I've always had this question running in the back of my mind when I'm shooting a gun that's new to me and I figure it's a good time to ask people that have a better understanding of accuracy/gunsmithing than I do.
I shot a couple of different rifles yesterday, all in the same weather conditions (60*F, 2-4 mph winds) from the same bench and rest. One Win Model 70 .270 (great trigger) grouped 2 moa in a perfect triangle, one Rem 700 7mm-08 (horrible trigger) put 2 shots 1/2" above each other and the 3rd shot another 1/2" higher and 1" to the right of the 2nd shot. The last gun is even more puzzling. Brand new Browning A-bolt, Mountain Ti .243. After cleaning the gun, one round was shot to dirty the bore and a test group was fired - 2 shots touching, running from 2:00 to 7:00 with the 3rd shot 1/2" further away straight "down the line". After cleaning and following the instructions on the Tubbs Final Finish bullets (3 finest grits were only used) the gun shot a larger group, with the 2nd shot 1" above and 1" to the left of the first shot, and the 3rd shot 1/4" down and 1/2" to the left of the first shot. Factory ammo was used for all 3 guns, and with the Browning, only 5 shots were fired at a time during the Tubbs process to allow the barrel to cool.
What do those group shapes tell me? I can post pictures if anybody wants.
Pictures are always good! I am by no means an expert, but I do not know how much weight can be put on "group shapes". There are quite a few variables to deal with when shooting groupings, especially when using factory ammo. As I'm sure you aready know your breathing, how you hold the rifle, even a heartbeat if your chest is against the bench is enough to change your shot placement. The rifle could just plain not like the ammo that you were shooting and you may need to try something else. Maybe I am way off, but there are my thoughts on it.
I would say that all of the rifles would tighten up w/ hand loads. Two years ago I got my daughter her 1st rifle, a Weatherby Vanguard youth model in 7-08. Didn't have much time so we were just going to shoot factory ammo. The target that came with the rifle showed a 3/8" group. Shot the factory ammo and couldn't do better than a 6" group at 100yrds. So, in a hurry, went home and pulled all the remaining factory, prepped the brass, seated 140 Accubonds to the magazine, and ran 1grn under max load per the book. Took the rifle out the next day to the range and printed an identical target to the one that came w/ the rifle.
I think I got a little lucky, but my point is that the factory stuff generally stinks.
The Winny and Remington are a buddy's who doesn't reload. He wanted me to get it on paper for him, and he'll do the final fine tuning for the hunt. He only sent 1 box of ammo for each gun, and not knowing if he had any more I didn't want to burn up the whole box. None of the guns have had any stock work done, I lightened up the trigger on the Browning as much as I could, but at 4# 4 oz it's still too heavy for my liking. A spring kit is in the mail and should arrive tomorrow. The Model 70 had a 2# 8 oz trigger, and the Remington checked in @ 6# 10 oz. Even with the horrible trigger the Remington still shot a smaller group.
The Browning is mine and I just wanted to run some factory fodder thru it for a baseline to see if there really is a change after firelapping the barrel. I've got some 85g Barnes TSX's loaded up for a ladder load test, .005" off of the lands. Today had 10+ mph gusting swirling winds and I didn't want to try a 300 Yd comparison in those conditions. I'll post a pic of the test when I can get it shot. The weather is supposed to turn bad on Friday, so hopefully I can get it shot before then.
I'm puzzled at what would cause diagonal stringing of shots. I'm shooting from a pretty solid bench, with a rest that mimics the Caldwell Lead Sled. I can shoot the guns free recoil and minimize my influence on them. That's strictly for load development, when sighting in for a hunting trip, I'll use a regular front rest and bag the back of the gun, and shoot from different positions as well.
I just worked up a load for a Savage .243 with the 85 TSX's. Factory stuff was about 1.5 MOA. The loads I threw together were .75 MOA with H4831 @ roughly 3220 fps. They weren't the fastest but they shot well.
Make sure you double check the torque on your action screws as they may be loose. Can't guarantee anything but its worth a shot.
That's the powder that I have loaded and ready to go. I started at the low end of the chart and stepped up in .3g increments. The action screws are 40 ip. I had the stock off to see what it weighed after I bought the gun - 1.5#. With a Leupold 3-10x40 scope, Butler Creek flip up scope caps, Leupold DD rings and steel mounts it tips the scale @ 7# even. I was hoping that it would be lighter than that, but it IS 3# lighter than my .257 Wby.