I will try and give you a few suggestions that I see from your load and loading technique.
1st thing noticed - The Lee bullet you are using, while correct in 'diameter and weight' is a 'pistol bullet' design and too large a diameter, IMO. The correct bullet would be the Lyman #403168(editted - it is the old Ideal standard bullet for the 40/60 cartridge). The 0.403 bullet with BP will expand to the 0.406 bore. It will have larger grease grooves and will most likely be longer in length to stabilize in the rifle. Tap on the link, then scroll down to 403168 bullet -
http://ps-2.kev009.com/ohland/Cast_Bullet/Ideal-Lyman_Molds/Ideal-Lyman_Mold_Descriptions.html#40_caliber_403 This bullet design is still offered by Lyman today.
2nd thing - As Greyhawk stated No powder coating with BP. No smokeless types of lubing with BP.
3rd thing - No felt wads in a BP cartridge. It is not a true grease cookie. Grease cookies are normally like a 3/16" thick semi-hard lube plug. Not the same approach as in a percussion pistol.
4th thing - IMO GOEX does not have to be compressed to 1/4" (0.25). My loadings are only 1/16" (to tops 1/8 rarely) compression.
5th thing - The 40/60 was a 60 grain cartridge and still should be close, not only 40 - 50 grains. I do not own a 40/60 but a 45/60. In a modified 45/70 case cut to 45/60 dimensions, I measured 61 grains of RS Pyrodex. GOEX 1F is less. That is Powder and Bullet with a 1/16 compression. I would only use 1F or 2F in the cartridges. (I use a different smokeless powder in mine). I will have to recheck the GOEX 1F measurements.
6th thing - These older firearms where designed around a different lead alloy than what we most often use. Much Softer. The Lyman 39th Ed suggests a 16/1 (Lead to Tin) alloy be used for the Winchester 210 grain - 40/60 bullet.
While I shot BP strictly in all my calibers for 20yrs, I now have to shoot indoors using only smokeless. NO outdoor ranges close by.
Hopefully Dusty and others with true 40/60 rifles will come in, to give you their loading techniques and suggestions.
Hope this helps, plus there are several books about BP reloading techniques. Also, Lyman's 49th Ed has a specific 1876 section, for us.