I've done it to multiple synthetic factory stocks, mostly to cut down on the loud (to my ears) "scratching" sound, when brush drags across them.
IMHO, it works only "fair" as a sound deadener.
On some, I've had to remove a buttplate to access the butt, and the entire gun from the stock to get at the barrel channel.
I've used a couple of different brand names from HomeCheapo and/or Lowe's, with no discernable difference between any.
None have generated enough internal pressure to bend or deform the stock material - when sprayed in, it simply foams up, expanding to overfill the stock void(s) - which overfill I slice off even with the stock edges, using a single-edge razor blade, a few minutes later, after the foam cures/dries.
The foam seems to add no appreciable weight to the gun - although I've never bothered to weigh any before/after.
.
