The classic method for shortening a polymer stock (if anything involving a polymer stock can be considered “classic”) is to shape an insert from wood, epoxied in place to serve as a new anchor for the buttplate/buttpad screws, or simply an adhering surface for glue-on pads. Works fine, ends up looking fine, and anyone with a combination belt & disc sander can make short work of it. Other techniques/methods work, and may be more or less work, but the old wood block insert method does work just fine.

Honestly, I do abhor polymer stocks, so I do typically replace them with wood stocks - Boyd’s stocks are pretty affordable, and it’s often fun for the kids to pick out their own color.

Forgive me if this gets a bit long, but I’ll offer some ideas I’ve used over the years when doing these jobs.

I forgot to mention when I first typed this, so I’ve come back here to briefly describe here how I cut the stock. Many ways to skin this particular cat, but most often, I use a simple holding fixture screwed to the buttstock in the buttpad/buttplate screw holes, then use the chop saw to make the cut. Radial arm saw or table saw work fine also. The jig is simple - a hunk of 2x4 on edge screwed to another laid face flat, long enough to provide a clamping surface on the saw table, or long enough to ride on the push guide if using a table saw. Just an L shape, held to confirmed 90degree angles. I draw a level line on the jig, center the buttplate screw holes over the line and drill - positioned such the stock is suspended just off of the saw table, laying on its side with the level line level between the top line and bottom line of the stock, and the cut will be parallel to the original buttstock angle (same pitch).

With a wooden stock, that’s largely the entirety of the project. I make the cut slowly with a fine toothed blade, through the stock wrapped in painters tape to help eliminate or minimize chipping - especially taking care to go slow right at the end of the cut when the curvature of the stock rounds off quickly. Lay the buttplate back on the stock to reposition the screw holes, mount the plate/pad, sand to fit, and go shoot. Setting up the cut is the longest part.

A Polymer stock may require an insert. But first... When you have the polymer stock open, it’s an opportunity to reduce the hollow “thunk” of the buttstock by spraying full of expanding foam. Once cured, carve out the desired depth of your wood block or relieve for another anchor type (more below), such the foam fills the stock and also provides backing to support the insert during the process.

The process then is pretty straight forward, and the “wedge” profile of the insert block becomes a little less critical (without filler, sometimes the insert can twist or tilt). Fill the stock with expanding foam, carve out the appropriate depth for the wood block, shape and fit the insert on the bandsaw and/or belt/disc sander, and epoxy in place. I do like to leave the insert slightly proud, such I can sand it flush with the stock wall on the belt sander - especially if using a glue-on pad instead of screw-on.

Very nice in this process is when the stocks include ribs down the spine and belly - the screw footings which would have otherwise been anchors for the buttplate/buttpad screws. These ribs are often parallel to one another, such they can remain useful for remounting the buttplate/buttpad - they’re not typically drilled deeply enough to retain any hole depth after cut to youth length, so I’ll redrill. Take note of the depth of the bottom screw here - the anchor rib won’t be very long and the bottom of the stock is angled upward, so it’s possible, even probable, to drill clear through to the outside of the stock, or have a long screw find its way to the outside.

If these ribs exist, but can’t be used for the new buttplate screw hole positions after shortening the stock, then cut back the ribs JUST short enough to allow your insert, using them as footings to support the insert. Or, although more work, inlet your insert to key around these ribs for better epoxy adherence (I’ll then cut perpendicular grooves in the top of the rib with a dremel to adhere the insert).

Inconsequential, but it makes my brain feel better - I have some ~5/8” marine board I’ve used instead of wood, keeping plastic in plastic.

More often, however, I prefer to pour epoxy footings - It takes a little extra time, but it works well. I’ll clamp the stock level, create a dam at the end of the stock with tape, then pour the epoxy footing. Flip the stock the next day, and repeat for the top side footing. If using foam fill, then simply carve out the filling to the appropriate size and depth of the desired footing.

As with any epoxy work, whether using a wooden or polymer insert, or pouring footings from epoxy itself, surface prep matters. I use a dremel and a roughing burr or large grit sanding drum to dress the surfaces, and will cut grooves and or holes in any structure available to give better grip to the epoxy.

Once it’s back in place, I wrap the rear perimeter of the stock in a single layer of thin masking tape and sand on the combo belt/disk sander to blend the pad/plate to the new shape. I try to avoid cutting through the paper tape, as repolishing a polymer stock to a uniform finish is virtually impossible. The roughed edge of a hard plastic buttplate can typically be repolished with some MEK (hazards acknowledged), but getting the right texture on the poly stock surface isn’t really a thing.

Unrelated, but maybe worth while - I’ll often stiffen and/or weight the forend while doing this work; re-establishing, or improving the balance of the rifle.

I typed this piecemeal during meetings this morning, but I’ll try to get back here this evening and post some photos of the end result of recent wood stocks I did for my son. Might even be able to do a polymer stock photo tutorial soon, with one of the many take-off’s we have laying around.