If you're wanting a good varmit bullet for the .243 go to a 70-75g bullet, otherwise stick to the .223. I've shot quite a few 55g v-maxes and Nosler Ballistic tips out of my .243 Win Rem 700 VLS and the wind seems to slap them around for shots further than 300yds anyway.
Velocity loss is quite a bit when dealing with a shorter bbl as the others have already stated. You can actually load a 55g bullet to 4000fps out of a 26"bbl, but I'd dare say that you're probably around 3100-3150fps with 42g of Varget, you can get it to 3350fps with 45g if it still will shoot....
H4895 will get them to 3500fps with 44.5g out of your rifle, if you are willing to shoot that much powder.
Hope this helps,
Dan
P.S. Hodgdon.com has data for the .243 Win for pistol loads, look them up. They are usually figured for a 16" or less bbl....