I owned both 1946 and 1951 M-39a's, at the same time - and shot them side-by-side with the same ammo at my club's range.
The older gun had it's issue open/iron sights, while I had mounted a Williams peep on the '51.
Except for their sights, and the -51's brass initial plate & gripcap - both rifles looked almost identical, with their 24" ballard-rifles barrels and PG stocks.
With the same ammo, at the same distances, the older rifle consistantly shot groups about half the size of what the newer rifle could - about 1/2"-3/4" @ 50yds for the '46, to 1" - 1 1/4" for the '51.
The ammo used was several different brands of both standard & high velocity LR, standard velocity Long, standard velocity Shorts, and some hyper-velocity LR, too.
The best groups came with standard velocity LR's, with Shorts not far behind.
I've never owned a Microgroove Model 39a.
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