I have had 3 rifles really throw me for a loop as far as finding a load and getting them to shoot:

Worst one was my father's Browning 1885 High Wall in 22-250. The rifle itself was beautiful but it was difficult to find a gunsmith to do any trigger work on a falling block rifle. Finally found one and got the trigger right but the best this rifle would do was about 1/2", which may sound pretty good but with a round like the 22-250 you should be ragging holes before you settle on a load. I'm convinced the barrel was just too long for that caliber. It had a 28" Octagon heavy barrel. We ended up selling the rifle and he bought a Remington 700 VLS and never looked back. Very good shooting rifle.

2nd for me was a Ruger 1 in the same caliber that I loaded for a guy on our deer lease. Pretty finicky little gun but found out that it liked the COAL very short. So short in fact that you could visibly see a small gap between the case neck and the bullet. Odd looking round but after all was said and done I accomplished several "one hole" groups with it. I later loaded a .223 in Ruger 1 and started with a very short COAL and found a load very quickly for it.

3rd is my Ruger 77 22 hornet. Had to shoot it quite a bit. I floated the barrel and worked on the trigger myself and got it right, but was still having problems. After some research I tried Lil' Gun powder and Small Pistol primers instead of rifle primers and found a really good load. Who would have thought that a shotgun powder and pistol primers would make a this little rifle shoot???