Looking for a book I read in the early-mid 1980's in the U.S., probably published then. A vietnam vet is transported to a quasi-medieval fantasy world and trained to fight for a kingdom -- might be the king somehow. Details: Cover shows a man on horseback wearing a feathered mask. He's short-sighted and loses his glasses; this makes him a poor archer. He comes in with guns, which work until they run out of ammo. Short noble ally who fights with a cestus on one hand and a claw on the other. When his family welcomes him home from Vietnam his mother catches him automatically crunching the ice in his drink to extract the moisture, which brings tears to her eyes (paraphrase). Long description of his first real swordfight -- how the heavy sword becomes weightless, the physical impression of his enemy, how lucky he is to survive. Overall strong physical and psychological realism and grittiness (modulo the fantasy setting at all, of course). I believe there was a single sequel, but might be wrong. Thanks for any help!