I have a s&w five shot breakdown #18622 . I would like to know what caliber it is. It's like chrome plated and no trigger guard

Honestly, it is going to be very difficult for anyone here to give you a true answer to this.

If the caliber isn't stamped on the gun (usually down the side of the barrel), guessing is nearly impossible. Your best bet is to contact S&W directly, provide them with the serial number, and they may be able to tell you all of that.

Edit- When you say "breakdown" do you mean break open? Does the barrel and cylinder break open and swing down away from the grip while still remaining attached?

My husband is a master gunsmith, I will ask him. But I have to know if this is a revolver or a long rifle.

If you take it into any gun shop, they can give you that info,

Here is a general size chart for caliber

Caliber - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Caliber)