Size Confusion With Different Brands

I'm getting frustrated trying to find best men leather jacket because sizing seems inconsistent across different brands. What fits as a medium in one brand feels like a large in another, and it's making online shopping nearly impossible.

I've tried several jackets from various retailers, and the measurements don't seem to match their size charts. Some run small, others large, and the cut varies significantly even within the same nominal size. It's particularly confusing with leather since you can't just throw it in the washing machine and expect it to adjust like cotton clothing.

Is this normal for leather jackets, or am I missing something about how they should fit? I'm used to pretty consistent sizing with other clothing categories. With leather being an investment purchase, I really want to get it right the first time rather than dealing with returns and exchanges.

I'm 5'10", 175 pounds with an athletic build - not unusual proportions that should be hard to fit. Yet I've had jackets labeled medium that were too tight across the shoulders and others that were swimming on me.

Should I be focusing on specific measurements rather than letter sizes? Any brands known for consistent, true-to-size fitting? Would really appreciate advice from others who've navigated this sizing maze successfully.