Twist ratio is important. The lighter grain shells use a tighter twist to increase spin in order to make them more accurate. A heavier grain is likely to break apart at higher rate twists, hence the 1:7 and 1:8. Mine (Ruger AR556) is designed to use 62 grain, 1:8 twist. I use 55g a lot and at fifty yards not much difference. At a hundred the 55g will rise a little more due to the lighter weight. I would guess it would be more accurate with a 1:9 on the 55g. Faster spin, more level flight.
I also have one of the .22 Ar's (S&W M&P). Same with it. 1:15 twist on it. The 40g works best versus the 36/38/42.
BTW, the Ruger can be bought new for $549. Sometimes a little less. Grabagun ran them a few months ago for $519. That's a complete rifle with flip up sights and a warranty. I've had a bunch of custom builts (actually one with a Palmetto lower) and factory rifles. Sold off all but the Rugers after I bought the first one.
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