FL training on Rings v Bars

I'm just about to transition my FL training from the ground and notice that either a straight bar or pbars are used for the rest of the progressions. Is it possible/advisable to use rings instead? Are there any disadvantages or advantages I should be aware of?

 

For the PE 13 & 14 a bar is a must due to the shift in center of mass the bar forces. I have used a thick dowel across rings out of desperation. It works, but isn't ideal, particularly if you find these to be a struggle at all.

When you're doing actual lever work there's no real difference, but as Cole noted it's important for PE13 and 14 to have a straight bar.

Thanks, gentlemen. I'll give the dowel a try and look for plan b if that doesn't work.