I read the Michigan regs found here:
https://www.michigan.gov/-/media/Project/Websites/dnr/Documents/Orders/Wildlife-Conservation-Order/ChapterII.pdf?rev=3c56ee93d9914dafbdfd7faf9c3f9bdd
Based on my read the .375 Win is legal and there is no mention of “taper”, just “straight wall”. That’s not the same as “parallel wall”. All the classic straight wall black powder cartridges like the .38-55 had a slight taper to facilitate ejection.
Given it is based on the 38-55, the .375 Win is no different, tapering from .420” at the base of the case above the rim to .400” at the case mouth. The .38-55 tapers a bit more from .419” to .392”, but the big difference for the purposes of the MI law is that the current case length on the modern .38-55 is 2.085” (and 2.125” for the old .38-55) while the .375 Win has a case length of 1.80”, just like the .460 S&W.
But here’s the kicker, the .350 Legend also tapers, from .390” to .378”, so it’s straight walled and also slightly tapered.
The .375 Win is legal, even if not specifically listed. If MI DNR is saying differently it’s wrong.
Technically, given how the law is written and based on cartridge case length:
“(d) A .35 caliber or larger rifle loaded with straight-walled cartridges with a minimum case length of 1.16 inches and a maximum case length of 1.80 inches.”
You could also trim a 2.105” .45-70 case back to 1.8” and it’s now legal. Hornady already uses short cases in its .45-70 Leverevolution ammo. Go a little shorter and you are good to go.
Bring a set of calipers to measure the case, if an officer questions you, and if he still cites you, insist he either take the cartridge as evidence or sign the case for identification later. You’re using a wildcat that’s legal. Call it the “.45-70 Michigan”.
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Idiots however seem to run amok in Indiana where their allowable rifle cartridges are specified in a list and based on .243” and .308” bullet diameter, eliminating anything that isn’t one or the other of those diameters,
I can’t help but think a hyphen word “to” got lost in the writing of the law.