Since I didn't see anybody address
You will see a lot of differences from people as to how much neck tension it takes to hold a round in an AR/15 case without slippage. I firmly believe after many tens of thousands loaded and fired - and with many different bullet weights, coated and non-coated bullets, etc., it is not real straight forward. First, a coated bullet (like moly or HIBN) will take more tension than a bare bullet (usually .001" more, minimum). Short bullets which do not fill the entire neck of the brass (like a 32-grain .223 bullet) will require a bit more than a long bullet seated deep enough to make contact with the entire neck (like a 73-grain bullet). The larger the bullet diameter, typically a bit less tension can be used. For example, I use less tension in my 6.5 Grendel than I do in my .223. I have also found that in my .20 Practical, a coated bullet will slip a little no matter how much tension you place on it - even to the point your neck peels like a banana trying to get the bullet in. Funny - those will still shoot pretty good - but to completely stop slippage - I don't use moly in my .20 because you simply cant get enough effective tension on it without crimping - which I still won't do. For "most" .223's, shooting non-coated bullets weighing 32-50 grains, a bushing .005" smaller than a loaded round, measured at the neck closest to the mouth as can be measured will usually do fine. For a 55+ grain bullet, I'd go .004". If I were using coatings on the bullets, I'd go .001" smaller. Keep in mind that when one sizes down, say, .005", there will be around .001" of "spring-back, resulting in .004" of true tension. If you are loading with work-hardened brass that has had, say, 8 firings and not been annealed, you will encounter more spring back than .001" - and it can take days or even weeks to "open up" that .002" or so. So- there are a number of variables that will change the advice quite quickly - but oftentimes folks don't mention whether their brass was just annealed, using bare bullets, and of what weight - or caliber. Then - there are non-leaded bullets which are longer than those with lead cores. You can get by with less tension on a 50 Varmint Grenade than a 50 V-Max. I think you get the idea.