[thefro526]

Simon, there are a lot of things going on here, and I can see why it's a bit easy to get lost in all of it.

When looking at motors, specifically those of the same 'size' or 'class' you'll see that they can vary wildly. What you're seeing with the 775-12 and the 775-18 also happens with various 500-series motors as well - just look at all of the different FP's over the years, the BB 555, 540, AM 9015 etc and you'll see what I'm talking about. Differences in the motor's internals will cause differences in it's characteristics some of which can make the motor more or less powerful. The most important thing to take away from this is than not all motors in the same size class are equal - if you really want to understand why exactly that is, you'd need to read into DC motors a bunch. (To be honest, I don't know well enough to even attempt to explain this in depth.)

Now that we've gotten that out of the way, it sounds like your problem is a fairly common one. 300-series, 500-series motors and 700-series motors are all cooled through a fan connected to the motor's output shaft. When you stall the motor that fan isn't spinning. Now you've essentially removed all of the motor's cooling (other than the case) it's very easy for them to overheat and burn up, especially when loaded. One of the most important rules to remember when using 500-series motors (FP's included) is that they shouldn't be stalled ever - if possible. I believe the same is true for the 775-12 as well, but I'm not entirely sure.

The 775-18 is a bit of a different animal. It's a motor designed to run at 18V that's now being run at 12V. Because of this, it's a bit better at handling excessive loading and stalling in some applications. In 2011, the team I was with used two 775-18's on that robot's shoulder joints and prior to the machine's redesign, we'd routinely stall the 775's for 15-20 seconds without issue. That being said, 775-18's did have a problem with case shorting when they were first introduced, and since then the general consensus among many people in FRC is to avoid them like the plague.

Now that we've gotten all of that out of the way, here are a few things that you can try to make your current mechanism perform as intended without major modification

Passive Assist - Depending on the exact specifics of your shooter's pivoting axis, you should be able to add some sort of assist without too much difficultly. This passive assist can be as simple as some surgical tubing mounted just right, or as complicated as a series of gas struts/springs/whatever. In any case, the assist should balance the shooter in such a way that it is 'weightless'. Once you get the shooter to where it's balanced fairly well, your motor has to do MUCH less work - ideally it's just overpowering the assist - which should keep you from frying more AM 9015's.

Active Assist/Position Hold - Again, depending on the exact specifics of your shooter, you can look into adding a mechanism to actively assist the motor and/or hold the shooter in position without having to power the motor. There are multiple methods of doing this: Disc Brakes, Pin Brakes, Worm Gears, etc will all do the trick.

OR, if you really want to, you can swap out the AM 9068/PG71 combination for a motor that's a bit more robust. I know for a fact that the BAG motors take stalling in this application REALLY well (it's a 'CIM' afterall) but, switching to one requires the use of a versaplanetary which may or may not be a pain in the butt.

Personally, I'd start with passive assist, as it's usually the easiest thing to play with. Add surgical tubing/springs/rubber bands/etc until you get the desired performance and you're set. By adding or removing assist, you can tune the mechanism to do some cool tricks like being 'self opening' or being perfectly balanced in a certain position. Also, if you get lucky, the PG71 might have enough internal resistance to where you only have to get close to the 'perfectly balanced' setup for the mechanism to stay in place.

I hope all of this is enough to get you pointed in the right direction. If you need anything else, feel free to shoot me a PM.