Browning's comment is only for their factory chokes and fixed choke barrels.
The aftermarket chokes have different recommendations. If 40yds is your maximum distance, a LM/Mod choke is all you need, and yes with steel add 2 choke designations for steel:
I/C = Mod
Mod = Full
So based on that, a L/M would be like a I/M and the Mod would be equal to a Full when using steel.
A 3" shell will do all you need and the money saved per case will pay for a full set of chokes in a couple of seasons.
I have several Terror chokes, in .670", .675" and .680" and they are tight patterning chokes, these will only be used during conservation season. I also have another favorite, the Briley EXR Extreme Range choke, it is about a I/C constriction, but what it has is straight rifling. It throws a very tight pattern and acts like a wad retarding choke with out the studs, I only use it on extremely windy days that the ducks skirt the decoys or just refuse to commit.
If you are shooting a Browning Maxus Invector + gun(the 3" gun I used last season to harvest 177birds), a L/M would be a good start, but if you are using #2 steel I would suggest you drop to a #3 load in a 3" hull.
I would gather you are using a 1 3/8oz load of #2 steel, if so, you have approximately 171 pellets while a #3 1 1/4oz load will have approximately 191 pellets with lower recoil: 1550fps vs 1425/1450fps. They will have about the same velocity and energy at 40/45yds, but with the 1450fps loads you get an easier to pattern load with lower perceived recoil and better pattern density.
The Kent 3 1/2" load is about $199 a case while my favorite 3" load is only $140, that is 2 extended chokes right there.
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