Should I use 6-48 or 8-40 screws when mounting a picatinny rail for a red dot? It will be on a 20ga 870.

Also, if anyone knows the length that'd be great. I can always cut them down and chase the threads I guess. I know the rear of the receiver is probably thicker.

Clamp it on the rib.

6-48 is standard and all I ever use unless one is stripped and needs to be bumped up to 8-40

Bbl I have is smooth as a catfish's belly

Picatinny clamp for barrel

I prefer the speed bead. I had my 870 drilled and tapped. It worked fine, but it's a little high. The speed bead mounts the sight lower where you can still look down the rib. It also mounts it closer to your eye, which makes the sight itself disappear and all you see is the dot. I don't like one mounted to the rib at all.

got it, I'll look into that

7x57 I'd look into calling William at Sumtoy and ordering one of his mounts. Just tell him what gun you've got and what red dot you're going to mount on it and for $50 he'll send you a mount that's rounded out on the bottom to fit your guns receiver.
The two of his I've used have fit the contour of my receivers like a glove and both sit very low.
This thread has some info and pictures of the Sumtoy mount
http://www.aldeer.com/forum/ubbthreads.php?ubb=showflat&Number=3613736
Good luck with whichever route you take! Post a pic of whatever you end up doing

Against my better judgment, I tried a Sumtoy mount recently. I can’t stand that thing. I had much rather have the Burris mount that all my other shotguns have. The Sumtoy seems to sit up higher than the Burris mount, as well.

N2 that Burris does sit lower. I found this pic earlier. Burris 336 on top Sumtoy middle picatinny bottom

The Burris mount may sit a little lower, but with the Sumtoy mount being contoured and the Burris mount being flat, the Sumtoy isn't as high as it looks in the pic. Either is better than mounting it to a rail.

This Scalarworks mount with a RMR sits very low. D&T and Cerakote on mount was down by Inverness Outdoors

The Sumtoy was a good bit higher when I compared mine side by side.

I don’t like the looks of a rail. Do they offer it the same size as the red dot footprint? I used Inverness Outdoors for a recent dip job.

The Scalarwotks is only compatible with Trijion RMR; the mount actually replaces the baseplate of the RMR so it can sit as low as possible. Not cheap; but a clean setup.

Word

Half the reason to put a red dot on is you don't have to get your head as low on the stock. Also, clamp on rails are like $20.

Not to me it isn't. I want the dot mounted low for the same reason I want my scope mounted low on a rifle. I want the dot to be where I'm looking when I throw the gun up. It feels natural for me to have a good cheak weld on the stock. You can't have that with the dot mounted high.

I use whatever is provided with the base I am using.
Most of the time that is a 6-48 unless the Burris 336 base is used which comes with 8-40 screws.
Sumtoy mounts are hit or miss on screws sizes.
I've seen them come with both size screws so I never drill holes until I have the base in hand.
6-48 allows room to fix threads if they strip also

if you have a base with no screws, screw length will depend on the base and the gun.
A 1/4" should work with 95% of applications though ... at least with a little trimming

Well I finally had a night not studying for exams or clinic procedures so I D&T'd my 870. I'm very pleased with how it turned out. Hardest part was squaring everything up on the drill press, then locking that in place, then triple checking everything. Mount is a Sumtoy. Told them what I wanted and they said can do easy (great dealing with them--no affiliation). I'm really happy how solid the whole setup is, and I'm glad I went with the 8-40 instead of 6-48. I put the base as far to the rear as I could before it started to gap. Plenty of adjustment left on elevation. Bore sighted it with laser and hopefully can get it on paper this morning.

The only thing I'd change is screw length. I have more threads that I can engage, so I might do that at a later dated, but would most likely need to dress them down.

Thanks for all the guidance--you know who you guys are.

For future reference 8-40 NS screws required #28 drill bit
8-40 screws torqued to 12 in-lbs with blue loctite