I haven't loaded a lot for the .243, but I'll soon be working up some loads for a Stevens that I aquired from a pistol match.
My previous experience with the .243 is that it truly can be picky. Especially regarding brass, as I seen some considerable fluctuations in case thickness, and neck thickness. (ie: case capacity too)
I suggest you refer back to your data and look at such things as case selection of the source.
If they used Winchester brass, that is one clue, as it and Federal tend to be lighter and hold more powder, and hence lower pressures with a specific load.
Also, primers can make a HUGE difference in small bore, large capacity cases such as the .243.
Also, case length will matter with your T/C. My experience with T/C contenders is that they have tight chambers, and tight throats, and short throats. NONE OF THESE ARE BAD !!!! It simply indicates that T/C is building their firearms to very tight standards. They make excellent firearms.
Mine and my brothers experience indicate that IMR-4831, is perhaps the best powder for the .243. Just an instance of a particulary good match of propellent to cartridge.
With length trimmed and neck reamed cases, he found that IMR-4831 delivered as much as 150fps faster than factory standard velocities with similar pressures as either of the 4350's or H4831. I'm told or lead to believe that AA3100 is very, very similar to IMR4831, so should be an excellent alternative.
However, I got superlative accuracy from a Rem. 788 with 80gr bullets (Hornady and Speer) and H4895 back in the mid '70s. No idea about velocity, as chrono's weren't widely available back then, but suspect that it was relatively low, probably 3,100fps or so.
But the many crow's and several deer I shot with this load never complained !!!!!