Is the Surefire 918 shotgun forend any good?  Is 125 lumens practical on a defensive shotgun, or should I just get the 60 lumen SF618?

I have the 918 for my 870.  It is somewhat heavier than the 618, so I think that is what you should consider rather than the lumens.  I took off my sidesaddle and mag extension when I put the 918 on because the total weight of the shotgun became more than I wanted.  However, I still would rather have the extra lumens of the 918 over the extra ammo capacity for a bump-in-the-night gun.

Do you happen to know how much each one weighs?  

What makes the 918 heavier?

M918 is a 9-volt system, M618 is 6-volt.  It's longer and heavier by one CR123 battery.  

I was shopping for a 618, but went with the M918 because I have an M900 for my carbine.  Instant parts compatibility and potential spares, plus one fewer type of bulb to keep around.  

I got mine from C4iGrant at G & R Tactical.  

The 6V (two-SF123A batteries or one SF223AC battery stick) model 618 can accept the P61 High Output Lamp Assembly (120Lu/20min) which although has a shorter runtime than the standard P60 (65Lu/20min) lamp assembly, I believe is well worth it for the output.

The 9V (three-SF123A batteries or one SF323AC battery stick) model 918 can accept the MN11 High Output Lamp Assembly (225Lu/20min).

You could order the 618 and P61 - and if you find yourself needing more light, or more light with longer runtime you can "upgrade" to the "Millennium" Lamp Module - the LM90 which converts your 618 to a 918.

Al

IMO the 6 volt unit is all you need on a shotgun.  Work has them on our 870's and I have one at home on my HD 870.  I have a 9 volt on my AR, that's where you need more reach IMO.

Can C4iGrant or BrightFlashlights comment on this?

I have the 918 FA on my 870 and have been very pleased with it. The only thing I would comment on, and this may never affect you, is that I run a tip off filter on mine. I noticed under shotgun recoil that that FM 13 filter will slide forward and eventually completely off the light.  I had this happen twice with it fully in the locked/cammed down position.

I sprayed some adhesive on the bezel and reattached after carefully marking it's correct position with a witness mark as to how i wanted it to open. I also taped off the areas where I wanted the adhesive to stop and have had no problems since.