Don't just look at the opening in the hide. When an arrow cuts the hide the tension around the wound pulls and distorts what would preserve the shape of the penetrating object. Peel the hide away so you can clearly see the thin muscle over the rib cage. This tissue preserves the shape of the projectile much better. Any firearm on entry will leave a roundish hole there. An arrow won't leave a hole at all. What you will see are the result of the blade edges as they pass through. It will look like a cross on a 4 plade a simple slit on a 2 blade but it won't be a hole like a firearm. The hole that results from the firearm, again on entry, will be related to the caliber. The large the caliber the larger the hole. There will also be a more noticeable "bruise" around the point of entry than with an arrow where there is very little bruising. Generally you would expect higher velocity rounds to bruise more given the same caliber.

Muzzleloaders are typically higher caliber than rifle rounds so pretty much anything close to 1/2 inch in diameter wouldn't likely come from a modern rifle. Over 1/2 inch and it's likely a shot gun slug. A 12 guage slug (non saboted) is nearly 3/4 inch in diameter when it leaves the gun. Saboted are typically close to 1/2 inch so telling the difference between a 50 cal muzzle loader entry wound and a saboted shotgun slug wouldn't be easy.