For a busted scope to come back on target EVER is a miracle. For it to come back OFTEN is next to impossible. In your first thread, you said yourself that after the first incidence YOU shot the rifle and it was on...sounds like operator error to me.

I'm not going to say it's a flinch necessarily, but I'd say a practice round with a few dummy rounds thrown into the mix is in order (load the gun for him and don't tell him there are dummies loaded). You'll be able to tell a lot about how he shoots by letting him THINK it's going to fire, then watch him when it doesn't.

He could also be "driving" the rifle if he is consitently shooting low. Many shooters will DRIVE the rifle forward and down after a shot in order to recover from recoil...on a surprise dummy round, this will be extremely evident and appear VERY differently than a flinch. Some shooters I have instructed have driven the muzzle down 12-14" on a non-recoiling shot. It's just habit, they expect a push, but need to recover quickly. It's not necessarily a bad habit, unless he's doing it BEFORE the shot.

If he doesn't really know how to use a scope properly, or the eye relief is not set properly for him (or he doesn't hold the rifle consistently it may be off from position to position and shooting session to session), that may also explain the odd behavior. A LOT of new shooters don't know about "shadow effects" caused by not having their eye properly positioned behind the recticule. The X-hairs are on target, but if there is a shadow on one side of the image, the bullet will run from it. If he's got a shadow all the way around, the eye relief is wrong and he would have a VERY hard time compensating for this error. If he ever mounted the rifle properly, then he may shoot well that time.

I'm not saying it ISN'T the scope, but I've never known a busted scope to come back on that frequently...it could be possible if the recticle is sliding back and forth in the tube, but I doubt that's the case in this instance...the cross hairs would totally disappear occasionally because they would slide out of focus.

Really, you need to have someone else shoot the gun. You already proved it's his fault once, but didn't realize it because you KNOW the gun or scope is at fault. If it shoots well for you and not for him...it's pretty clear.

By the way, to drop 18" at 50yrds, you've only got to drop your muzzle about 0.3-0.4" from your line of sight. 18" at 50yrds sounds like a huge miss, but remember, that's only half a yard at 50yrds, or 1.5ft at 150ft. The angle of deviation for 18" at 50ft is only about 0.57 degrees. At the muzzle, that translates into only around 1/3".