Bought a Martin Stingray, and now my horses back appears to be sore. As in, she tries to bite n kick out when I throw saddle on, been riding her approximately 3 weeks in it. I was warned before I bought it that it may require "shims". I have no idea how to tell if she needs a shim, or if the saddle is causing this, or what? Anyone have any experience/thoughts/ideas on this? My mares back/withers seem relatively normal to me, not appreciably small, but certainly not wide or overly prominent in withers. Opinion appreciated :)

Do you have pictures you can share with us of her both saddled and unsaddled?

i know several who’ve gotten along will with the martins and they do shim them when needed.  They weren’t my cup of tea though and I didn’t find that they fit a lot of what I was riding.

The information on Martin's is so inconsistent. I called Martin and they told me that they are all the same tree (BTR, FX3, Stingray, etc) then a dealer told me thats not true. One saddle fit company said no horse should need wider than an 8" gullet. My mare is a standard 7" and then another dealer told me she rarely keeps 7" in stock because most need much wider.

So I am just as confused as you. However, if its making the back sore, I wouldnt be using it thats for sure. 

If you post some pics that would help. Is she sore over her kidneys or more up in her shoulder? 

I have an 8in crown C I shim on my 4 year old. If it's not shined properly it bounces and hits her back making it sore, when my other mare out grew it she was sore in her sholder. 

see, thats the same inconsistent info that I got from Martin...

I am going to chime in here.  So I am on my 7th Martin saddle.  I have had 3 Crown C's, 3 FX3's and I have a BTR on order. This journey started over 10 years ago.  I had a lazy L that was making me sore on my sit bones.  I tried a friends Crown C so I sold the Lazy L, took my mare to Brian in Greenville to see what size gullet to get her and he went through that whole you need wider wider, my  mare was 15.1 and 1100 lbs typical quarter horse with a really good wither.  I ordered an 8.5 and got 2 shims(he suggested 9!).  I hated it.  always kicked my feet up felt like I was running down hill.  Talked to a couple very well known trainers at a race about their crown c's (they now ride BTRs) and they both said same thing they went back to the more narrow gullet.  So I sold my 8.5 and got an 8 because i was on the fence.  Had it for a while, still shimming and decided to sell it and get a 7.5 but then the FX3 came out tried it loved it much better so I got a 7.5 FX3.  Loved it much better so I bought another 7.5 Fx3 the newer one had the SRS so I sold the older fx3 and ordered the newer FX3 7.5 again.  Once I got into the 7.5 I did not have to "shim" anymore which was nice.  They both fit my qh well untill I now have 2 babies, 3 and 4 yr olds.  Now I have big dry spots on my gelding but not on my filly SOOO I wanted to try the BTR.  I always do the test ride program from Martin and Loved it.  it was a 7.  So I am now selling 1 of my 7.5 wide fx3 and waiting on the BTR 7 wide because it seems to fit him better and I like the FX3 7.5 on my filly.  

I kinda feel like a Martin expert at this point.  All that BS about wider wider etc have you noticed 2 things. 1) sherri cervi does not shim any more and has the stingray which promotes a "standard 6 3/4 tree.  Idk if you can even get wider than a 7.  WHen I ordered my BTR 7 was the widest offered online.  2) Same with the new Lisa Lockhart fearless by Martin.  

I have 3 horses 2 with really good withers and the 7.5 fits both perfect seldom dry spots on one and never dry spots on the other. THe BTR more narrow is for the gelding he is built more stocky not so much wither and the 7 seemed to fit him better on the test ride. 

I also took my gelding to the local Mardin dealer and now they agree standard is better because they dont "rock" back and forth and sit up more evenly. 

Sorry this is long maybe it will help.

Thank you, I am not the OP but I found it helpful. I am battling the same thing right now. My mare is very average also. 15.1 and decent wither. She has a super wide loin, thats the only real tricky piece so she needs a wider bar angle. Sat a 7" fx3 on her and was happy with it from my standards. But then a dealer kept telling me she doesnt even carry them really because they dont sell. I really dont care either way but just want to understand the method to this madness.