May I suggest again using a cross-cut sled on a table saw and shimming the butt of the stock square with the blade.
Setup should take about 10 minutes, 15 if you have to change to an 80-tooth blade on the saw first.
All you are asking how to do is to make a parallel cross-grain cut in wood. No compound mitres, no strange profile... a straight cross-grain cut.
Here is a link on one way to make a small sled.
In the case of something oddly shaped like a stock... some Plasticene or other non-air-curing putty on the sled and built up under the stock works better than a whole boatload of shims, it's like a soft cradle for the stock. Cover it in saran wrap to avoid getting grease on the stock. Masking tape (good stuff, like 3M) over the cut area, and don't push the cut. Give yourself 1/8" by cutting it long and finish to the line on a stationary belt sander. I do one more stage after that, and push-sand on a mirror (or glass, or marble) with 400 grit wet/dry stuck to it to make it perfect.
Here are the last two stocks I cut down:
Mauser 96 on the left with a spacer of Osage Orange/Maple/Osage Orange, Winchester 101 on the right with a spacer of Honduran Mahogany. Even at that magnification, there are NO gaps between any of the joints.
It's Woodworking 101. You just have to stop thinking like a machinist.
Bren R.