So I've had the chance to use this in Dunwich now as Zoey, and I was pretty dubious at first but I think I really like it. I didn't even combo it with any of the ways of getting extra ammo. Ok, the obvious thing to say - in most cases the Flamethrower is better and for various pretty clear cut reasons. But anyway, I had one Flamethrower already and so I got a BAR just to mix it up and try some new stuff and was pleasantly surprised.
The first time I used it got snagged by an Avian Thrall in a very early turn and that really sucked, big time. I ended up spending all 8 ammo in one go to overcome the negative modifier which felt really bad at the time, but it highlighted the flexibility of this gun - not only is the damage flexible, so is the combat boost. Now, ok sure using all 8 ammo in one go is well yeah pretty wasteful, but with the Flamethrower I would've had no way out of that other than to just take a bunch of pot shots and hope I got a 0, or alternatively swap it out for a blade and lose the ammo entirely that way too. Basically, got two rules wrong there. Flamethrower wins that one.
I did get to use it in a more piece-meal way later and the ability to take out a creature with exactly the right amount of health all in one action is pretty strong, stronger than I realised. If you end up spending lots of ammo on a tough monster, you also get a much higher modifier - so it actually scales really well for larger creatures as you're more likely to make the damage stick. To be fair though, taking only one action only really applies to big monster of four or five health, and then you're likely only to get one of those out of the ammo supply.
Another nice point is that with the Flamethrower, a couple of times I found myself wanting to engage more enemies to fully utilise the damage, but that would incur an attack of opportunity. Without the damage efficiency the value of the Flamethrower goes down (though it's still great obviously). Perhaps I should not have swapped out those Taunts earlier? But then we're talking support cards which essentially increases the cost of the weapon. BAR on the other hand doesn't suffer from that problem - exactly as much as you need all in one action. The sheer damage output of the Flamethrower means it still holds up really.
The comparison is more relevant for the Lightning Gun as you really need that second weapon to deal with the monsters with four and five health. This is of course means you need a Bandolier, so support cards again. With two four health monsters you can kill them both in two actions with the BAR. With Lightning Gun you can kill one and damage the other in three actions. Flamethrower says "hi" at this point of course, which is why it's still better, and what it potentially loses in the action efficiency it makes up for ammo/damage supply.
Lastly, I frequently find myself at four XP after a mission too, so this one just feels significantly easier to pick up, even though it's only one XP cheaper.
So the M1918 BAR then? Yeah I actually quite like it.