On most factory rimfire rifles the Picatinny rail is flat — 0 MOA of cant — so a centered turret points roughly parallel to the bore. That is fine for plinking and a dead end at distance. Rimfire drops so hard that without help you will run out of clicks long before the far gong.
MOA and mil are just angles
Both describe an angle, not a distance; they only become inches when you name a range. 1 MOA is 1/60th of a degree; 1 mil is 1/1000th of a radian. At 100 yards, 1 mil ≈ 3.6 inches. Pick the unit your brain likes and commit — matching the rail's units to the scope is a convenience, not a requirement.
What a canted rail actually does
A canted rail tilts the whole scope down relative to the bore. Re-zero on that tilt and you have effectively pushed your available travel up the dial: a 20 MOA rail buys ~20 MOA of headroom, a 30 MOA rail ~30. The bullet drops the same amount — you just have the dial to chase it.
How much cant for Long Gong?
For 300 yards on steel, 30 MOA is the sweet spot. A 20 MOA rail can reach 300 but leaves you at the ceiling of the dial, where tracking is least reliable and clicks may not match the math. A 40 MOA rail gives breathing room up high but can push your 50-yard zero toward the bottom of the dial. 30 splits the difference for standard-velocity match ammo (CCI SV, SK Standard Plus, Center-X, Eley Match) on a scope with ~60 MOA of internal elevation.
Why a 34/35 mm tube gives you more elevation than a 30 mm
The rail is half the story; tube diameter is the other. Inside the main tube floats the erector assembly that carries the reticle; the turrets shove it around and springs hold it loaded. How far it can swing before it bottoms against the wall is your total adjustment range. A bigger inside diameter gives the erector more room. Going 30 mm → 34 mm adds a few millimeters of internal clearance, which translates to meaningfully more up-travel. A 34 mm scope with 100 MOA total gives ~50 MOA up from center; on a 30 MOA rail that is ~80 MOA effective — more than enough for 400+ while staying in the linear middle of the dial. One-inch tubes belong on short-range plinkers.
Practical pick
For a dedicated 300-yard rimfire build, a 34/35 mm tube on a fixed 30 MOA rail is the confident answer. A quality 30 mm scope absolutely works — you will just live closer to the ceiling with less margin for ammo or weather swings. Buy a good rail (Area 419, EGW, Talley, Vortex Pro, Spuhr), torque it to spec, and stop thinking about it.
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