I shoot bullets sized to .311" in both my old Sav. 30-30 and Sav. 99 in 300 Sav. with or without the gas check applied with equally good results.
Also the Eddystone has a 1:10 twist rate and is designed to shoot heavier J bullets in the 200+ gr. range best,it's probably why your finding the heavier 200 gr. GC bullet shooting better than the 165's.
Here is some info from a post by
Jerome Cooper
Col Harrison, writing for the NRA loading handbook did a thorough investigation on the performance of 30 cal cast bullets. One of the parameters that he established is that 1917 barrels with 5 groove rifling (which way the twist goes is not important) has lands that take up a major portion of the circumference of the bore.
As such the rifling requires a bullet with a shorter "body" (less than half the length of the bullet) that will ride in the grooves and a longer "nose"...which will ride on tops of the lands and be guided by them. The ideal bullet design (no pun intended) was 311334 (not any longer in production) Lyman still makes 311332 a close copy. Saeco's 301 version of the old RG-4 is also an excellent choice. RCBS and Lee also make designs of various weights that will work well too. Other molds such as Lyman 311291,311041 and 311284 as well as 311299 and 314299(if a larger diameter bullet is needed) have been reported to shoot good groups.
There is some variance in the measurements of 1917 bores so it is a good idea to slug your bore if possible to see exactly what internal dimensions you are dealing with before choosing a mold, or making adjustments to it later if necessary. Depending on BHN bullet hardness; the use of Lino or water quenching etc. velocities of up to 2200-2300 are easily attainable and will afford good target to hunting capability. With the current trend in ammo prices it makes getting into casting an interesting venture. The 1917 can be an accurate rifle if you get into making it one.
As to paper patching that a whole different ball game,you will find a wealth of info here on the subject in more detail than I can ever put in one post.
http://castboolits.gunloads.com/forumdisplay.php?62-Smokeless-Paper-Patching&