If you are only planning to shoot at 100 yards, then you’re prob right - it doesn’t matter. Then again, it would be an unconscionable waste of that platform. I’m not assuming that’s the case with you as I have no idea of your shooting patterns.
Otherwise, gathering MV with a chrono will tell you (or enable you to calculate) the two things you need to know to determine how consistent you load really is: SD and ES. It will tell you if your load adjustments (charge weight increases or decreases, seating depth adjustments, primer swaps, etc) were the right adjustments to make (did the SD/ES go up or down with those changes and to what extent?)...bullet holes by themselves at 100 yards can’t give you that information. Anyone can shoot a decent group at 100, unless they aren’t trying or there is something wrong with their ammo, scope or rifle itself. But if you want to know if that load is likely or not to hold a tight vertical dispersion at say, 800, you will need MV data to see how consistent the load is coming out of the muzzle, round over round.
If I do my job as a hand loader, any load I create should be such that I can grab a random round from that load’s production lot and, when I shoot it, it will perform just like any other randomly selected round from the same load / prod lot in similar conditions across all of those rounds. If they are consistent, my probability of making a first round hit at distance is much higher than not, assuming I do my part. You won’t know either way how consistent your ammo is unless you measure for MV.
Plus, I’d want to know what impact different AOs and conditions have on my velocity so I can adjust and still hit the target in as little rounds as possible. If you never go anywhere else with the rifle and live in an area with minimal environmental changes throughout the year (San Diego), this aspect is irrelevant.
Lastly, I can track how the system is performing over time (not as critical with a. 308 but important with magnums and other cartridges with short barrel lives). For example, I took my Mk13 out one day in Feb (about 100 rounds down the barrel from when it was new at that time) and discovered my muzzle velocity for my load increased by 90 fps relative to what it was when brand new when I initially developed its 190g smk-based load. I hadnt taken a MV reading in a while so elected to do so. This was done during zero confirmation with Labradar. That new information changed my entire dope card. I usually shoot that rifle at targets starting at 800 out to 1500 at roughly MOA sized pieces of steel. Without labradar I wouldn’t have realized this and wondered why I’m air-mailing my targets. Who knows how many rounds I may have fired before I got on target (don’t want to needlessly consume barrel life, esp with a rifle with a projected barrel life between 1200-1500 rounds). Just one example of personal experience arguing for the collection of MV data that comes to mind. There is a reason why you see so many bench rest shooters with chronos - do you think they are competing for tightest group at 100? Watch any of Frank’s (a world class shooter) YouTube vids on the snipers hide YouTube channel. Chances are he has a lab radar set up if the video involves shooting and testing a rifle or ammo. Why do you think he would feel the need to collect MV data?
To me, rifles like the M40A1 are meant to be shot at long range (long range starts at 600 yards in my mind). If you don’t know how consistent your load is using MV as a means of measuring consistency, it will take you longer and cost more in time and money spent to get there all things equal.
Who knows, you may get lucky on your first try or someone else’s pet load will work great in your gun. I sincerely hope to you do. I’m not that lucky (wish I was)....My lab radar was $600.00 about 4 years ago. That thing has more then paid for itself since.
