Among my crossbows is a trusted and reliable Barnett Jackal. I've killed three deer with it and my 10 yr old daughter has killed 4 deer with it. With this post are some pictures of her with jackal kills. Couldn't add the other two pictures of her cause it said the file was too big.
To answer your question from an experienced Jackal owner. Yes the scope rail is made of metal. The string should be changed at a bow shop, unless you own a bow press or can make one. I could be wrong, but I believe all compounds need a press. Yes it needs rail lube. I use a rail lube called scorpion venom on all my crossbows... it's the best I've ever used. It shoots 315 feet per second and uses 20 inch arrows. It has anti-dri fire and a smooth pulling trigger. The limbs and stock are camo. The string wear is minimal. I'm on my second serving string cause I shoot alot. On my current serving I have over 150 shots, yet the string looks like new... that's cause of the awesome rail lube (scorpion venom) I use. This crossbow is accurate with both mechanical and fixed blade broadheads. With the speed of 315 fps it allows a wide range of broadheads that can be used with superb accuracy. Many of the super fast crossbows need to shoot strictly mechanical cause accuracy is lost when using fixed blade heads at over 350 fps.
The Jackal is lil on the loud side, but then again I've never heard a quiet crossbow. Still at the speed of 315 fps no deer even flinched at the sound of the crossbow before the arrow was through it. The accuracy of the Jackal is top notch. I can steadily hit pop bottle cap sized bullseyes all day at 45 yards, so can my 10 (now 11) year old.
I do cock the crossbow back for my 10 yr old cause the draw weight is either 130 or 150 pounds. However with the rope cocker it's a synch.
I love my jackal. It's a great crossbow that's very well made by a great company that shows pride in their work. You honestly can't go wrong with a jackal. I'm 39, my daughter at 10 killed 4 deer with it... that has diversity written all over it.
I hope this info helped you out. Good luck in your choice.