I do not think any optical type sight whether scope or red dot will last very long mounted to the slide. Might wanna reconsider that option and for sure ask the manufactuered of it if they think it capable of holding up.
About 1/2 of the bullseye shooters that have optical sights use a slide mount without any troubles. Some of the top shooters, as it seems to be a matter of personal preference. there is a single type that is preferred by most shooters, with no problems. I follow a bullseye forum with a number of top shooters and they have run that thread a couple of times with the same results. Go to bullseyepistol.com and check out the information there for very good advice on everything involved in bullseye.
My primary wadcutter match .45 has an Illusion Red Dot scope mounted in Tasco rings on a Weaver style base mounted directly to the slide. It's held up for over 10,000 rounds and got me into the NRA master class, so I guess the combo works. The Illusion 1" scope is not readily available now days, so UltraDot would be my choice. Discounted, they can be found for about $100.00. Note that the Weaver style base is a specialized base, and has a slight taper milled into it. Also, mounting a scope on the slide means you have to adjust you recoil springs to your load, and the loads are only light target loads, not hardball or other high velocity/high performance loads. The balance is so fine that you may have malfunctions in cold weather if you don't bump up your load slightly. It will require a firm, consistent grip to prevent catching cases, but then again, that the nature of the 1911 in general. The extra weight on the slide slows down the recoil cycle, but for me the firing cadence was much like my hardball/Service Pistol cadence. You should also add an extended ejector. The scope base and extended ejector can be gotten from Gil Hebard Guns, 125-129 Public Square, Knoxville, Illinois 61448. I've dealt with them for 30 years and they have been consistently good folks to deal with.
Kragsman: You can still purchase scope mounts for the 1911 that replace the left side grip. They are designed to allow access to the slide for normal functioning. f you choose to try one of those you will at least be able to see how she does without drilling the slide first.
The reason I went with the slide mount is that it mounts the scope as low and close to the bore as possible. All the grip mounts I tried put the scope much higher, and gave the pistol a top heavy feel. Again, you can't shoot hardball with a grip mount as the recoil will break the mount. (Personal experience in addition to manufacturer's recommendation.) All in all, which method you choose is a personal decision, but you might check the preferred method of the top shooters at Camp Perry (home of the National Pistol Championships). I think you will find the slide mount predominates.
Now that Tasco went out of business you might find a optima cheap cheap cheap. I picked this one up for 40 bucks on clearance and the base was $7.99 through CDNN with this base you do not have to do any drilling it uses the sight dovetail. Jim Ballester Molina pistol with optima sight
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