What kind of groups at 100 yards are sufficient for deer hunting?

Well the deer has around a 6" kill zone so 6", but every one is different. I like to get my groups as tight as possible, and usually don't settle for anything over 2". But you also need to be confident in the load you are using also.

I would agree 2.5MOA accuracy is whats required for a 100 yards shot. Reason I say that is one, you want good shot placement with a ML. Blood trails can be spotty for the first 50 yards. And in a hunting situation, freehand, against a tree, using your treestand as support, thats going to cause you possible further error. So a little margin sure won't hurt. So yes a deer has 6" kill range, but that doesn't mean your going to be shooting off the bench at a deer sandbags and all.

What are you comfortable with? After you have the best group possible then try some free hand shooting and some off shooting stick shooting. It will really change your idea of shooting at 100 yards.

I know a lot of good shots. Stick them at a bench rest and they can really shoot some amazing groups. Take that bench rest away, and they fall apart. I watched a friend once miss a deer that was standing broadside at under 35 yards. After he shot and it ran off we checked and there was no blood or indication that he even hit. He then admitted he was so shaken at seeing a deer standing like that he could not concentrate. As I watched him shooting, he was shaking so bad I had to laugh. I even wondered whether he would hit the deer.

With a scoped rifle I like at least a 2.5 inch group. With open sights I insist on a 4" or better off a bench rest. After you can do that then start shooting the way you would if in the field. If you can hit a 6" circle every time that is the distance you can shoot from. Some people have real problems at 100 yards. It's nothing to be ashamed of. It is just that they do not practice.
They are the ones that have a deer come on at relative easy ranges, and they miss. Their comment is, I make shots like that at the range all the time. The truth is they do not make shots like that all the time at the range. No bench rest there, no calm day, no stress.

Sight your rifle in. Get the best groups. Then practice. After you can make hunting type shots, practice some more.....

A "group" is not really an issue when deer hunting with a muzzleloader since you will only have one shot. If you can sit down and put your first shot in the "middle" of the kill zone at 100 yards, then go hunting.

I personally am not happy with over a 1" group at 100 yards which I can do with both my Knights (Extreme and Elite) and my BP Xpress. In my Knights, I shoot 130 gr Pyrodex and 250 gr Barnes Expander and in the BP Xpress 200 gr Pyrodex and 300 gr Hornady SST. As I said before, this is probably overkill for hunting situations, but I have fun at the range trying to make it happen.

How far will you actually be shooting at deer? As one poster here puts as his tag line "A man needs to know his limitations" or something like that.

I would try to make sure at the range you are darn close to how well you do with a centerfire rifle. If you can't group well with either practice, practice, practice. After that it is all up to the hunter because most bp guns are as accurate as centerfire rifles. Yes they have less range which brings up my first question but all other principals apply.

That poster would be me [8D] I think retrieverman has it right - its really only a one-shot group that counts. I know a guy who does just that. Goes to the range and fires one shot. (Fortunately range is close to the house) Comes home and cleans gun. Goes back out a couple of days later and fires another shot. Repeat as needed.

I would say whatever range you find yourself able to stay inside of 4-6" off-hand on a regular basis.

For many folks, inside of 4-6"/off-hand is much less than 100 yards.

Bench/stick shooting is fine, but not always the opportunity.

Sure use the bench to establish that the rifle is shooting well and then start off-hand practice.

For bench shooting, I would think the same inside 4-6" rule would apply only you should be able to obtain 75-100 yards iron sights and even farther with optics depending on your rifle, load and optics.

And of course, a great deal of practice to be able to shoot this accurate or better on a regular basis.

My .02