If it is a USGI Synthetic it does not need a Liner...But if you are working on a Aftermarket stock that is fiber glass this same method should work just fine....For Max Accuracy I always reinforce the forearm...unless I am working on a MacMillian stock. Here is the text from a post I did a while ago.
Reworking The USGI Fiberglass Stock for Accuracy
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I thought I would step by step outline the steps I take to rework USGI Fiberglass stocks for NM accuracy...It CAN be Done, and you can do it! This is how I do it, I hope this helps some of you. I hope that perhaps this might be made into a sticky as a resource for others. I have done 3 stocks this way and they shoot as accurately as any of my oversize NM birch stocks.
What you’ll need: Your Dremel tool, Fiberglass Cloth, West Systems Epoxy Resin system, Wet-dry sand paper, Modeling Clay, Packaging Tape, Plastic wrap, paste wax, wooden shims, and one USGI fiberglass stock.
Prep Work: Being careful sand ALL the finish and Paint off the areas to be filled and epoxied. Leave a rough finish so the epoxy will bond to the existing substrate (stock). Now with your Tape, tape-off all areas you don’t want epoxy to get on.
1. The first task will be to fill the Selective fire cut-outs in the rear of the stock and the front of the receiver. I use the techniques outlined in duffs Book and create an epoxy plug, I also fill in the Front cut-out so I have increased bedding area and support for the connector lock pin. After the Epoxy cures I file it smooth and blend in to the outlines of the stock so it does not create a high spot to interfere with the bedding.
2. Now we are going to stiffen the forearm in order to make it as stiff as a Match stock. The first step is to cut the fiberglass cloth to fill the forearm in a “U”from the Front of the receiver front to just behind the front ferrule. I use 2 or 3 layers here.
3. Now trial fit the cloth, I use a Few wooden shims cut to fit into the forearm and press the cloth against the bottom and sides. And give a tight fit, I coat a piece of plastic wrap coated with paste wax to wrap around the wood forms so the wood will not be epoxied in.
4. Now Mix up a batch of Epoxy...enough to soak the Fiberglass cloth will do. Now lay in the fiberglass into the forearm and make sure the fiberglass is soaked with the epoxy and laminated together. Working carefully, place in the Plastic wrapped wood forms and shim it so it forces the cloth against the sides and bottom. Now we wait till tomorrow.
5. Remove your form and plastic wrap, you should have a smooth layer of fiberglass pressed and compacted against the side and bottom...looking good so far but not done...Now using a bit of acetone, wipe the inside of the stock to remove the residual paste wax that transferred to the cloth from the plastic wrap, finally sand down the inside of the forearm to rough up the inside and make sure no residual wax is left to destroy your next efforts.
6. Now we are going to fill the stock with epoxy to the level of the inside ledges to give maximum strength and rigidity. Level the stock so the ledge and stock are level. We are now going to mix up enough epoxy to spoon into the forearm to fill the forearm level with the ledges. I use Modeling clay dams fore and aft. Repeat for the bottom and each side. I also fill the inside stock cut-outs at this time. Again you have till tomorrow to wait, this whole step will take 3 days. When you are finished do the minor fitting required to make sure nothing rubs or binds per Duffs instructions.
Your stock is now incredibly rigid and ready for your paint job and perhaps skim or glass-bedding if you are so inclined. This technique capitalizes on the strength and rigidity of both the resin and the fiberglass cloth to give a maximun advantage to the work with the lightest weight possible.
I also remove the Stock Sling and reattach it with Shortened T-nuts and Black oxided Hex head screws that allow a great ridgid attachment of the sling on the forearm. The hardware is available for a cpl of dollars at ACE hardware. I epoxy in the T-nuts with West System Epoxy.
Hans
