IME/IMO, the M14 drops from many capable of .5-.8 MOA to 1-1.5 MOA at rounds 4 and/or 5 in the group. I rebarrelled mine when a 2nd hard-found load opened up to 1.25-1.5 MOA all the time.
Both the prior barrel and this one will drop 2-3 rounds almost on top of each other out to 600. My spot that sorta goes beyond 500 is just too hard to set up targets with the time I've had available the past two years to really test beyond that.
Just measured the targets from the last zero check less than two weeks ago. The CDB and shot 2 at 200 yards were 1.6 inches with #3 noticeably low and centered below the 1st two shots. The WDB 3 shots at 300 grouped 1.63 inches. The two shots at 400 were 1.4 inches apart but .23 mil high. Was hotter that day than any prior serious zero check session I have done with this thing scoped on an SAI 3rd-Gen mount (two bolts, the rear one in their stupid modified replacement for the strip clip guide. The Gen I (?it's completely unmarked) has the rear bolt 1/4-inch or so further to the rear which enables the standard strip clip guide to just be drilled and tapped...).
Both were medium-heavy barrels. For scoped use and bipod I use a USGI fiberglass stock with two lengths of steel bedded and screwed into the forearm for stiffness. Don't know if it helps but it makes me feel better. With scope, sling, cleaning kit and empty mag it's two ounces shy of 16 pounds. (!)
So with forbidden 3-round groups, this one just did .77 MOA at 200 and .53 MOA at 300. The 2-round not really 'nuff to be called a group was .34 MOA.
The loads were at least a year old, 42.5 4064 under a Hornady 178-gr A-Max, CCI primers and I don't remember which ones were the FC cases (full prep and weight-sorted) or Lapua (on their last legs). Don't remember which, if any, were annealed before this loading...maybe only the Lapuas.
Oh, my unitized gas system broke apart but the shim(s) and proper nut timing seem to be holding things together just fine. Might get it re-welded the next month or two...
Now, the highpower bullseye target is about 2 MOA at 300 and 600, so for the prone stages we tend to be minimally satisfied with 1.5 MOA performance on 5- and 10-round strings. Military testing showed that 10-round groups go something like 1.2 times the size of 5-round groups, so a 1.5 gun on 5 rounds tends to be 1.8 MOA. That leaves only .1 MOA "wobble area" to clean a target. But since the matches tend to be won with the points from standing scores, IME shooter skill there is more important than losing 1 or 2 points at 300 or 600.
Now this last set of data points with this rifle was only 8 shots, with probably another 20 fired between the 300 and 400 zero checks. My plan is to do 10 rounds at 100 two times over two different sessions, using separate aimpoints, so I can try to isolate any barrel warming/dispersion patterns.
BTW, do any of those M14 chassis systems leave the barrel free-floating (within the limits of an attached gas cylinder, of course)?