Jerseyboy,
Dunno how old your .243 ammo is, but if its listed at 3070 fps its REALLY old, since that was the original factory standard WAY back when--and was chronographed in a 26-inch test barrel. The present-day industry standard for 100-grain bullets in the .243 is (if I recall correctly) 2960 fps from a 24-inch barrel.
Plus, quite a few of the powders used back then deteriorated relatively quickly when stored in uncontrolled conditions. Which is why I wouldn't judge ammo made 40-50 years ago when fired today.
The 6th Speer manual was published in 1964, and included the chronograph results of a bunch of factory loads fired in more typical barrel lengths than the industry-standard 26-inch of those days. The Winchester 100-grain factory load got 2893 fps from the 22-inch barrel of a Winchester 88 lever-action, which is right in the range I'd expect from a 4-inch shorter barrel.
