If you use a telescopic sight for hunting, especially hunting dangerous game, the failure of you scope can be a matter of grave concern. For the North American hunter, the failure of a scope will probably not end your life, but it might end your hunt.
The obvious long range solution to the possibility of scope failure is to have a backup scope available. However, when you are 2 hours from camp, and have just spotted a 14 point Boone and Crockett buck, that solution will just not do.
The best short range solution is to have a set of backup iron sights on your rifle. Many factory standard hunting rifles come with iron sights, to which the buyer adds a scope. The problem comes in when you want to use the iron sights while the scope is mounted on the gun. If you are using a scope mount that requires the use of allen wrenches to take the scope off, you are unlikely to be carrying a set when you most need them. Additionally, generally you discover that your scope is broken or unusable only when you've spotted your target and are ready to fire. Few game animals will wait patiently while you break out the tool kit and proceed to dismount your scope.
The solution to this problem that the market has come up with fall into a few main categories:
See through mounts, that have a tunnel through the scope mounts or rings that allow the use of the iron sights and the scope simply by moving your head. The major draw back of this arraignment is that the 1-2" difference is sighting plane makes having a good cheek weld to the stock almost impossible.
Iron sights built into the top of the scope rings work well in that they can be used without major alteration of the shooters position on the rifle. Since most scope rings are only a few inches apart, these type of sights are relatively in-accurate, and as such are only useful for short range shots (probably 50 yards and less).
By far the most common solution is to use a system of scope mounts or rings that allow the scope to be moved out of the way of the iron sights. These are generally7 know as "Quick Disconnect" or QD systems. The systems that are on the market that I know of are:
Warne: Warne makes a set of rings that have a lever that when rotated a quarter turn will release the rings from the scope base. They also make a rear sight base that incorporates a peep sight, so no additional rear sight is needed. They cost ($49 dealer, $70 retail) from Brownells for the peep base and ($70 dealer, $100 retail) for the rings. They also make a 30mm ring, for about $10 more. These rings are made in styles to fit Warne/Kimber, Ruger, Sako, BRNO, and tip off 22 bases. The standard Warne bases cost about $9-13/ea.
Pinkington makes a conversion kit to allow the Redfield/Leupold/Burris/Browning bases to be quickly removed. This conversion kit replaces the screws that traditionally hold the rear ring with a single lever. Once the rear base is free, the front is twisted out of it's base. The potential for damage to the scope tube while using it to twist out the front base can not be ignored. These are available from Brownells for ($21 dealer, $27 retail)
Finn Aagaard uses this system on his light rifle, and is satisfied with it. Given his experience, I'm not going to argue with him.
Dave Talley makes a double lever scope ring set, that requires that a gunsmith make custom bases to fit the rings. These go for ($140 dealer, $175 retail) from Brownells. Add $20-25 for 30mm rings.
Brownells list 'Pivot Mounts' that are ring-base sets that are hinged on one edge of the ring-base set. The locking mechanism appears to be a flat spring that keeps the hinge closed. ($5.50-$7 dealer, $10-12 retail)
Leupold makes a unique (?) QD ring-base set: the double lever system has the levers on the bases, not on the rings. These QD rings go for ($22 dealer, $31 retail) while the bases are in the range ($48 dealer $68 retail)
A.R.M.S. is reported to make a QD ring set that fits on Weaver bases. About $70.
In Europe, the Suhler Einhakmontage or "claw mounts" are very popular, though to be properly fitted takes a significant amount of a gunsmiths's time, and thus can be very expensive. I don't have specific brand information on these mounts, if you do, send it to me, and I'll see that it gets put in this FAQ.
If anyone knows of any other makers of QD scope systems, please contact me. Also, anyone who has used any of these systems, please feel free to comment on the product, I'll try to incorperate these comments into this FAQ.