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Old December 26, 2010, 08:36 PM   #1
Senior Member
Join Date: June 28, 2006
Location: NEO
Posts: 202
How to SBR my SIG P556

I have received several PM in the last few months regarding SBR’ing a SIG P556 pistol and as such, I am responding by producing this short tutorial as a ‘how to’.

1. Complete ATF Form 1 – This is the absolute first thing you must do before anything else. There are other threads dedicated to how to complete this form, so I will not go into details here.

2. Wait for your stamp – For the purpose of this thread, I will assume that you are in possession of your stamp and can now begin the legal process of building an SBR.

3. Fitting the stock – There are actually several way you can go with this depending on your budget and personal preference.

a. By another SIG 556 Classic and swap out the lower. Essentially you are done; you are now the proud owner of a SIG 556 classic SBR (I’ll get to engraving your upper receiver and what to do with the ‘spare’ SIG 556 now in your possession in a little bit). BTW, this is the only way that I am currently aware of if you are interested in having a ‘Classic’. I believe there were a couple of after markets considering making Classic lowers, but it is really cost prohibitive.

b. Assuming you decide not to go with suggestion ‘a’ above, or that you did and now have a 16” ‘pistol’, either way the following will demonstrate the steps necessary to add a stock to a pistol lower.

i. Unload and clear your gun. Once completed, check it once more. Set the magazines and all ammunition back in the safe, drawer or in another room.

ii. Unload and clear the gun again.

iii. Separate the two halves – If you have the older style ‘pins’ you will need to use two large screw drivers, turning counter clockwise simultaneously. This requires care not to scratch, nick or otherwise damage the screws. If you have this setup, I highly recommend purchasing this. If you have the newer style lower, simply push the two pins from the left side towards the right, similar to any AR.

4. Remove the rear plug – Again several ways to accomplish this.

a. Buy a tool from this guy but remember, you’ll probable use it exactly once, unless you plan on doing more than one. In which case….

b. Make your own tool – again several ways of doing this depending on if you plan on keeping the rear plug or simply want to ‘get it off’. I was merely interested in the latter and hence used the following method.

i. Use some clamp protectors and clamp the lower receiver to a bench vice with the plug pointed up on an angle.

ii. Carefully heat up the plug (I used a MAP gas touch) – caution, SIG uses a plastic plug on the bottom of the sling adaptor just below the plug, stay away from this. The reason for the heat? Again depending on when you purchased your gun and how the plug was installed, either a whole tube of red Loctite was used or none at all. Yeah, SIG was (is?) not known for ‘quality’ on most of the 556 product line.

iii. Heat for 2 – 3 minutes. Did I mention carefully on the plug itself?

iv. Inset large needle nose pliers into the two holes in the plug.

v. Use a set of channel locks to ‘clamp’ the needle nose pliers in place.

vi. Apply even force to the channel locks in a counter clockwise rotation.

vii. Remove plug and discard, done.

c. If you are interested in a ‘less destructive way’, try goggle, there are more ways that I can shake a stick at. Necessity is the mother of all inventions.

5. Install stock – Again, depending on your budget, taste, style, individualism etc… there are several ways to go.

a. Purchase a MIL spec AR baffle tube, screw on and install a stock of your choice – done.

b. For a folding stock, I recommend ACE ltd.

i. this

ii. or this

iii. or this with this.

6. Final step – Engrave your upper receiver. Since you are now the ‘manufacture’ of your new toy, you must have your name, city and state engraved on the upper receiver as per ATF. (unlike an AR, the upper receiver is the 'registered' portion on a SIG) Here is a good link regarding engraving.


I hope you have found this useful and as usual, I assume no responsibility, risks or liabilities. This is mealy for educational purpose only.
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