I think some reviewers disagreed with the SRU's decision to delay the entry, because they might not have followed the plot and conversations carefully. The show made it clear that Keenan had a shotgun in his hand and that SRU is concerned about ricochet in a confined space. I thought the scenario was fairly realistic. I own a shotgun and shoot skeet and trap. My understanding is that shotgun practice is only allowed outdoors and not in any indoor ranges. You can imagine why, since a load of pellets can spray all over, I am not sure if the scenario would have been extremely different if Keenan had another type of weapon, but I imagine that ricochet would be a concern also. Regardless, the scenario presented was Keenan holding a shotgun in a confined space. I did not feel that I had to suspend disbelieve in that scenario. Also, the fact that the kid died and not the cop seemed realistic. It depends on the wound. Maybe the kid's major artery got nicked.
I think the show maybe also wanted to make a point that this show was trying to be realistic and not the Hollywood-style fair where a cop can aim and shoot the gun out of a perp's hand. The point is that if there is lethal entry then all hell might break loose.