Drilling and Tapping Shotgun Barrel for Scope Mount

[maintenanceguy 510]

I want to mount a scope base on a 20ga shotgun.  I ordered the base and was planning to silver solder it to the barrel.  The base came today and it's aluminum - so no silver soldering. 

 

Plan B is to drill and tap the barrel but I suspect I don't have enough metal to do this safely.   I'm hoping somebody knows the real formula or rule about how much chamber wall must be left undisturbed.

 

The thickness of the chamber wall - right where I hope to drill 3 holes is .143" thick.  The screws are 6-48.  Ideally, I'd like to have a thread depth at least the same as the thread width of .135".  This leaves only .008" chamber wall which seems way to thin.  I'd even need to use an end mill instead of a drill bit to get a flat bottom hole and I'm not sure a bottoming tap would actually reach all the way to the bottom of that flat bottom hole.

 

But...then again.  I'm not reducing the chamber except for three tiny spots and those spots are reinforced by the machine screw so maybe I'm okay.

 

Anybody know the "real" rule about this?  And if it's not a go, is there some option other than silver solder to stick an aluminum base to a steel barrel?

 

Thanks.

[maintenanceguy 510]

Anybody?

[louu 399]

Pretty much every slug gun I've seen has had the scope mounted to the receiver. 

[PK90 3,570]

Receiver only.

 

Sent from an undisclosed location

[Parker 213]

Unless the barrel is rifled, I don't see the merit or need for a scope unless your eyes are going. (I know all about this problem.) Soldering bases to the barrel are a better idea than drilling and tapping an already thin tube. 

 

Better: Drill and tap the receiver (side mount or go top mount if there is enough metal.) That's how every proper slug gun was set up in the 70's when slugs were allowed for smoothbores here. Remember that each time you remove the barrel for cleaning you may lose zero, depending on the fit of barrel to receiver and lockdown nut. (There's info out on Shotgunworld for shimming receivers with aluminum or brass if you do this.) That's why cantilever mounts are so in vogue for mounting scopes today.