Will I able to deduct the full 40000 I paid for a barn I built in June of this year with the 179 section deduction

No, unfortunately, you will not. While most equipment that businesses lease, finance, or purchase will qualify for the Section 179 Deduction, there are some exceptions.

Per IRS Publication 225 - "Farmer's Tax Guide": 

To qualify for the section 179 expense deduction, your property must be one of the following types of depreciable property. 

1. Tangible personal property. 

2. Other tangible property (except buildings and their structural components) used as: 

• An integral part of manufacturing, production, or extraction or of furnishing transportation, communications, electricity, gas, water, or sewage disposal services; 

• A research facility used in connection with any of the activities in (a) above; or 

• A facility used in connection with any of the activities in (a) for the bulk storage of fungible commodities. 

3. Single purpose agricultural (livestock) or horticultural structures. 

4. Storage facilities (except buildings and their structural components) used in connection with distributing petroleum or any primary product of petroleum. 

5. Qualified real property. (Special rules apply to qualified real property that you elect to treat as qualified section 179 real property. For more information, see chapter 2 of Pub. 946, and section 179(f) of the Internal Revenue Code.) 

6. Off-the-shelf computer software that is readily available for purchase by the general public, is subject to a nonexclusive lease, and has not been substantially modified.

Unfortunately, a barn is not considered a single-use structure for purposes of Section 179.