So I posted this question the other day and unfortunately the previous forum will not let me reply to anyone. So im starting a new one in hopes that I can respond to people here. 

Hi everyone,

Thank you so much for the helpful input. For those who didn't understand my explanation lets see if I can hopefully word it differently. So I am planning on making 3 separate double barrel cakes that are all 6 inches in circumference and will be a semi naked design. This is for a wedding and the location is about 30 minutes away.

I have tested the delivery of these cakes on 2 separate occasions and have had little to no luck. My first attempt I completely assembled the cake at home and when I got to the location the cake was tilted in the box. I attached a central dowel to the cake drum which was about 1/2 inch thick. I then doweled the entire bottom half (which was 3 layers) then I placed a smaller cake board down and put the other 3 layers on top to complete the double barrel. 

My second practice attempt I decided to assemble 2 separate cakes (each 3 layers tall) then put them together at the location (for a total of 6 layers tall). I also added an extra egg to my recipe to make the actual cake a little more dense. I succeed in stacking the cake but it just seemed too wobbly and scared me a little. However it did NOT fall over this time.

Do you think my buttercream isn't stiff enough (4 sticks of butter, 8 cups of confectioners sugar, salt, and vanilla)?

or

Is there a problem with the actual construction of my cake?

I hope this cleared things up. Thank you so much everyone!!

What size was the bottom board of each dbl barrel?  The larger the better (within reason = like use a 10" base).  It can be removed once you arrive at venue - it doesn't have to become a part of the display :)  It doesn't sound like it's a b'cream problems.  I have used this  recipe  https://www.cakecentral.com/recipe/22469/2-icing  for years on wedding (& other occasion) cakes with very few mishaps (that usually had nothing to do with the use of b'cream) - especially if used as a crusting icing.    You said.........Is there a problem with the actual construction of my cake?.........   It just sounds to me lkie you have too tall of a tower with not enough base to steady it.  I'm guessing these 6" cakes end up 12" tall when all is said and done - right?  I don't see any way to strengthen it......unless you start w/a metal rod down the center anchored into a thick large base.  But then you will forfit at least a couple of servings..........  don't know, haven't done anything like that. 

As I think on this longer I'm envisioning somekind of contraption of a 10 or 12" roundbase with 12" tall metal or heavy plastic pipe 6 1/2" wide.  Place the cake on the base board then slide the pipe down around it.