Picatinny Rail Shim
During my Vulcan air cylinder adventure a few days ago, I discovered that the Picatinny rail is NOT indexed in a specific position relative to the barrel/air cylinder bands. It is held down by two screws, but the mating surfaces are completely flat. No locking pins, nor aligning grooves are present. It’s just a flat interface between rail bottom and strap top. The screw heads go into countersunk holes atop the rail, but there is a LOT of tolerance as to position you can tighten things down. About 3-4 MIL left or right is possible. Mere friction keeps the rail from shifting.
1. Do NOT loosen or remove the Picatinny rail unless you are willing to realign the rail.
1b. Again, Do NOT loosen or remove the Picatinny rail unless you are willing to realign the rail.
2. If you do take off the rail AND have the barrel bands loose at the same time, you need to take care when reassembling to get things back into alignment. Keep the bands loose. Attach the plain side plate first. Then use its upper edge of the plate to align the Picatinny rail. Tighten down both Picatinny rail and side plate so they square the bands and rail BEFORE tightening the barrel bands.
3. Even if you do the above, you will probably have the rail in a slightly different alignment relative to the barrel than when you started. There is no a hard index to enforce alignment.
So, yes it is a pain to get things back into alignment. On the other hand, this curse is also a blessing if you have windage or lateral offset errors.
Since I was forced into this situation due to a barrel band that would not let go of the air cylinder (even when empty), I had to realign my rail anyways. So I have taken advantage of the situation to experiment with shimming back of the rail itself for elevation and deflecting the rail sideways to correct windage. Some brash shim stock beneath the rear mount does the elevation. Rail locks down immobile quite easily. Thus far it is working well enough that I am planning on grinding or bedding a with proper angle so all mating surfaces are full contact
If you are happy with your rail, DO NOT DO THIS.
But if you are already stuck with a rail that has been taken off the gun or have an already misaligned rail, this opens a new avenue for keeping the scope optically centered and adjusting elevation/windage without adjustable rings.
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it is more important to learn how to mount a scope, if you don’t do that, you will end up with very expensive and many scopes, and still unhappy
or just completely ignorant and you would never understand why you missed a shot. hehehe
turning the turrets IS A BAD THING.
And that’s why my setup includes use of the vertical reticle instrument. It stacks the scope and barrel vertically at the gun.
Couple that with the also having the POI and POA stacked at the zero distance and you minimize lateral offset.
The stock might end up slightly off vertical if there is a mount offset, but that the effects of that offset are minimized by stacking the scope and barrel vertically.
Your point is true if you have a mount offset and failed to vertically stack the scope and barrel.
The POI would drift left/right as you change range.
OK now shoot it at 8yards and check if it is not shooting left or right.
🙄
Success! After about six hours work I now have a steel shim that sets elevation. Windage was also adjusted via the Picatinny rail side/side motion.
With scope at optical center. POI is on target with 2 moa down and 1/2 moa right on my scope.
I intentionally made the back of the rail about 0.005 inches bit too high so subsequent scope adjustments
leave more elevation range upward from zero. Then again, I usually just use hold over rather than dialing.
My shim was created from a 5.5 inch long piece of steel flat bar. Rough ground in the slope (about 0.040 inches difference between rear and front)
Made it 0.110 in thick at rear and 0.070 at front. Your needs will be different.
Took a few hours of hand lapping flat, test firing, grinding and lapping again to get it right.
Pretty pleased with the results especially since I had to realign my rail anyways.
——-
I have an improved procedure for getting things realigned when everything is all apart. (barrel bands loose, picantanny rail off, side plates off, trigger block loose, shroud off, UNCOCKED.)
1. Use the picatinny rail as a reference for when the barrel bands are rotated into correct position. Put the rail atop the flats but don’t use any screws.
Hold the rail down tight on the flat portion of the band. Check how it rests upon the flat of the other band. If it is not perfect flat on both flats,
slightly spin the bands relative to each other to get their flat portions perfectly fitting the flat bottom of the rail.
Also check that the rear, top barrel cover/spacer is firmly sandwiched between the rear barrel band and the breach. That cover stabilizes the
distance between the amidship assembly and breach block during cocking. If it is loose, you need to push the bands backward and recheck alignment with the rail.
2. Screw down the rail to hold the bands in this alignment. Take up tension gradually moving back and forth between front and back screw. This will hold alignment
of the bands for next steps but not be the final rail position
3. Align the trigger block centered with the bottom flat of each barrel band. Tighten its 8 mounting screws in alternating patter much as you would a scope.
Now the bands are locked without rotation relative to each other.
4. Remove the screw covering the barrel index hole. Use a flashlight and verify that the barrel index is centered within the hole in the breach. If necessary, adjust barrel position to get it right.
5. Gradually tighten the barrel bands. Take up tension gradually and alternating pattern. At this point the bands, barrel and air cylinder are probably as well align as possible.
6. Put the plain cover on
7. Put the u-slot cover on. Now the side plates, trigger block, and bands themselves are holding the alignment and you can loosen the Picatinny rail for final positioning
8. Loosen the Picatinny rail screws. Align the rail using the side plates as reference. A reasonable position is centered left/right at both front and back ends of the rail.
If windage is off with test firing, you may consider adjusting the rail to correct for windage.
Or you can go Fuxineering and install a shim under the rail to minimize usage of scope turrets.
Guy
I zero for 25 yards. That gives me pretty darn close for my entire range of highest interest.
What distance are you optically centering to POI for ❓
Just to be clear…. I do NOT recommend loosening the rail. This is info only if your rail is obviously misaligned or needed to be removed.
The odds of getting the rail back in the same position again are slim. There simply isn’t a part of the gun that forces the rail to be
in the exact right position.
There are only two things I can think of that can help.
1. Use a flat piece of material against the plain side plate to help align the rail.
or
2. Try to take advantage of what centering effects the countersunk holes have by lifting the rail up
and alternately tighten the rail screws from completely loose to snug. The idea being to keep
the heads of the screws self aligned in the countersinks as the rail seats.
I think the amount of play makes strategy 2 less reproducible.
Since mine needed to be pulled completely off, I’m now working on lapping some wedged shim.
This will be tricky since the front end of the shim will need to be ground quite thin.
To take care of all my needed elevation, the rear of my rail needs to come up over 0.05 inches.
That was just by adding more brass shim pieces until POI was where it was needed.
If I was smarter, I would shoot without any shims and with the scope at optical center.
Measure the change needed on the target and use the ratio between the rail contact length
and distance to target, calculate the exact shim needed.
Shim thickness = error at target * rail screw spacing / distance to target
All in the same unit of measure of course.
Mine scope is perfect zero with 20 moa rail at 50 yards. But it’s 1.4 mils to the right
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Nice post, thanks.
Thanks for the info guykuo :thumb:
I’ve changed scopes and lasers several times using tight fitting quick disconnects but never checked the rail mounting screws.
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I looked at a video review on the taipan or however it’s spelled and noticed the at/barrel clamp is a one piece design. That way it’s pretty much guaranteed to be lined up correctly. I thought it’s a excellent design on their part. If they had the Vulcan piece in between the clamp and the breach block that would put the icing on the cake.
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