We have had many customers go with the proof 7 saum prefit along with defiance short action. No complaints at all - like Longtine mentioned 168's work really well, one well known customer of mine is running 175 ELDX with great success as well in the short action defiance.
If we can help with your build let me know, we have both of these items in stock here:
https://jaoutdoors.com/product/ruckus-ultralight/
Defiance Pre-fit Barrel’s These will work for the following defiance actions: Ruckus, Anti, Anti-X and the tenacity with integral recoil lug (tenacity MUST have integral lug) The Proof Research Defiance Ruckus Pre-Fit Carbon Fiber Barrel features unprecedented accuracy, performance, and...
jaoutdoors.com
Like others have mentioned, the XM route truly is a bullet proof setup for the SAUM's, especially if you intend to run the longer bullets, keep in mind you need to be careful which defiance XM length action you order. There are two variations, one being strictly BDL style or the latter being AICS DBM fed.
with that being said the custom builder route is FUN --- with MANY high-quality builders available today you really cannot go wrong with that route either!