Harry: You can certainly shoot GC designs without the gascheck but I've found that the load needs to be quite mild and even then, accuracy is usually less. JAYB and I loaded some rounds for his BRNO 243 and they shot tiny, tiny groups at very mild speeds. I think that off the hood (bonnet to you) of his Land Rover we were shooting 3/4" at 80 yards. His bullets were most definitely gaschecked.
I shoot a lot of cast bullets. I mean, a LOT. I have over 100 bullet molds of all sizes from 5mm to .72 Caliber and I seem to keep acquiring them. Long ago, when I got sick and tired of BR shooting technocracy, and was at a point where I could almost intuitively know what load would shoot out of a given rifle, I started looking for something else to do. An old timer in Las Lunas, New Mexico got me started on cast rifle bullets.(I had been casting for handguns for sometime) I loved it. My initial groups were just mediocre but it was MY mediocrity! I cast the bullets, I sized them, I fit them to my chamber and the results were more on my shoulders than anyone else's. Suddenly, more than at any time in my shooting career, I became the variable in accuracy. I worked very hard at this casting thing and ended up winning some cast bullet BR matches as well as branching out to odd ball calibers and obscure bore sizes. So it's been 30 years and the best bit of advice I can give someone starting out is this:Cast bullet shooting is a Discipline, not just an option.
To do it correctly takes preparation and planning. If you are sure you want to shoot cast bullets in your 243 I am willing to help. It's not hard but there are details to attend to. Let me know.~Muir