You all seem rather familiar with armor, seeing as the majority has made some. (an assumption)

I have always been concerned with the visibility when wearing a plate helmet. From your experience, how much does a helmet hamper your visibility, and what type of helmet provides the most visibility?

I'll let our Armor Master answer this question.

Each helmet is different. Usually you get used to what ever helmet you use. Understanding the visibility and blind spots in helmet types can be useful information. Mostly it just takes experience.

You can get used to nearly any type of visor. However, you need to get your 'time in' with it.
Even though you can't see the whole picture, you get used to how people move and react. Even though you may not see your opponents feet, you can see his hip movements which should give you good clues to what is happening next.

Currently, all Knights frown on bar grills and do not support their Squires wearing one.

Most if not all fighters would let you try out their helmets to see what could work for you.

I suppose I should try on various styles of helmets before I actually make my decision, but after a small amount of research, a Barbute appeals to me.

Here are some images of barbutes:
-A standard barbute
-A neat, black barbute
-A more open-styled barbute
-A more 'fitted' barbute

The black one with the narrow slots might actually be usable in our group as-is.

We have used versions of the other barbutes. We just make different visors for them.

Many of our helms, including mine are modified barbutes. The barbute has the advantage of letting you "size" it to some degree by bending the T slot more opened or closed. However, they often do not provide enough face protection and require some type of plate to be added over the nose/mouth region.

The black barbute is the closest to what I had in mind, how does the mouth/nose opening on it look?

Is there any particular visor type that you aim to put on the too-open barbutes, or do you just go with whatever works?

A mass majority of our helmets are purchased online. It's one of the main expenses. Look around online and see what's available. I haven't seen any barbutes with slim openings for sale.

Here are two barbutes that are for sale I found in a quick google search:

-"European Barbute" The opening on this one seems like a good size, I might not be a good judge of that, however.
-"Barbute Helmet" This one's opening is noticably bigger, I'm assuming I would need a visor for it.