I had thought there was a pretty severe accuracy penalty too, but when I checked it ingame it came up as +0... not a bonus, but not really a penalty either. Referring to the wiki shows a -2 on the mod, negating a +2 on the rifle. I had an A mod I was thinking about slapping on it, but I was kind of holding out for a sniper mod.
Think about it this way - the Nuclear Burst Cannon I made starts out with 8 round bursts. The mandatory 2 levels of TH required for MAc bring it up to 10. Burst Cannon doesn't negatively affect fire time, and I was rolling with 3 levels of Fin by the end, so I'm putting a massive amount of rounds downrange in very little time - and since enemies tend to be clustered when they spawn, even the rounds that aren't hitting their target are often hitting something.
The biggest drawbacks to a burst cannon are that it drains the magazine very quickly due to the massive burst size, and reloading time is dangerously long(not to mention how quickly you'll go through cells). This makes it generally only viable for a switch to put a large amount of plasma on a dangerous target in a short time, as described above, but doesn't really allow sustained firing, much less chainfiring. Of course, Ammochain completely negates the first drawback, and the fact that I put it on a Nuclear rifle negates the second(and third). What this means is that I actually can chainfire, and holy god does that make for huge bursts(15! per trigger pull) once it gets up to full speed. I don't care how crowded your level spawn is, you will always run out of targets before you even come close to running out of ammo, and with the nuclear regeneration rate, you'll be at full charge before you find more.
@Zicher, I'd been attempting with Int->Int->SoG, for pseudo ESP, or SoG->SoG->Dualgunner, assuming the double damage was worth the slight increase in fire speed, but perhaps I was misguided. I'll have a look at your method tonight - it builds well into MSs, so perhaps I'll get a shot at Marksman Platinum as well if I pull it off.
I am, of course, playing AoMr, as that's required by both badges, Arena Gold and Marksman Platinum.
Edit: Mission complete, at least for Arena Gold - wasn't able to make it even as far as the Anomaly, though - Phobos 6 and 7 were both maze levels which were not kind to me. At any rate, thanks for the tip - attack speed is quite powerful. I suppose that makes my case for a MAc + Fin3 Burst Cannon even stronger...