In (Across the Course) Service Rifle, we shoot both mag length for rapids, and single feed for the 600 yards stage slow fire prone. Since that 600 yard line is slow fire, and we single feed, we can load 80 grain VLD designs that wouldn’t fit in the magazine.
Those exercises teach us two things. That properly reamed chambers do pretty well with the typical 69 to 77 grain magazine length ammo, but.... We definitely benefit from testing those seating depths on the single feed 80 grainers too.
I ran some factory loads with 77 TMK at the 600 yard line when I was traveling and didn’t have time to load. Do I give up some performance and windage? Yes. But not as much that I would give up and stay home.
Go try a seating depth development and you will see if that makes any difference to your goals. We can usually see two nodes on those 80 grain tests in an AR Service Rifle. If we start at jam and work down, the first node is usually within the first 5 to 15 mils, followed by another about 30 to 40 mils down. Sometimes being in those nodes makes the group really shrink, sometimes they are just enough to make you worry less about being perfected. When comparing the group size of an 80 grain load at the optimum load versus the factory or magazine length load, you will definitely see a difference. In general terms, the difference is in the X count. We are talking about a group size that would run maybe 0.5 to 0.75 MOA optimized, versus 1 to 1.25 if not. Calling the wind correctly is a much bigger effect and even a well centered 1.25 MOA group size would easily put you in High Master class.
So the bottom line is you should go test if you really want to know the impact. Even different bullet styles would require you to re run the seating depth development, so there is no way to predict how big a difference this will make.
