Sorry for the long post. Once I start, I can't stop. I tend to think through some of my responses as I write them...and I generally learn something in the process. I hope someone else might, also.
This is just an IMHO thing, but I'm not a fan of flashlights mounted under the barrel for HD shotguns. Here's why:
Two quick reasons that I'll mention up front, then dismiss right away: 1. The combination of the mount and actual standalone flashlight sitting all the way up at the end of the barrel really changes the balance and effective weight of a longarm that's already fairly heavy when fully loaded. 2. I know I'm in the minority when it comes to slings on defensive shotguns (that's just how I was taught to use 'em and what I became accustomed to), but an under-the-barrel light mount, at least the ones I've seen, preclude the use of a sling.
Now, to me, the more important reason is number 3: offset from barrel. 'Cause, to be realistic, with an HD shotgun we aren't talking about an under-the-barrel light mount: we're talking an
under-the-magazine-tube mount because any respectable HD shotgun will have a short barrel--no more than 20"--and will have an extended magazine tube for greater capacity. Which means the end of the barrel and the end of the tube will be pretty close, too close to mount a standalone tactical flashlight under the barrel without mounting it under the magazine tube.
If you want a light mounted on your HD shotgun, then I presume you're preparing for the possibility of needing to move out of your safe room and either clear or secure a building. If you're going to hunker down behind the bed, call 911, and prepare to blast whatever comes through the bedroom door (generally the best strategy, BTW), you probably don't want the additional weight a flashlight adds to your weapon. You don't need a sling, either.
But if you're gonna move around inside a house with an HD shotgun using a barrel-mounted light, you have a couple of problems.
The first is due to the length of the shotgun. It's a superb securin', breachin', and blastin' tool, but the 18-inch-barrel defensive shotgun is 38" to 39" long, and is not the best option if having to clear the typical residential 3-foot-wide stairwell, or bedrooms leading off the typical 4-foot-wide hallway.
Before anybody says anything about no-buttstock pistol grips: no, I don't like 'em. A shotgun with a regular buttstock held in assault-fire position sticks out no farther from the body than a "stockless" alternative. But you can't go to a point-shoulder firing position if the gun has no buttstock. If you ever need to--accurately--fire a few rounds of standard-load 12-gauge in quick succession, you need to have a stock tucked into your shoulder.
"Securing" generally means you are confident your residential third-level security zone (the actual doors and windows) have not been breached. You are moving out of your safe room to either confirm security of entry points, obtain additional information about the situation, or to post yourself at a predetermined place from which you can best stop an anticipated criminal breach. This assumes all "friendly" occupants of the house are accounted for and in the safe room.
A 12-gauge is primo for this. However, almost all urban areas have some ambient light from adjacent houses, streetlamps, etc. And you know the interior of your house better than the bad guys. Using a tactical light inside the house if it has not been breached will telegraph your position to those outside.
"Clearing" generally means you are compelled to investigate and render neutral a structure the integrity of which is not known; it may already have been breached by one or more bad guys. This is dangerous stuff, and even the pros' SOP is to never attempt it without two operators.
I can think of only one good reason to clear a house I believe had been breached: there are unaccounted-for family members. You may decide that defending possessions is important enough--or even exiting the house to clear your residential second-level security zone (pets might be out there)--but I'll leave that up to you.
Not only is a shotgun an imperfect clearing tool because of its length, it also isn't the best tool for delivering a precise shot. If you pie-slice a corner to find some guy with a ski mask and a knife holding your daughter 15 feet away, you may need to take a very precise shot. You want a single, accurate round for that, not an expanding cluster of pellets.
Rule of thumb: Never carry a long gun without having your sidearm at the ready. At close quarters, it isn't difficult to move inside a longarm and take it away. Your handgun is part of your weapon-retention system (as should be your hand-to-hand training), and you may need it to execute precise ECQ-distance shots.
Back on the specific topic, let's go back to that under-the-magazine-tube flashlight mount. It looks something like this:
From the top of the barrel to the light, we have 3- to 3.5-inches. That's a lot greater than the sight-offset of a standard AR-15. And if you've had any defensive carbine training, you know how easy it is to shoot smack into your cover because you can see a good sight picture...but failed to allow for the offset.
To clear a room, you want to expose (and telegraph) as little of yourself as possible while you "pie" the room, slice by slice, from one extreme edge to the other of the view presented by the doorway. Geometry and distance are your friends.
In a four-foot hallway, a shotgun allows you very little distance from the doorway...in some spots fewer than 5 inches when you allow for the depth of your shoulder. Now, envision yourself using a flashlight mounted three inches or more from the top of the barrel and your sight picture.
How do you slice that pie?
Without the flashlight, you would keep your elbow tucked tight, and cant the shotgun outboard so that the sight picture leads into the clearing area.
If you are relying on that bottom-mounted flashlight, you can't bring it to bear in the first 30 degrees or so of your sweep without either letting your elbow swing out to become vulnerable, or sacrificing your shoulder mount. Neither are great options.
If you're gonna go for a barrel-mounted light, my preference would be something like the
LMS-1202 light/laser combo from Lasermax.
It's designed for the light and laser to mount to the sides, centered in between the barrel and magazine tube. Also consider that a separately-activated laser could be a huge force multiplier in a nighttime situation.
Let me clarify something else about a tactical light: it isn't used like a standard flashlight and should never be in a constant-on state. That's one of the inherent problems with just sticking a flashlight on the end of your barrel.
The purpose of a tactical light isn't to brighten the landscape and show you what's out there. Its purpose is to identify targets. If you've had any low-light training, you know the mantra is "flash and move." Your shot will typically be taken just after--or a millisecond before--the light has flashed off.
Light on: only for target identification; light off: move and shoot.
A constant-on light is not a good idea. If you have a constant-on light, you may as well be painting a target on yourself.
A simple bottom-mounted flashlight on a shotgun implies that you'd need to reach out and turn that sucker off and on with your offhand. It looks great in movies when you have soldiers moving through a dark, cavernous warehouse with lights on all their weapons flaring and sweeping all over the place, but that ain't real life.
Any self-respecting tactical light needs a momentary on/off pressure switch. Regardless of how it's held or mounted.
Last up, if you have a nifty $40 solution to a $200 problem, I strongly suggest you go out to the range and simulate, as best possible, the use of that light in practical conditions. Turn it on and fire 50 or more rounds of 12-gauge in three-round rapid bursts. If it holds up, it's a great buy. My bet is that anything purchased at Home Depot for forty bucks ain't gonna stand up to repeated 12-gauge blasts when mounted on the front of that recoil.
