Start off with a min load and work them up. Trust me Even If you load up 1 or 2 rounds to make sure your gun can handle it. I agree with the other guy if you want more power in a 243/6mm buy a 240 weatherbee mag(good luck finding a gun), or a 243 wssm, or 6mm remington.

I have been reloading for 15 years and my father has been reloading since 1956. We both just got schooled on this one. I just bought a used Ruger M77 in 300 Win Mag from a coworker. His father in law bought the gun new in 1975. He fired it 3 times, could not handle the recoil took it home and bought a 270. It sat for 20 years in his gun cabinet. Then my coworker bought from him fired 14 rounds and said too much recoil. This gun has only been fired 17 times(yes its mint, perfect no scratches, have the half boxes of ammo 17 missing, no BS).

Using once fired win brass tumbled, cleaned, and trimed. I loaded 2 diffrent loads each to the min load with 180 grain speer bullets. I loaded 10 rounds with Hogdens 4895 and 10 rounds with Imr 4350. With rifle loads I weigh out every load and I use an old Rcbs Jr press(very slow). I went to the range and fired the 4895 rounds first.(At the range I fire 1 round a minuit to let the barrel cool down). Damn this gun kicked as hard as my dads 338 win mag. My dad laughed at me and said get used to it kid(I am 29). Then I fired the Imr 4350 and Damn it kicked so hard it almost fell off the sand bags. Then I could not open the bolt. I had to wait 5 minuits for the barrel to cool down before we could get the bolt open. It was a bitch but with 2 people we got it open(I am luckey that I only had 1 round in the gun) I split the neck and started to push the primer out of the case. The back of the case was deformed from the pressure. This minimum load was listed in the speer #14 book.

We had the gun checked and the bore is .0015 smaller than it should be(tight bore). This explains why the previous owners hated this gun. Tight bore= increased pressure and recoil. Commercial manufactures load rounds to a happy medium so if one gun has a tight bore and one gun has a larger bore they both will be safe.

So start off lower and work your loads up. Use your head and dont trust anyones loads. Use them for refrence only.
Jay