I have a new horsetackworld circle y saddle with a 5 star 7/8 pad that I have used on a few mature barrel horses that I have ridden. I am now riding my 4 year old Firewater Ta Fame Stud and he is really starting to turn hard. My saddle really slides when I go around the 3rd barrel especially. I use a weaver smart cinch with it now. I do notice some sliding when I get on the stallion at first but not just a significant amount. I make sure everything is good and tight before running but I still feel the saddle slide like crazy on that hard turn. He is a little downhill at the moment in a growth spurt and doesnt have a significant amount of height to his withers but he is pretty muscled. I am wondering if i need a thinner pad?? Shims?? or a different girth?? When I sit the saddle on him bareback the fit looks right. Any help would be appreciated. 

I have a bob and just stopped using my 5Star and went to a saddleright for the same reason. Also went from a 30" cinch to a 28" cinch. The saddleright pad does not slip at all. Might be worth a shot! I do still love my 5Stars...

I second the Saddleright pad, I prefer them to both CSI and 5 Star pads on most horses. 

A good friend of mine has a colt who is as round as a barrel with absolutely no withers, she couldn't get any treed saddle to stay on him. She switched to treeless and has had a lot better luck. 

SPH pads might help. The front is cut so that the shoulders dont create drag on the pad when turning. I am having good luck with them. Pad width can also effect how the saddle fits and slips or not.

If it's the treeless I use the treeless pad by reinsman that Felica sells. I tried 5 stars and best evers and had nothing, but issues. I love my reinsman pad and I use a short ventech neoprene girth. I ride my 4 year old in this set up with no issues

I would guess the 7/8 pad is to thick for that saddle/horse combo. I would try a thinner pad.

Saddleright is a good choice in my opinion.  I also use a Nevertight cinch.  They are super helpful as well.  

This could be another thing to consider...the angle of the bars.   When the angle is too wide, the sides of the bars can't "catch" the horse's back as well, therefore, it is less stable when changing directions....or mounting.   

I have struggled and struggled with this.  Even having to give horses an extended break until I found a different saddle concept from what I already had tried....

If you have a saddle with too wide of an angle, adding padding - or subtracting it - won't completely help what's actually happening.

I know of one guy whose shims are different than any I see, and when you buy a corrector pad, the shims come with it.  The shims have a gradual cut and layer that dont really shim the gullet, but shim the extra space in the angle of the bars. (the bottom pic shows how his shims would fill in the too wide of an angled bar).

He has even modified the tree of saddles that customers love, to solve that common problem of too wide of a tree angle.  Before you tried that, I would suggest trying out his The Corrector Pad and reading literature on his website about saddle fit.  (I forgot whether or not we are allowed to cut and paste here or post links - it's one or the other that we stopped being able to do, or I would include it here).

What are the SPH pads?