To answer your questions, ChiCom 7.62X51 Ammo is NATO caliber China may or may not be in NATO what difference does it make? They do produce ammo for the M305. It is available when in stock from Canada Ammo for $449 for 1000 rounds including shipping. I just went through 1000 rounds and I want to buy more. The signs of incipient case head separation occur on the web just above the case head. You will sometimes see a bright ring around the brass. You can not depend on this sign to tell you if the case head is going to separate during firing. If it does you could be in very deep Kimchee my son. You must take a bent paper clip that you stick inside the shell casing and feel the case wall near the case head. You will feel a slight irregularity in the smoothness of the case wall at the web. I use a RCBS case master gage to detect this problem. If I detect incipient case head separation I toss the brass.
Sadlak pistons do just fine in a ChiCom Norenco M305 at least they do in mine as I have both the grooved and the non grooved piston. I do believe that all Norenco rifles are no more than rack grade. As for reloading I have been reloading for about 10 years now and I have tried IMR4064 ,IMR4831 and RE15. I seem to get tighter groups with RE-15 at 42.5 grains loading. The 168gr match king Sierra pills (bullets) seem to shoot the best out of my Norenco. I have also tried Burger 168 grain hunter bullets but they do not seem to do as well in my particular rifle. The M305 is very very load, powder, and bullet sensitive if you want accuracy. Normally a Rack Grade Springfield will shoot somewhere in the neighborhood of 2 to 8 inches or so which is all U can expect from a battle rifle. I got lucky with mine as it shoots into 2-3 inches at 100 meters with ChiCom ammo in the Sage EBR chassis. Oh and the last thing, U can purchase a Norenco Chinese made M305 from Marstars Canada for $599. Yes they have gone up considerably in price but also so has the quality when U compare them with the earlier offerings from Norenco. Norenco is a company in China that bought all the machine tools from Springfield armory when the US stopped issuing the M305 and went to the M16. The Norenco receivers are all drop forged, The US made M305 has a cast receivers.
The US Marine Corps and the US Army have found that the ranges encountered in Afghanistan and Iraq are just too far for the 5.56 round to be effective. The Sage EBR Enhanced Battle Rifle is issued to at least one man in a rifle company in the US Army as the designated marksman, The US Marine Corps does the same thing only they call their designated marksman, the designated sniper. The Sage EBR with the shorty 18 inch barrel is issued to the US Navy Seal teams.
The M305 is not a rifle for a beginner. There are many pitfalls and safety issues that can get you seriously injured or killed if U do not know what U are doing with an M305. My advise to anyone contemplating buying this weapon is to read everything U can on it.