What Stick said is definitely the fastest path to success: just work up loads for that rifle.
If you're trying to pick the most likely culprit, it's probably the long throat. Larger chamber, same gas volume, less pressure, lower velocity.
For that same reason, if you're loading for the rifle, you will reach max pressure with more powder and at a higher velocity than if the rifle had a shorter throat. Some would consider this a benefit.
Long jumps don't necessarily mean less accuracy. But they can change barrel timing enough so that the load that shoots well in your buddy's gun won't shoot well in yours and vice versa.
The solution to this really is just to work up loads for that particular rifle. Most factory chambers aren't cut all that concentric so most gunsmiths won't want to rechamber them, plus Remingtons tend to have short barrel shanks anyway so there might not be much room to cut deeper.
If you're set on messing with the barrel (instead of just loading for the one you have) go to bugholes and buy a new barrel and pick the 308 reamer of your choice.
