Center Support Pin

The center support "pin" is actually a metal tube that surrounds a full-length rivet.  I have seen a 3/16" rivet used successfully, with a section of 1/4" tube.  Factory rivet size is 4.5mmx38mm (approximately 3/16" x 1.5" ).  Flatten both ends of the full-length rivet.  You may have to drill out the inside of the tube if it needs to be de-burred.

You can also use a 5/16" bolt that is trimmed and drilled on both ends, then either tap both ends w/ 10-32 threads and fasten to the receiver using screws, or drill out both ends and simply use 5/32" rivets.

Factory-built center support pins can be ordered from GlobalTrades and AA-OK/Hivel

Here is a pic of the factory support pin from a Bulgarian kit.  Note the way the pin is pressed on the front.  This pressing into the rivet should not be necessary for "normal use" home builds.  I think it is there for allow proper clearance of the magazine, based on the wear marks.

Making Your Own

Material

I ordered a 2 foot section of 4130 steel tubing from aircraftspruce.com.  This should last a few rounds! 

Measurements

Measuring the tube means subtracting the thickness of the rails and cutting to size.

1.25" inner width 

-  0.049" rail thickness x 2 rails (0.098") 

= 1.152"

For a "normal" rail, cut the tube to 1.150" and you should be fine.  Note that rail thickness will vary.  My home-built rails are built on .049" square tubing.  Bulgarian factory rails from K-Var are slightly thinner, so plan your cut a bit larger.  

If in doubt, cut longer.  You can always trim down with a file.

The Inner Diameter (I.D.) needs to accommodate a 3/16" (or 4.5mm) rivet shank.  The material I used:

5/16" O.D. 4130 steel tubing

0.065" wall thickness

0.183" I.D. 

part# 03-00900

The I.D. needs to be 3/16" for the rivet - 0.1875".  Just clamp the tubing in a vise between two pieces of wood, and run a 3/16" drill down it - slowly.  Too fast and the drill will bite and spin the tube against the metal.  When complete, you'll have a properly sized I.D. to fit the 3/16" rivet.

The OD requirements depend upon the bolt configuration.  Some bolts have a larger bottom lug (the one that pushes bullets out of the magazine) and thus will need an appropriately smaller tube.  Some 80% builds place the hole too high up and cause bolt interference.  You can file the tube down by placing it in a drill press and filing it, or on a lathe.  Clearance should be measured before the tube is cut.

Too Large:      Corrected size: 

Placement

Just slide into place!  Note that the X-Y dimples will make it a tight fit so you may have to wiggle it into place.

Installation

Insert the rivet, and press!  I was able to get it really close with a vise, then I finished by (CAREFULLY) peening the head with a 3 lb. hammer.  It worked, so I'm not complaining.  Note you can do the rear trunion rivets this way.

Complete.