Need advice: custom Picatinny or Weaver rail base


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Odd Job
December 29, 2012, 10:35 AM
Although this is for a crossbow, the problem is going to be solved by good old fashioned firearms-related ingenuity :)

I have a Cheetah crossbow which I want to put a Picatinny or Weaver rail on (doesn't matter which). This is where it will go (note this is polymer and I can't make any alterations to it):

http://i55.photobucket.com/albums/g154/Odd_Job/RailBasedimensions_zps0ca20186.jpg

My current thinking is to make this in two pieces: a custom "base" onto which I can secure a rail with a flat back.

1) The base of the rail must be at least 8mm/0.31" high
2) The base of the rail must be at least 115mm/4.52" long
3) The base of the rail must be no wider than 14mm/0.55"

The base will have to be drilled to accept the two pins which are 3.47mm in diameter (0.136")

I'll have to get the base drilled and tapped to match the holes in the rail.

I can also remove that knurled nut in the middle and make use of that hole as a further means to secure the base to the crossbow. That is a 4.85mm machine screw (0.191").

How would you do it? Is there anything out there which already has a base that could fit?

Advice much appreciated.

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Haxby
December 29, 2012, 12:59 PM
Google
BKL-456 14mm
That might be what you want, or it might be a starting point.

Odd Job
December 29, 2012, 03:10 PM
It is too wide, the 14mm is the "inside" dovetail width and I need the overall width 14mm max. I am looking for a one piece base also...

rcmodel
December 29, 2012, 08:37 PM
Probably your best bet is a blank rail and a friendly machine shop!

http://www.brownells.com/optics-mounting/scope-bases/rifle-bases/picatinny-rail-blank-prod26705.aspx

Sorry I can't be of more help.
But your MM measurements are giving me a head-hurt.

We old Americans just don't do MM/inch conversions in our head yet?
And I for one, never will!

rc

Odd Job
December 30, 2012, 04:30 AM
That looks like a plan, RC, thanks!

Sorry I can't be of more help.
But your MM measurements are giving me a head-hurt.

Look again, I have the measurements in mm and inches hehe

Odd Job
April 11, 2013, 12:06 PM
Update: this got solved with the help of a friend at the local rifle club. I got one of these 12" blank Ergo rails from Midway UK:

http://tinyurl.com/d93koqg

My friend was able to turn that into two of these:

http://i55.photobucket.com/albums/g154/Odd_Job/IMG_2138_zpsc6e7961b.jpg

Here is the machined and drilled rail next to the mount assembly:

http://i55.photobucket.com/albums/g154/Odd_Job/IMG_2139_zps7be7bcac.jpg

Here it is, a nice fit:

http://i55.photobucket.com/albums/g154/Odd_Job/IMG_2140_zps64919585.jpg

http://i55.photobucket.com/albums/g154/Odd_Job/IMG_2141_zps6181c209.jpg

http://i55.photobucket.com/albums/g154/Odd_Job/IMG_2148_zps5a58e451.jpg

http://i55.photobucket.com/albums/g154/Odd_Job/IMG_2149_zps2ce1521a.jpg

I'm very happy with that. The second rail (not shown) is even better because the back end has been ramped to match the mount assembly angle.

That rail will go on the other crossbow, possibly with a reflex dot sight.

rcmodel
April 11, 2013, 12:11 PM
Well, that turned out nice!!

Good work!

rc

Odd Job
April 11, 2013, 12:21 PM
The guy is very good, the rail looks like it was made for the crossbow!
My next job will be to get some black polyethylene blocks fashioned to fill the hole where the knurled nut used to sit (no longer needed as the rail will not tilt).

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