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 AngusArms.com. He will be offering gunsmithing tips and tricks on Fridays.
Among a host of issues that can result in your pet M1A producing crappy groups is a loose-fitting op rod guide. The guide is designed to maintain alignment between the operating rod and the gas system. It is located on the barrel behind the gas cylinder and attached with a 1/8-inch roll pin. Finding loose guides on M14 style rifles is commonplace and not necessarily a problem where reliable function is desired but varying op rod contact with the gas piston will affect barrel harmonics and ultimately accuracy.
Ideally, we want the guide to be a press fit but the reality is that many guides on factory rifles have a slight amount of play. Tightening usually involves knurling the barrel surface, driving the guide in place with a block of wood or aluminum, adjusting its position relative to the gas cylinder and securing it with a new roll pin. The use of epoxies such as Flexbar’s Rocksett or Loctite provide additional insurance against movement. The hand knurling tool I use in the shop was produced by Eagle Rock Technologies of Bath, Pa. It can be purchased through MSC Industrial Supply. Using a punch to dimple the barrel as an alternative to knurling should be done very carefully to avoid damaging the bore. Heavy dimpling can be detected with an air gauge and would be counterproductive when working with match grade barrels. Next time you strip your M14-style rifle, check the op rod guide for wobble.