I found a couple cheap 2.5 gallon American oak barrels. They were used twice for tequila storage. Will this have any effect if I use them for wine?
Nick,
Yes it will have an effect on your wine. It will make the wine taste like tequila.
You need to ask yourself, "Why am I putting wine in a barrel?". Most folks do this for one of three reasons....
1) Impart Wood Flavors to the wine - Being that the barrel has been used to store a "high alcohol" fluid with a much higher solubility than wine, I do not believe that you will get any wood flavor from the cask.
2) Micro Oxidation - This will certainly occur, but at a much higher rate than, say, in a 50 gallon barrel.
3) Concentration/evaporation - Like micro-oxidation, this will occur at a rather high rate..
The biggest issue you have is one that you might not have already considered, and that is the size of the barrel. When it comes to casks and wine, the most important thing to consider is the amount of interior surface area that come in contact with the wine VS the total volume that the cask can contain. In other words, Surface area to volume (S/V) is important.
The larger the cask, the lower the amount of S/V. The smaller the cask, the HIGER the S/V. A common rule of thumb is to cask wine 1 wee for each gallon of capacity. In your case, this would only be 2.5 week. Not a whole lot of time to obtain any wood flavors, but you will get (limited) evaporation, and some micro-oxidation.
That little cask of yours is really meant for the hard stuff (which requires and can hold up to a much higher amount of micro-oxidation).
Still, there is no harm in trying. It is only 2.5 gallons of wine. I would, though, plan on 2.5 weeks only.