Curing Scope Rail Pullout From Recoil
Posted: 09 Feb 2013 00:13
The Rossi M92 is provided with 4 tapped holes to mount an M92-specific scope rail, for use with low-power scout scopes. Many hunters have mounted scopes on M92s, only to have recoil rip the rail clean out of the threads after repeated use.
A look at the Rossi setup clearly shows its deficiencies in the mounting arrangement. The four holes that mount the scope rail to the barrel are all located at the rear third of the rail. Since the scope is also attached at the opposite (front) end of the rail, this acts as a lever, which amplifies recoil force and prys the screws right up out of the barrel. Adding to this, the four tapped holes in the barrel are not very deep, and the fine thread 6-48 screws cannot take much stress before thread failure occurs. So what to do?
Some people have epoxied their scope rails down to the barrel! While it appears to work OK, I am not one to permanently alter my brand new gun with adhesive! While looking at the gun I had a brainstorm...why not use the rear sight DOVETAIL as a 5th mounting point? It is very strong, and also sits forward of the other four mounting screws. I envisioned milling a rectangular "pocket" in the bottom of the scope rail, one that would mate perfectly with a filler blank inserted in the dovetail. A 5th screw could then be run between the rail and dovetail blank and presto! The dovetail would provide a strong resistance to lifting the rail up off of the barrel, and also keep the rail from laterally sliding back and forth and shearing the mounting screws.
I bought a couple of Marbles brand dovetail blanks and headed over to my friend Brian, whom is a master machinist. Brian liked my idea, but made it one step better...he machined things so that you can't even see the additional mounting point when the rail is on! And...this setup does NOT permanently modify your gun.
Look at the attached photos and I'll try to provide some basic guidelines on how to do this. You WILL need a good machinist to obtain the precision alignments necessary:
1) Take a standard dovetail blank, chuck in a lathe and turn a .370" dia. cylinder out of the upper part.
2) Trim the cylinder down so that total height from bottom of blank to top of cylinder is .115"
3) Reduce the width of the blank on a precision grinder so that it is .450"
4) Drill through and tap the center of the cylinder to accept a 6-48 mounting screw.
On the Rossi M92 scope rail:
1) Locate .345" center-to-center from the forward-most factory mounting hole. End mill a .372" dia hole at that position, to a precise depth of .070" below the bottom of the barrel radius in the rail...measured at the THICKER side of the rail. Note that this hole is milled perpendicular to the BORE of the rifle, and not to the barrel, which is tapered. Use the upper surface of the rail for alignment of the mill, as it is parallel to the bore.
2) At same position, end mill a .500" dia, hole, but not quite deep enough to cut the entire circle. You might have to experiment with this depth, as it is designed to just clear the outsides of the modified dovetail blank.
3) On the same center, drill a .144" clearance hole for a 6-48 screw and countersink the top side just enough to match your particular screw head. Do not countersink too deeply! Before mounting the rail to rifle, get a NEW 6-48 bottom tap and clean out the tapped holes in the barrel. I found that mine were cut very shallow from the factory, which can cause thread binding and/or further weaken the already minimal holding power.
The cylinder post you milled in the dovetail blank should be a tight fit into the socket milled in the rail. I needed to tap my rail down with a soft mallet to get it to seat, which is perfect. This tight fit allows very little front/back stress to reach the mounting screws. The dovetail now takes up most of the lateral and up/down recoil forces on the rail! Clean all threaded holes with alcohol and use blue Locktite on all screws. Make screws good and snug, but don't overtorque.
Many thanks again to my friend Brian, whom is truly a world-class machinist, and one of the best "idea men" I have had the pleasure of knowing. He took my simple idea and made it truly elegant.
Let me know if this cures your problems with scope rail pullout.
Comments welcome and encouraged!
-Turkey Huntsman
San Diego, CA
A look at the Rossi setup clearly shows its deficiencies in the mounting arrangement. The four holes that mount the scope rail to the barrel are all located at the rear third of the rail. Since the scope is also attached at the opposite (front) end of the rail, this acts as a lever, which amplifies recoil force and prys the screws right up out of the barrel. Adding to this, the four tapped holes in the barrel are not very deep, and the fine thread 6-48 screws cannot take much stress before thread failure occurs. So what to do?
Some people have epoxied their scope rails down to the barrel! While it appears to work OK, I am not one to permanently alter my brand new gun with adhesive! While looking at the gun I had a brainstorm...why not use the rear sight DOVETAIL as a 5th mounting point? It is very strong, and also sits forward of the other four mounting screws. I envisioned milling a rectangular "pocket" in the bottom of the scope rail, one that would mate perfectly with a filler blank inserted in the dovetail. A 5th screw could then be run between the rail and dovetail blank and presto! The dovetail would provide a strong resistance to lifting the rail up off of the barrel, and also keep the rail from laterally sliding back and forth and shearing the mounting screws.
I bought a couple of Marbles brand dovetail blanks and headed over to my friend Brian, whom is a master machinist. Brian liked my idea, but made it one step better...he machined things so that you can't even see the additional mounting point when the rail is on! And...this setup does NOT permanently modify your gun.
Look at the attached photos and I'll try to provide some basic guidelines on how to do this. You WILL need a good machinist to obtain the precision alignments necessary:
1) Take a standard dovetail blank, chuck in a lathe and turn a .370" dia. cylinder out of the upper part.
2) Trim the cylinder down so that total height from bottom of blank to top of cylinder is .115"
3) Reduce the width of the blank on a precision grinder so that it is .450"
4) Drill through and tap the center of the cylinder to accept a 6-48 mounting screw.
On the Rossi M92 scope rail:
1) Locate .345" center-to-center from the forward-most factory mounting hole. End mill a .372" dia hole at that position, to a precise depth of .070" below the bottom of the barrel radius in the rail...measured at the THICKER side of the rail. Note that this hole is milled perpendicular to the BORE of the rifle, and not to the barrel, which is tapered. Use the upper surface of the rail for alignment of the mill, as it is parallel to the bore.
2) At same position, end mill a .500" dia, hole, but not quite deep enough to cut the entire circle. You might have to experiment with this depth, as it is designed to just clear the outsides of the modified dovetail blank.
3) On the same center, drill a .144" clearance hole for a 6-48 screw and countersink the top side just enough to match your particular screw head. Do not countersink too deeply! Before mounting the rail to rifle, get a NEW 6-48 bottom tap and clean out the tapped holes in the barrel. I found that mine were cut very shallow from the factory, which can cause thread binding and/or further weaken the already minimal holding power.
The cylinder post you milled in the dovetail blank should be a tight fit into the socket milled in the rail. I needed to tap my rail down with a soft mallet to get it to seat, which is perfect. This tight fit allows very little front/back stress to reach the mounting screws. The dovetail now takes up most of the lateral and up/down recoil forces on the rail! Clean all threaded holes with alcohol and use blue Locktite on all screws. Make screws good and snug, but don't overtorque.
Many thanks again to my friend Brian, whom is truly a world-class machinist, and one of the best "idea men" I have had the pleasure of knowing. He took my simple idea and made it truly elegant.
Let me know if this cures your problems with scope rail pullout.
Comments welcome and encouraged!
-Turkey Huntsman
San Diego, CA
........beats the crap out of using Krazy Glue !! 