A number of years ago, when issue ammunition was still used for DCRA style competition, there was interest in setting up target rifles for 5.56, and some were made up on No. 4 actions, like Syxx15's pictured above. They tended to shoot very well. I suppose the catch is that the value of the other parts and labour is the same whether a salvaged No. 4 action or a custom target action is used. I believe it is mentionned in the surplusrifle article, but keep in mind that when a LE is breeched for .303, there is a gap of about 1/16" between the barrel face and the boltface. When a rifle is set up for a rimless cartridge, it is a good idea to eliminate this gap, so that the barrel and bolt are almost touching - no point in having more brass than necessary hanging out and exposed. For a 7.62x39 conversion, I wonder if it would be possible to use the original barrel: pick up the threads and turn and thread the whole shank, shorten from the breech, and lightly ream the existing chamber. Use a Savage style barrel nut to lock the barrel and set headspace. Haven't tried this, or even checked measurements, to know if is feasible. Spoiled no. 4s are so available and cheap that they are great for experimentation.