Gus Norcross, originally trained on National Match rifles and pistols by the National Guard Marksmanship Training Unit specializes in Garands, M14s and 1911s at his small shop on the coast of Maine. His website is www.angusarms.com. He will be offering gunsmithing tips and tricks on Fridays.
When we think about tuning M14-type rifles to improve accuracy, factors such as bedding, barrels and triggers usually come to mind. Most never consider the fit of the handguard to be a problem, but eliminating contact between it and the stock can vastly improve accuracy. Keep in mind a prime directive of M14 accurizing: nothing forward of the receiver should contact the stock except the bottom lip of the front band.
GI handguards were made from fiberglass, with the underside painted silver. Current commercial offerings are plastic and less prone to cracking than the originals. The only function of this piece of plastic is to keep your hand away from the hot barrel.
Accuracy problems occur when the guard is too long, causing pressure between the front band and the receiver, or when the bottom edges are making hard contact against the stock.
If the guard is too long, shorten the receiver end with a belt sander. Relieve the edges of handguards that are touching the stock by laying a piece of coarse sandpaper on a flat surface and sanding as necessary. Test fit frequently and keep the edges straight. A 1/8″ to 3/16″ gap between the stock and guard is fine. Removal or replacement of the handguard clip can be tricky without the right tool. Badger Ordnance manufactures a set of handguard clip pliers available through Brownells as part number 093-200-050.