With this in mind, what do you find to be the most important part of the reloading process to achieve accuracy out of an AR?
I have an AR-10 chambered in 6.5 Creedmore. It is the Super Sniper from Wilson Combat and I just installed a fresh 24" barrel. I am struggling to get the consistency I am looking for. For example, I did some testing with two brass manufacturers (Lapua and Alpha Munitions). I was testing 5 different bullet seat depths for each case type in groups of 5. All the brass was new and this was the first fire. I shoot the Laupua 136gr Scenar-L with 38.5gr N550 powder and BR4 primers.
In a couple loads, i was able to get a decent group at about .9 MOA but for the most part, groups were not real consistent. I put this down, in part, to being first-fires and hope that once i reload them with a headspace more compatible with my chamber, the groups will become more consistent.
A part of me wants to get consistency compatible with a Bolt gun but i know that is wishful thinking.
What do you find to be most important for consistency in an AR platform... Bullet seat depth, setting the correct shoulder, or something else?
I don't think there is a magic bullet, no pun intended. The first step to me is using quality components, particularly the bullet choice. Some barrels quite simply like certain bullets more. One of my ARs loves 69gr Sierra Matchkings for example. Consequently that is the only thing I run through that gun now. I would think this is probably the top factor overall. While you might like X type of Berger, your gun might like Y type and weight of Berger instead.
Other than that selecting the right powder matters. While X variety of powder might be an overall excellent choice, there might be other stuff that works better with the specific combo you are using.
Building that information up is not easy as sometimes it means buying 1lb of 10 different powders. If you read carefully and set up criteria of what you want to achieve, and look at how certain powders act with certain bullet weights it can help narrow down choices a lot. IE if you see one powder that works really good in other people's/companies tests with light bullets, but it works only so so with heavy bullets, generally speaking don't think you will magically get a different result for the grain weight of bullet that you shoot.
I would also say do not chase velocity. Yes you need enough to make things work, but depending on what you are trying to accomplish the internet would have you think that 'more throttle' will fix everything. Do not be afraid to back off of loads.
If you are starting from scratch it's one thing, but for example with my 69gr SMKs, I know they shoot well in that barrel. If I want to test another powder for that gun I keep things consistent and see how other powders work with that bullet. That said I still work up loads with brand new things from time to time. With that the key is being able to notice when you are on to something and when you are not. Having an efficient testing method is important.
If you are not using expanding mandrels then start. This made a big difference in being able to control my SD/ES.
Also one thing that I proved to myself is that the shooter matters. I bought a new Labradar a couple of years ago and I ran tests on loads that I previously developed to see 'really' where I was at. As part of that test I would intentionally square up in a proper prone position and I would control the pressure I was placing on the stock with my shoulder.
In other tests with the exact same load I kind of did a 'free recoil'. IE didn't put very much pressure on the stock and kind of let it do it's thing (to a point). The end result was that I could see a pattern of how that impacted SD and extreme spread. Simply put I was able to induce it one way or another based on what I was doing behind the gun. That is not to say some loads are not better than others, but how are you going to know if it's you or if it's your load? So in other words if a person shot Erik Cortina's single digit SD loads, out of his gun, on a perfect day, that shooter can induce a whole lot more SD/ES based on how they shoot.
Basically the totality of the results of the system is what matters. If you know you have good brass, then move on to something else. Trying to compare Lapua to Peterson probably isn't going to give you the improvements you are after, at least in the context of the overall reloading process.
Final note is knowing what is good enough based on what kind of shooting you are doing. Trying to get .1 something groups isn't always the best use of time and resources. That said if you get something that consistently shoots .5 MOA or whatever, and it satisfies your shooting discipline then just roll with it.