Now that I run my SBS 500 suppressed, I am shooting through the can. (It has a tritium XS sight installed)
I would like to put a Holosun 407K on it.

Too much of a PITA/risky to do yourself? If so, which vendors would be best for this?

Probably not enough metal there to be able to mill it, why not mount a rail section?

ETA - No drill mount.

Thought about something like that, but how sturdy & reliable are they really? Also, do they work with a side saddle?

Obviously they can be milled, Mossberg sells them that way now. I'm not really sure what the best move in this instance is. If it wasn't a SBS, I'd say just buy a new one that is already milled and tapped. I'm not sure how deep you can mill them. You could ask Mossberg and maybe you will get an answer. Or you could find a new "OR" model in the store and measure it. I have no idea which vendors would mill this for you.

For whatever dumb reason Burris Speedbead never made a mount for Mossberg, so that's not an option.

Those no-drill mounts are trash. They don't even work the first time, you can wiggle them with your little finger. I'm not sure why they didn't disappear from the market 20 years ago. Those things came out decades ago for the growing popularity of longer range turkey hunting and putting scopes and tube red dots on shotguns. They kinda-sorta worked good enough for that. They are not something you want to depend on.

If your Mossberg is from the last 10 years, it should already be drilled and tapped for a rail anyway, so it would be extra stupid to buy the no-drill mount. You don't even need a rail, you can buy red dot adapters such as this, you just have to find the one that fits your red dot. https://www.amazon.com/dp/B0CGVXV2HB?tag=arfcom00-20

Myself, I am going to buy a new 590S 18.5" optics ready model 51607.

Mine is not milled. It's an older police trade-in. I'm going to give that Weaver mount a try. If it's junk, I'll return it. For $35 it's worth the experiment.

Just use Esstac shotgun cards and then you have a removable side saddle that you can slap whatever load you want up there - slugs, buck, breaching, geesemen - whatever the threat needs.

Full disclosure, I'm a gunshow Esstac dealer, I do not deal online & this isn't an offer to sell.

No it isn't worth it, I'm telling you it is grade a junk. It's a sloppy bolt through the trigger pin hole, and one screw in the ejector screw hole. The whole mount is thin and flimsy. You can't even tighten the bolt since it locks up the gun. You could honestly get a more sturdy mount by T rex duct taping your re dot to the frame. You would be better off just drilling and tapping yours, you can do that in your garage with basic hand tools.

I'm going to try the Weaver & an aluminum mount. If they both suck, they get returned & I guess try to find a company to mill the receiver.

Probably better off having the receiver drilled & tapped for the standard Picatinny rail, then you can mount anything on top.

LOVE my 590S. Haven't put an optic on it yet, but going to. That gun eats ANYTHING.

My 500 is older than ten years but still came with the 4 screws on top of the receiver to pull and put a piece of pic rail on it.

They've probably been doing it a long time. I don't know what year it started

Defender Tactical has a RMR mount that fits the drilled/tapped factory holes on the top of the receiver.  It has built in iron sights (uses Glock sights) and co-witnesses with the red dot.  While not as sleek as the Mossberg factory option, it is definitely a nice aftermarket solution.

If that's an option it's probably the simplest. With Remingtons I've seen it done a bunch I assume there's plenty of metal on the Mossberg guns. If you found a specific mount you liked you could probably have it mounted directly to the receiver as well if you didn't want a rail for some reason.

If you can find one in stock, EGW makes a nice Pic rail for Mossbergs.

https://egwguns.com/mossberg-500-590-835-930-935-all-years-shotgun-picatinny-rail

Googled vendors for this & came up with nothing that looked good.

Was much easier to find a vendor to cut & thread the barrel

You're over-thinking this, if you've got a drill press just drill and tap the receiver for a rail/mount; the receiver is aluminum so it's not hard to do (I've done it to two of them now).  You can do it with a hand drill (see second video) but a drill press is a better idea since it's easier to secure it better.  If you don't have a drill press or you aren't comfortable doing it yourself just have a gunsmith do it.  Make sure that your screws fit properly/don't protrude into the receiver before putting Loctite on them and you'll be fine.  


First example that popped up on youtube:


Example of a guy doing it with a hand drill on a Benelli (just to show how easy it is):



ETA- You can still find the old B Square mounts on Ebay sometimes and Aimtechs are available from Optics Planet and some other places.  They work well enough it you get your screws tightened properly, not too tight to cause binding, and Loctite them so they don't come loose.  You can add a bit of 3M double sided tape to help ensure they don't move if you're worried about it.

I slapped a Weaver saddle mount on it. Instead of adding a riser to the Kicklite stock on it, I decided to order a Magpul SGA stock & riser. We'll see how it goes. I did that because I would rather still be able to reach the action release w/my shooting hand.

Yes, I know about the Shoten release. If I don't like the SGA, I can sell it & get the Shoten release to go with the kicklte

Broke the setup in last weekend. Works well. I'll keep it as is

@RangeToy

I bet the same vendor that cut the barrel could drill and tap the receiver for an optic rail if you ever need that done.