I've been a gunsmith for about 10 years now, and precision bolt rifles, ARs, and 1911s have always been my specialty... For years, I have always drilled/tapped scope bases the hard way... Doing extensive measuring, marking, finding the perfect centerline on the receiver, then finally, drilling and tapping... Obviously, this is a tedious task, as a slight mis-measurement can totally ruin a customer's receiver... Even if you mark the hole positions correctly, drilling each hole exactly straight, and tapping each hole exactly straight, takes skill. I seen this Williams Scope Mount Drill Fixture for sale on Brownells, and decided to add it to my inventory... someone broke into my workshop a while back and stole a ton of my reloading dies, tools, ammunition, and 2 guns... So I took out a loan to restock my workshop (as well as beef up security...). The tool is here: SCOPE MOUNT DRILL FIXTURE | Brownells Costs about $100, but it is WORTH EVERY PENNY! You position this on the receiver, and close it against the receiver, and the tool's design, working off of the concept of "scissors" or sliding parallels, automatically centers itself over the receiver, regardless if the receiver is round, hex, octagonal, etc.... I use a piece of heavyweight drawing paper to put between the tool and the receiver to ensure nothing gets scratched.... After it is in place, you install one of the two included hardened, carbide bushings, set the clamped unit into the drill press, and drill your holes. Then, (if you used the smaller bushing), install the larger one to guide the tap, and tap the holes. The bushing ensure the threads are started perfectly. There are other bushing sizes available, but the two that are included should be all you need... One for 6/28 and other for 8/32, if I remember correctly. The tool's "arms" or "guides" can be positioned in 9 different holes, which covers almost any combination out there... I can't think of a single gun that this thing wouldn't fit. So far I have drilled and tapped scope mounts for a Remington 870 shotgun, Mosin Nagant bolt rifle, Winchester 94 Pre 64', a Smith and Wesson 1000 shotgun, and a Mauser 98 (Interarms), as well has re-drilled damaged base mounts on a Marlin 917-VSF and enlarged the holes on a Winchester 70 Laredo.... I couldn't ask for better performance! All firearms turned out perfect. So far, this is one of the most outstanding tools I have added to my inventory - it saves me so much time, effort, and aggravation, I just had to get on here and praise it! The ONLY negative thing I can say about the tool is that it doesn't come with it's own storage container... A tool this nice should come in a plastic, padded case or something... But, that can be added.