Any tips on taking out Level 3 Sentries?

I recently started playing as a sniper to try and better understand how to avoid getting my brains exploded as a HWG. Been having a ton of fun, been getting some pretty amazing strings of kills/assists, all that, and haven't run into too many unsurmountable obstacles. Except for some nigh invunerable sentry guns.

I was under the impression that the only two unlimited range weapons in the game are the sniper rifle and the soldier rockets. Due to this, they are the ideal way to clear sentry guns if the engie doesn't consider all the angles of approach. Very gratifiying to snipe an engie, then have a nice long chargeup and pop their sentry down. Except sometimes the gun doesn't die, or even get hurt.

My most recent example: On PL_Goldrush, stage two offense, an engi put his gun where I could easily see it from the rooms above the first cap point. I killed the engi, charged the shot, and landed a shot that should have killed the sentry. Nadda. No damage. I just shot the thing three times from three slightly different angles (not a ton of room to work with) before the engi got back. All three fully charged shots, all right to the middle of the gun (as in not the missles or the legs or the barrels, the meaty center bit) I've shot plenty of sentrys, but I've run into more than a few that just won't get damaged.

Am I doing something incorrectly? Is this latency? I hope not, since a sentry doesn't move about, so where I think it is and where the game thinks it is should be the same place, right? Any ideas?

edit: spelling

Im not sure what to tell you here except in my experince levl 3 guns take 2 100% shots to destroy

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I'm not sure whats up with that either, but its faster to do a 100% shot and then just follow up with a no scope to finish it off. You will still hit it as long as you don't change aim.

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3, isn't it?

Stay out of range and pump some shots into it.

Go Demoman.  Or go Medic and build an Uber so that you and a teammate can take it out together (preferred method).

Snipers should be offensive placeholders, and defensive suppression.  I love when snipers try to take down my sentries.  So futile against an attentive engy who knows how to use his builds to shield from headshots.

When attended by an engineer, give up or go demoman. An unattended sentry, you're still one of the worst possible choices but you can do it, but you don't need tips to kill an unattended sentry.

Pretty much you need to lure the engie out. Give the sentry a shot and see if this causes the engie to come out in the open. If he's behind, try to shoot his other buildings. If none of this works than go demo, or just wait for the engie to get distracted by something (like a spy).

Also work in tandum with a soldier, time your shots so it hits directly with a rocket and you will bring it down in no time.

There's normally an option, if you can't take out the Engie, take out the dispenser. He's not going to be fixing the SG for long when he runs out of metal. As long as you're not within their range, SGs are sitting ducks for Snipers.