I am about to start some development work and was after some others experiences before starting out.

G'day Express;
You can make some absolute screamers out of those 55's in a .243!
You can leave a 220Swift in the dust in same weight bullet if you so desire, not the best way to treat your barrel though, sure warms-up fast! I did it once just to see, and that was enough! Got excellent accuracy but have since backed it down to 3400 to 3500fps range. Still sub-moa in a Ruger 1B.
Good luck, and have fun!
MM

I had real good luck with both the 55 grain Sierra's and the 58 grain Hornady's. I finally chose the Hornady's because my 10 group average was .62" over .71" for the Sierra's. I was loading them to about 3,600 fps which is relatively mild in my gun and the groups started to open up as I got toward 3,800 fps.

Seat the 55's DEEP in the case, to an OAL of 2.600" MAXIMUM.

If you try to seat these any longer accuracy will undoubtedly suffer. These light, fast varmint bullets need bearing surface of the case neck to "ease" them into the throat. Otherwise they will obturate (bulge) into the gap at the end of the cartridge case and chamber end. (Custom chambered rifles likely won't have this problem).

The 53 grain H380 load works well with the 55's in the .243. Nosler's recommended 52.5 grains of H414 is another great load.

But if it were me, I would use IMR 3031, at about 40.3 grains for plenty of velocity and better accuracy than the two above mentioned powders.

Dan