Are you shooting a recurve or compound? This whole high foc or 650 grain arrow is a bunch of bs in my opinion when applied to the common archer using modern equipment. I am not trying to say Ashby Method has no merit but let’s unpack this a bit. Most of his game animals were in Africa using a recure bow from my understanding.
When it comes to archery here is my two cents. First off the comments and people saying just do this or use this tip because it works for me is a huge mistake. Many times I meet hunters in camp who I have guided or folks at the archery ranges and they have no clue how their equipment preforms for their situation. Its all anecdotal like my buddy said if i shot a 400 gr arrow with this broadhead I would be fine because he has killed an elk before.... I use these statements to offer up some perspective and also give people who read this an approach to find their own set up that works for them and demonstrate how I approach my archery set up. I will also add some rebuttal's to the question at hand here regarding the ultra heavy arrow questions... as well as FOC.
as far as my set up goes I will list it here for reference.
New set up
Hoyt rx7 75# limbs 27.5" draw
472 Grain arrow (estimated foc 17%, I shoot a victory rip with 4 fletched aae max stealth vane and a wrap, insert is an easton 75gr half out)
Current speed is 281 FPS
Since this is an elk forum, I will speak in context of elk. If we were hunting in Africa or strictly antelope/whitetail I would also say your approach might be a little different. Another point to note is I'm also speaking about the average male. I am on the short side when it comes to draw length so I have to take into account the fact that I don't get the most efficiency out of a compound. My options are limited... I cant just shoot a 5550gr arrow and expect it to preform as well as someone with a 30" draw who may have said it works for them... Women and kids etc also have to use a slightly different approach. What this all boils down to is finding that happy balance between speed and weight of an arrow. When balancing these two we need to understand that there is a law of diminishing returns when we add too much weight or we increase speed by decreasing weight. The end goal here is maximizing your Kinetic Energy and Momentum of your arrow. By doing this you ensuring your arrow will ethically kill an elk and achieve maximum penetration. When building my arrow/bow set up it all starts here. Things like FOC, broadhead choice etc will all come later. At this stage I am determining my optimal performance. I encourage you to play around with this easy tool, its not perfect but will help you understand the balance of speed and weight. ( note; as a short draw I have had to overcome this with the use of a turbo cam in past or in my current set up a 75# limb)
This Realtree calculator calculates your kinetic energy and momentum for you.
www.realtree.com
Here is the estimate on my set up..
Kinetic Energy: 83.92 FOOT-POUNDS
Momentum: 0.593 SLUGS
I have found this to be my sweet spot given my draw length and the speeds I could realistically achieve with my draw length. Sure I could improve this with a 500gr arrow going 300fps but that is just not going to happen for me at 27.5" draw. On the other hand when I play around with my set ups given my draw length I find that increasing or decreasing arrow weight actually decreases my KE and Momentum. In my tests I tried a 572gr arrow and my speed actually dropped all the way down to 248fps. Same goes with trying to achieve 300fps. I would have to drop my arrow weight all the way down to about 412gr. The performance was below for the heavier set up
Kinetic Energy: 78.1 FOOT-POUNDS
Momentum: 0.63 SLUGS
so in this case we can see I gained slight improvement in momentum but also lost KE. For me this does not justify going to an ultra heavy set up. Furthermore we must take these considerations a step forward. Just evaluating on these numbers wont suffice. Before I talk about shootability I will also briefly mention we should discuss KE and Momentum at extended ranges. I use the Archers Advantage program with my sight tapes and downrange performance calculations. talking in context with elk... I like to be above 55ftlbs of KE when discussing lethality. When looking at these and applying a range to them my current set up ; I am "lethal" out to an extended range of 95 yards. Would I take that shot, no but It is good to know that inside 50 yards I have plenty of retained energy. Using an ultra heavy arrow, I found that I dumped speed at an alarming rate. When looking at a range outside 40 yards my KE dropped so fast that I would not consider a shot beyond 40 with a 600gr plus arrow. So my point here is know your set up and know your effective ranges.
Now lets talk about shootability. I love being in that 270-280 range fps. To me it allows a lot of flexibility to the shooter. One my trajectory is such that I can shoot different yardages and have a degree of error. Lets say the elk is at 35 and I shoot my 30 pin... I am still lethal. I find with ultra heavy arrows one mistake in a range could cost you an animal with a complete miss or a wound. two years ago I ranged a cow at 35 yards but she moved off a bit during my shot sequence. I ended up shooting her at 47 yards. Because of my speed my arrow was still lethal based on me shooting my 40 pin. I shoot a lot of 3D here in Colorado and I love practicing longer shots, so for me I would argue having and arrow with a flatter trajectory and good energy is the perfect balance.
In terms of arrow build I flirt on the high FOC side at 17% along with a 4 fletch set up. I do this because I am shooting an Iron Will s125. For me I love this broadhead and have had great performance. Using the higher FOC and 4 fletch I do believe it controls my broadhead better but I must add i still debate to myself is it the FOC/fletching combo or just a placebo effect that I shoot better. Who knows but I do know my groups are better and my arrow durability has been good with the half out. Another thing to note is I do notice some flight issues due to the FOC at longer ranges. Couple that with my 4 fletch the arrow past 70 yards doesn't want to fly according to my sight tape. Inside 70 my 281fps sight tape is dead on but after 70 my arrow corresponds to a 279fps better. I attribute this to parachuting effect and the FOC causing arrow to dive a little.
I know this was extremely long so I will wrap up my thoughts. This is how I approach it and it is just my opinion. There are plenty of other ways that work too. I like the blend of speed, shootability and leathality. With an ultra heavy arrow or FOC i do no think you can achieve all these. In terms of my set up I also recognize its a bit over kill. I like this because I will take this same set up on my moose hunt in 2023 to Alberta and in addition, if i do mess up on a shot I have first hand experience in what my arrow can do. All in all, i encourage you to play with your set up and find what works for you and what you like to shoot. I think i have taken a few of the principles form the ashby method and applied them to a more modern approach balancing speed and arrow weight.