What I remember.. Read in some anthology mid 1970's but I had an impression it was a good decade or so older (slang expressions etc). The story began with a teenage American looking out from his reinforced family bunker at the mail APC trundling up the street, painted in white 'truce markings'.
It leaves something in his family mailbox so he exits through a slit trench to retrieve it. As he crawls the kid across the street tries to nail him with a sniper rifle. Two or three ammo magazines and a black grenade, he concludes it's a sample and, as he is the only 'consumer' in the house, he pockets it.
Later that day he returns home from a planning brief for the weekend battle and there's a stranger talking to his parents. He eavesdrops and learns there is a lot of government concern due to some protest faction mailing out some neural grenades. He examines his and notes some crude welds and mods on it. The stranger goes on to say if neural grenades get used it will be anarchy.
The youth are hypno conditioned from an early age to regard this (dystopian) society as normal, fighting and killing from pre-teen to early adulthood, then all are conditioned again to forget all that and be productive by working in munitions factories. If these teens snapped out of it and saw the world they had to live in to make money they would probably go on a killing spree at all authority figures.
Protagonist then noisily enters the house and gets introduced but denies receiving any new grenades. The visitor asks about the forthcoming battle with another district ( or town) but the adults eyes all glaze over while the lad discusses it, only comment is from his mother who tells him if he gets wounded again then no blood on her sheets.
At the weekend battle in a massive stadium, one team fires off mortars too soon and wipes out a squad or something before the start, causing an outraged blast from the siren in the umpires tank.
Due to these shenanigans the kid is eventually on the losing side, they're getting slaughtered as foxholes are overrun one by one. In desperation he pulls the pin and throws the grenade.
No Spoilers now because I really can't remember what happened then or anything more about this story, or other stories in the same book. Help please
Edit: thinking further on this I'm fairly sure that, after the forgotten events are over, the protagonist and his erstwhile enemy aka "the kid over the road who takes potshots at him" are both leaning against a car outside in the street.
Discussing what to do now the fighting is no longer necessary and if they should disarm