Re: Accurizing a factory barrel
TomVan, I would suggest you take a few fired cases and measure the runout of the neck. if the case runout exceeds 5thou, there is a very good chance that the chamber is not cut square with the bore.
That barrel will never shoot well. You might find a load for MOA or a bit better, but a tack driver, forget it.
Chamber relative to the boreline is the single most important factor in accuracy. The best barrel in the world with an off center chamber will shoot as well as an SKS.
If the runout tests holds up, then I would try every single factory load available. You might get lucky and find one that shoots. If not, then handloading will be a must do.
Another check is to measure the throat of your chamber. The bullet must fully engrave in the lands before leaving the neck. If not, accuracy will be poor. With the short 150gr bullet and a long throat, this may be one reason for your poor performance.
Remington may have set up their reamers for the 180gr and heavier bullets, so throated with more freebore then your load can handle. Remedy would be to set back the barrel and get that bullet closer to the lands.
The action and stock should be bedded, or least checked to make sure that the barrel is not rubbing (free float if you haven't done so already) in the stock. Reduce the trigger pull to a nice light letoff.
Ensure that your scope is accurate and not suffering from excessive parallax. You should also have the resolution to see at least 1/8" at 100yds. Very hard to shoot small groups with low/no magnification.
Shoot off solid front and rear rests on calm days. Finally, shoot only a few rds and let your barrel cool. Your barrel can start to walk after only 2 rds if shooting in warmer weather (a lot of powder and heat from that case).
Watch your flinching, and body form. No matter how slight, it will greatly affect your consistency. Not good for shooting tiny groups.