Hole over chamber neck clearance

[Junior1942]

Guys, is there a rule-of-thumb about clearance between the bottom of a drilled scope mount hole and the top of a chamber or bore? This particular hole, should I drill it, would be for a 6-48 screw located directly above and maybe 1/4" in front of the chamber neck of a M44 M-N. There's about .340" between the top of the neck and the top of the receiver.

Edit: I just double checked my measurements, and there's .392" of metal thickness where I need a 6-48 hole. If I drill about a .192" deep hole will at least .200" be enough barrel metal in a high pressure area?

My 6-48 bottom tap has about a .060" tapered tip with no cutting threads. At .021" per screw turn, I need about .126" for 6 threads + about .060" for the tip = about .186" hole depth.

[Junior1942]

I just measured how much the two 6-48 front screws protrude from the bottom of a Mauser scope mount--about .070" or about 3 1/2 screw turns. T'ain't much!!!

[Phil]

Hi Junior,

I think it is in Hatcher's Notebook where they turned a M1903 barrel down to about .062" over the chamber and fired a number of service rounds with no problems. Blew it up with a proof load though. Found it, page 202. The barrel was originally 5/16" thick and he did turn it to 1/16". Took three service loads just fine but popped it with a proof round. So you should be ok if your measurements are correct and they sound about right to me.

Gonna scope it eh? I made a mount for a M39 that fit in the rear sight bed. Works like a charm! Boy those things will shoot!

Good luck,

Phil

[leftiye]

Just a guess, but most scope mount screw holes go at least .125" (1/8") into the metal.

Again, just my opinion, but unless you weaken the barrel (which a screw hole doesn't do much of), or go to pressures that will cause steel to flow- LOL!, then you've got beaucuop metal to keep the gasses inside your gun with the almost .4 inch of thickness. I'd even bet that as little as 1/8 inch would probably be okay (between the bottom of the hole and the chamber), but I wouldn't ever go that thin. A hole is a MUCH different proposition than having the whole barrel that thin!

I'd get a little longer screws or drill the countersinks in the bases deeper. Get another "bottoming" tap, and grind off that part that doesn't have any threads so that you can thread all the way to the bottom of the hole. Start with the one you've got and finish with the modified tap.

[Junior1942]

Thanks, guys. I thought that over .200" was plenty of steel twixt hole and bore, but I wasn't sure.

Phil, this is for an H-Town Imports $15 M-N scout scope mount. The Chinese designed it as if the scope was doing the recoiling, not the rifle. The rear of the mount has to be tied down somehow or the scope/mount twists counterclockwise and loosens when the rifle recoils. I put an 8-40 screw through the mount and into the military rear sight, but I want a 6-48 at the very rear for insurance.

The $28 Simmons red dot sight I'm installing is top heavy. It's a big ol' sight + the battery and its compartment are at the very top. When the rifle recoils, to the right in the photo, Newton's Law says the scope wants to stay still. The cross screws in the center act as a pivot, and the scope and mount twists counterclockwise.

Below, see a picture of the work in progress. The 8-40 screw I installed in front prevents downward movement. The mount is now cut off just past the 8-40 screw. The plug screw located 2" to the right of the cross screw is now an 8-40 screw through the mount and into the military sight. It prevents upward movement. I'll install a 6-48 screw through the very rear of the mount and into the receiver as insurance. This rifle is lightweight and recoils sharply. And the sight is mounted high and is top heavy, acting like a lever.

http://www.castbullet.com/shooting/photos/mn16.jpg

[Junior1942]

Well, I got her mounted. This photo shows how top heavy the red dot is. The battery is inside the dial at the very top. Under the center of the sight you can see the 8-40 screw tapped into the military rear sight mount. I didn't add a 6-48 screw tapped into the barrel, far right, because I think the 8-40 in the center will hold just fine. If it doesn't, I'll replace this model sight with one which has a much lower center of gravity.

http://www.castbullet.com/shooting/photos/mn18.jpg