Not an ARCA rail, but I put an Area 419 picatinny rail- 5 inch- on my LH 457 varmint last week. The ARCA is wider isn't it? I haven't used one. Anyway the picatinny fit fine side to side but I had a small issue lengthwise. At least on my stock there is a slight outward bulge, so the rail will not touch the wood along its full length. I tightened it down such that there is about a one millimeter gap between the rail and the wood at both mounting holes and the rail is on the wood about the center three inches. Seems stable so far.