I have the first version of the MM LW barrel ( no mention of hammer forged at that time) on my .22 Mrod. It shoots around 1-1.5 MOA with JSB 18.1g doing 865fps.
I bought an NSA 0.217" sample pack, cranked up the power and tried all four weights. The 19g doing 840fps (JSB @ 920fps)) was about 1" groups at 50 yards, more than adequate for pigeon busting at 50ish yards. The heavier weights were closer to 2" groups. The slugs dropped the pigeons much more convincingly than the JSB.