First, in a hypothetical advanced combat supplement, I'd simply do away with abstracted ammo entirely. I have been gaming for 20 years and keeping track of bullets in a clip has always been fun for us--apparently we are unusual, but we like keeping track of details manually, and yes, we obviously like math, physics, science, etc. You guys should see our original MC character sheets. We used to draw little boxes for every bullet and then repeat that process for every clip. Oh, the memories!
Anyway, let's say we want a mechanism for somewhat more logical, less random autofire WITHOUT completely ditching abstracted ammo. Here are some thoughts on how I would do it:
1. First, and I'm sorry, but I can't get past this entirely, I'd divide every abstract reload into quarters. In other words, I'd change the minimum "burst size" from AN ENTIRE MAGAZINE RAARGGHH (makes machinegun noises) into a quarter of a magazine (actually fairly reasonable and still mitigates the need to track individual bullets). On a character sheet, this is easy, a "reload" is represented by a square with 4 smaller squares inside. Anyway, I think this could be a compromise between firing off a whole magazine at a time with Let Rip and keeping track of individual bullets. The GM can still cause complications with weapon jams and the like, but a player also has a better idea of how much ammo he's going through.
2. I'd ditch Spread as a quality entirely. I don't think you need it with the mechanics I have thought up.
3. Keep the Modes, I have worked those into this system.
4. When you make an autofire attack, you use the same rules that you have now, except you DO NOT add to damage dice. Instead, for every point of momentum generated, you hit with an extra bullet from the burst. This round is at full damage, full effect, and rolls for hit location normally. This is VERY similar to how the original game worked. You MAY NOT spend Momentum to add to damage on autofire attacks--Momentum only would allow you to hit with more shots. Each d20 added to the attack roll subtracts 1/4 of a reload and I would allow anything of Mode >1 to fire at Mode 1 if the player wants to.
4a. I'd also ditch the Ammo (Quality) quality. Instead, I'd make RELOADS that have those properties and for fancier ammo types, I'd increase the cost of the reload, but not its encumbrance or type. If I load an Armor Piercing 1 reload into a weapon, the weapon has that property regardless of whether it's using autofire.
4b. If you wanted to keep Spread on some level, then I'd redefine it--Spread X means that each point of Momentum generates X *additional* hits on the target, representing an extreme rate of fire. This could be a good way to represent LMGs and HMGs (and certain Bauhaus weapons...hint hint) that have high rates of fire without losing the mostly abstracted ammo rules that I am suggesting. You might say, "Ok, that's a lot of dice rolling though." I have two responses: (1) dice rolling is FUN, but (2) ok, then how about you just roll full damage once for each burst, and then just apply that FULL damage/Effect to every hit location, which is actually only a very minor change from Spread as currently written.
5. The corollary of this is that single shots DO let you add Momentum to damage, which in an abstracted system, represents single shots being more precise than a crazy burst.
So there you go, in a nutshell. You have a great core mechanic in this game. You can leverage Momentum spends for all kinds of things, including autofire, in a way that I think is far more satisfying than the current rules as written (RAW, henceforth). And also, I think that the MAIN mistake with the Let Rip and abstracted reload mechanic is just that you went too far. "A whole clip or nothing" is far too binary for a game like MC, especially given the fairly detailed, bullet-by-bullet nature of the original game system. I look at this way: if a game is going to feature lots of automatic weapons, which MC definitely does, then it needs a more robust autofire system than a game where that hardly comes up (1920s CoC, for example). Why? Because you want satisfying mechanics for a common event!