I have used
this for Canoe repairs, because I think it has some structural advantages. I grind the area surrounding the hole (on the canoe) until the glass fibers are exposed, and then apply a repair patch to form a supporting surface and use regular resin to affix/support the cloth. I also grind any protruding material down below the surface level once it's rigid.
The Bondo is applied to overfill the repair slightly higher than the surrounding surface.
Once it achieves the hardness of cheese, I cut it back down to surface level using a
Ped-Egg, which is just a miniature variation of a
Surform file.
For repairs larger than the holes you mention, the Surform may be better than the Ped-Egg. It should be reasonably longer than the repair area, so it bridges the repair and does not cut below the desired surface and cause any dips.
If your rifle has any tendency toward being barrel-heavy, this could be a good opportunity to add some lead shot to improve balance.
When I use a single shot follower, I also line the blind mag well with a plastic bag and fill it with lead shot to reduce recoil somewhat. No need to add resin for this step, so the shot can be removable.
Read through the reviews and make your own decisions. Stirring the resin well to get everything up off the bottom should resolve any tendency toward runniness. Don't skimp on the mixing time.
Greg