I had to buy a new spring guide to fit my new spring I bought. It is a P.D.I. spring. It is wider than the stock spring in my M24 rifle. So I had to buy a V3 first factory piston to fit it. I personally went ahead and bought a piston head, spring, spring guide, and piston. So I wouldn't have any problems with internals. Many people told me that a P.D.I. spring will work buy itself in the gun just fine. But that was obviously wrong. You can buy a smaller diameter spring. I think First Factory Sells them, to fit the stock guide and piston.