One the most common problems with the Pump Rem's is the for-arm rocks a bit,so if you change your grip you will bounce all over the place.The first Big gun I ever owned is a 7600 Rem Gamemaster in 30-06 and it shoots great 1-2 inch groups @ 100 yds with rem Core-loks 180's.
Try resting the gun in between the pump and the reciver(hold on to the for-arm)this will give it a much steadier rest.Also don't rest it on the barrel that causes problem also.
Give us more info also please.
BBJ