I use the redding neck bushing die. You seat a bullet in the brass you use then measure the outside neck around the bullet then order a bushing 1,2 or .0003 thousands smaller than your measurment and the die and bushing will resize your brass that amount no more or no less. just for example if your loaded neck measured .249 and you order a bushing .248 it would shrink the brass a thousand less. and if they were still loose you would try a ..247 bushing in the die! You should go ahead and order three bushings under your measurment so you would have them.
heres a link on .243 imp dies. I ordered a sinclair international cataloge and got mine there.
http://www.midsouthshooterssupply.co...sku=0007638420