If I got this suggestion right, you are suggesting a kind of arming time for the projectile after it has left the waepon? Like "modern" rifle grenade work in real life and for example in vBf. So the projectile needs a fly time of a fourth of a second or something, before exploding on impact. So it can still be used on infantry and so on, but would be useless at distances maybe closer then 20 m in Bf-scale. I dont know if this is realistic for the piat, but gameplay wise it would stop "noob tubing". It also would prevent you from being killed by yourself, if you hit a wall very close by.
I am not sure if this would help gameplay or not. I have never seen killing infantry with zooks, schrecks or PIATs much of an issue. The only issue I see here, are people, who run close up to you, only to kill you with it in a very close encounter. Atleast in my experience this happens so rarely, that it isnt much of a problem.