The M12 Fuel 3/8 stubby I ordered showed up today. So FWIW to anyone who cares, I did some testing with all the electric impact wrenches I currently own. 3 3/8" impacts and one 1/2" impact. They are the Craftsman C3 3/8" Impact Wrench (ID2000) on a 18 nominal volt, 2.0Ah battery. The Craftsman C3 1/2" Heavy Duty Impact Wrench (ID2030) on a 18 nominal volt, 4.0Ah battery. The Matco 3/8" 16v Infinium Impact Wrench (MCL1638IW) on a 14.4 nominal volt, 2.0Ah battery. The Matco M12 Gen 2 Fuel Stubby 3/8" Impact Wrench (2554-20) on a 10.8 nominal volt, 6.0Ah battery.
Manufacturer ratings:
Matco: 70 ft/lbs
Craftsman 3/8": 130 ft/lbs
Milwaukee: 250 ft/lbs
Craftsman 1/2": 300 ft/lbs
Not part of the test (because I sold it), I previously owned the gen 1 M12 Fuel 3/8" impact and found it worthless. Any of the 3 3/8" impact wrenches above are superior.
First, a thought on size. The M12 Gen 2 fuel is a clear winner as far as length is concerned, hence the "stubby" nomenclature from Milwaukee. It is huge in every other dimension. As with all brushless tools, the motor is much bigger than a brushed counterpart, so the body on this is beefcake. It is bigger round than even the 1/2" Craftsman. With the 6 cell XC battery I plan on using with it (M12 tools are noticeably more powerful with an XC), it is also the 2nd tallest, taller even than the C3 3/8" impact. Though it is not nearly as stubby, one could make a case the Matco is actually the smallest. The body is small, it's not very tall, and it is fairly stubby. This tool is just light and tiny in the hand. I always liked that about this impact. It fits in places really well. It also has a small battery, (although a 3 cell M12 pack would be smaller).
Testing on power. Without being specific or scientific, I did some testing on some lugnuts and a GM 3800 crank bolt tonight in the garage. The 3/8" impacts are all very close to each other in power. It took a lot of swapping back and forth to actually figure out which had more power than another. I eventually concluded the C3 3/8" has more a little more power than the Matco (as I had concluded in the past), and I concluded the Milwaukee (on its highest power setting) has a little more power than the C3 3/8". The Craftsman 1/2" was in a league of its own with substantially more power than its smaller 3/8" counterparts. It may not have the nearly the power of these brushless 18v behemoths from Milwaukee M18 and some others, but there is nothing wrong with the old C3 1/2".
I was VERY disappointed in my Gen 1 Fuel M12 from a few years ago, and although the stubby is much better than that one, and it's the "best" of the 3 3/8" impacts I have here, I'm disappointed in it as well. It is rated very nearly double the C3 I have been using at work, yet it barely has more power. It will be nice that it is smaller and will work with all my other M12 tools at work, but I wish I was getting a power bump over the C3 as well. I'll bring the C3 home, and all my work tools will be M12, all my home tools C3. The venerable Matco I'll sell or give to somebody since I don't really need it anymore.
I wish I had the new brushless version of the Matco 16v Infiuium 3/8" to test, but we lost our Matco dealer a couple years ago. It is rated 250 ft/lbs like the Milwaukee, and I have a feeling it's closer to that spec since it has more voltage than the Milwaukee and Matco actually under-rated the brushed version (70 ft/lbs, but it would remove lugnuts torqued to 100 ft/lbs without too much struggle). It's insanely expensive, but I have a feeling it would be the best compact 3/8" cordless impact wrench out right now.
Picture rankings in order of power (weakest to strongest) and rankings in terms of size (smallest to largest. Matco and Milwaukee tie).