One thing to try MIGHT be 150gr bullets. Don't know why, but it works for some people. As a general rule, sportered and especially shortened No4s and even more No1 rifles don't shoot well. We can get into a huge discussion over the reasons, but it won't help your situation...
Could you go into more depth on short barrels jmoore , I don’t remember having ANY problems with my Winchester Jungle Carbine.