My shooting buddy and I have used Trail Boss in his 500 S&W for practice and familiarization loads. And also for just plain fun.
Seven grains is a bit too light and we get a lot of soot on the outsides of the cases and on the chamber walls because the pressure inside the case is just not enough to make the brass seal against the chamber walls. Thus, blow-by that has been mentioned already. 8.5 to 10 grains of Trail Boss is still a little dirty, but acceptable and yields 700 to 800 fps (as I recall) with a 325 grain bullet. We can watch them fly downrange!
For over thirty years I have fired 357 Magnum cases exclusively. I have only ever owned one box of 38 specials. But I shoot a LOT of bullets at 38 Special velocities using very fast powders. Bullseye and Unique mostly. My 44 Magnums have never seen a 44 Special, but a lot of 44 Special loads.
Shooting Special loads in magnum cases (38 or 44), I have never gone VERY far down in power. I stay in the high Special range, lest velocity fall so low as to allow a bullet to stick in the barrel. So, there's my testimonial.
Here's a caution. Heavy crimping will shorten the life of your brass. The working of the case mouth will make it get brittle and eventually you will notice a crack (usually right after belling the case mouth - another reason to use a single-stage press rather than a progressive) in the case mouth. At that time I would consider cutting the length of the Magnum case to the Special length. You will give it a second lifetime.
We found, when shooting less than full cases, that we get higher velocity when the powder is near the primer than if the powder is at the front of the cartridge case. (We pointed the gun up and then slowly leveled the gun to the target and chronographed the results, then pointed the gun down and slowly leveled to the target and chronographed that speed. Even with Trail Boss, which was said to be insensitive to position, it made a significant difference, 50 fps or more or 10%). If you are shooting for accuracy in light loads, keep that in mind.
Suggestion: When I started shooting, only the rich or those in the shooting sports industry could afford chronographs. Now, a C-note can get you a decent instrument. For the handloader, it is a handy tool and a wonderful toy. You can also monitor your powders' performance. For example, you can see if you are getting velocities below 44 Special minimums out of your Magnum cases. I would consider myself in danger of getting a bullet stuck in the barrel if I went below any published velocities.
P.S. I saw this thread and just had to add the link:
http://www.rugerforum.com/phpBB/viewtopic.php?t=64827
or, if the link does not work, paste this into your browser rugerforum.com/phpBB/viewtopic.php?t=64827
The comments there might be illuminating.