Ah .... US respondents have (understandably) failed to grasp the OP's real issue here - which is all about primary extraction and hard extraction in manual versions of a rifle mechanism designed around semi-auto operation. While a conventional turn-bolt action incorporates a primary extraction cam to break the case-chamber seal as the bolt-handle is lifted, the AR mechanism entirely lacks this feature. In normal operation, gas operation gives a really hard and fast kick to the bolt which wrenches the fired case out of the chamber. Do the same job by hand using a modestly proportioned side handle and you can see the bolt stick pretty well closed fast if you have a tight case. This partly depends on the nature of the case's brass alloy, but mostly on pressures generated by the load.
What Nathan wants to know is which set of data he should use bearing in mind that while Brit straight-pull ARs are technically 'bolt-action rifles', they're not so far as extraction goes in the same camp as a Remy 700 etc. He should have asked this question on a British forum, there being plenty of straight-pull savvy members on UKVarminting.co.uk for instance, Mark Bradley and Dave Wylde both of whom build straight-pull ARs professionally as well as using them in Service Rifle, Practical Rifle and McQueens competitions.
In general terms, some versions of the NATO / US M855 62gn milspec factory round work fine in these rifles, while others provide problems. I found Brazilian CBC Magtech M855 worked excellently, but most UK manufactured RG was right on the margins with some batches being really 'tight' in the chamber - not that I ever had much desire to use RG ammo in a well-built rifle with a good match quality barrel. Since NATO ammo is loaded at around 60-61,000 psi, you can see that the lower figure is the maximum, or a tad above, that which the straight-pull user should be aiming for.
I mostly loaded 80s in my straight-pull AR shooting days, but I obtained very good results and easy extraction with 23-24.7gn Viht N135 and the same charge weight range of Hodgdon H4895 with 69s. Referring those back to the Sierra manual which lists H4895 with the 69gn SMK shows I ended up running just around / above the max load listed for the AR and maybe half a grain down on the 'bolt-action' max load. However, this was 9 years ago and I was using the then standard production Lapua case - very thick-walled and with significantly lower capacity than today's Lapua Match 223 case or Winchester models. They (the old Lapua) would produce higher pressures with a given charge and produce sticky extraction at an earlier stage than current Lapua or most US manufactured brass. (I'd also say don't use RG brass, although some do. Also, HPS Target Rifle Ltd's German MEN Brass is very high quality, but is also thick-walled and hence lower capacity needing reduced - by ~0.5gn - loads.)
Running my actual load combinations through QuickLOAD showed my finalised match load pressure figures were in the 56,000-58,000 psi range and that's what I would work towards whenever trying a new combination.
In summary, use the AR data initially starting not much (1.0gn) below the listed maximum for whichever powder you're using, and work up in small (0.2gn) steps. You'll probably find you can go a little higher than that while retaining easy extraction - but do what the rifle tells you both in group size and bolt operation. The other factor is of course the chamber - how well it's been machined, and how well polished. A 'rough' chamber will cause problems at MUCH lower pressures than a really good one. You also need to bear primer piercing in mind - many AR match trigger / hammer assemblies see seriously cratered and even pierced primers at below absolute max chamber pressures. We don't get the hard cup milspec primers dveloped for semi-autos in the UK, so use SR magnum or BR models. I used the Remington 7 1/2 BR model for years with good results.
