If you don't know, FN accidentally used the wrong pistol grip specs when they created the mold for the lower receiver. If you want to use a pistol grip other than the one that comes standard you must either modify the grip or modify the lower receiver. Tonight I modified my lower receiver.

(I was totally inspired by DOE's post about this.  Thank you for the fantastic technique!  http://www.ar15.com/forums/topic.html?b=6&f=24&t=313547)

This is the pistol grip mounting point on the SCAR.  The black line is the material I need to remove to get it to the standard pistol grip spec.









This is what happens if you install a standard pistol grip to a SCAR:



At this point I used a Dremel and a file to remove the excess material from the mounting point.  I was rather meticulous so the process took me a little more than an hour.  I used the Dremel to remove most of the material, but it isn't possible to get the surface perfectly flat with a Dremel.  The file allowed me to get the surface perfectly flat.  The black marks from the permanent marker was crucial for monitoring my progress.  I absolutely did not want to remove too much material or change the angle of the mounting point.  As I got close to the standard spec I kept trying the Hogue grip to see how close I was.  

(Side note:  The cheap POS A2 grip that comes with a normal lower parts kit fit with very little modification.  I am certain they do not internally mate very well with the mounting point, not that it matters.)


All done.

looks good didn't know FN messed up that badly.

Youd think, FN would have factored that in, or at the least made changes to the jig to get it right, especially for the price they expect

Nice pics.  Thanks for the post and tutorial.

Seems to me that modifying the lower cost part would be the way to go, but what do I know.

Looks easier to modify the lower. I do agree that if possible modifying the grip would be better. Still a good mod though.

Two months ago I modified a Hogue grip to fit the standard SCAR lower. I've decided now that I prefer having a rifle that can accept any grip.

+1

Modding the grip is not as easy as it seems. The screw interface area and the grip tang is about 1/8".  If you thin down that area, you and Mr. Murphy will have a date at some point.  I once saw a recruit fall on his rifle and break the grip.  Wouldn't have believed it if I wasn't there.

Good job CARifles!  

I modded my lower.  It's easy to do, especially if you go the template route done here.  I'm not sure why FN just didn;t make them compatible to begine with, but it's an easy fix.

Mmmm Hogue goodness!

Whatever the reason for the incompatibility, you can be absolutely sure it was intentional, not accident. FN has more experience with M16s than anyone but maybe Colt.

My guess is that with the steel screw into plastic lower, they wanted more thread contact and hence needed a deeper hole which means a longer mounting tab than used on the M16 pattern.

Mod the grip.

I think one of the engineers took one of FN's $2 pistol grips they install standard on their M-16's and got the PG mount dimentions from that as opposed to using the dimensions from the M-16 lower receiver. They failed to realize that the inside of a standard pistol grip does not make contact with the mount, nor does it need to. There is a good amount of space in there. Aftermarket pistol grips have tightened up that space considerably and will not fit on a SCAR.

In my humble opinion my rifle is now better than a standard SCAR because it will accept any pistol grip, not just the standard $2 brown FN grip. The mounting point is not significantly weaker than before the mod. At the end of the day the only thing I modified was a single plastic replaceable part. It's not like I hacked off part of the steel serial-numbered receiver. I understand where you are coming from. Believe me when I say it took 4 months for me to build up the nerve, and I also did modify a grip 2 months ago.  But to each their own.

I spoke to the product manager on the US side (Bob) of FN.  He did not sound like it was "intentional".  He was not aware that the a2 grip could not be substituted and had to refer to the "military side" for clarification.  They somehow got a hold of a reverse engineered TDP for the grip.  Mod the lower grip tang and used a cutting disk on the grip screw if it contacts the lower trigger flat if you are using the factory grip screw and have "issues".

I'm sure he told you that, but Bob Ailes wouldn't know such things and he is way downstream of those that do. You'd be asking a seemingly esoteric engineering question to a salesman.

Not an FN Engineer, but I can tell you with a high degree of certainty that it was lengthened due to the comparatively weaker plastic threads. It's an obvious solution to a obvious problem. The M16 TDP is readily available on the company intranet, so it's highly unlikely there was any accident or guesswork involved.  

They know precisely the dimensions as they should exist in the aluminum M16 lower and they know that using the same data on molded plastic, the same thread pattern and depth, will reduce the strength of that interface. Lengthen the tab to allow a deeper hole to allow more thread contact.  

The incompatibility of the A2-spec grip for the .mil customers could not be less important here since it's not a high-wear or consumable part. Actual grip part commonality with the M4 or A2/A3/A4 was never a design parameter.  The subjective, common "feel" of the A2 grip was the only intention here.

I understand what you are saying about lengthing the interface.  On the Scar, the only interface with the grip screw is a square washer located in the slot of the grip tang.  That small 1/4" is the only interface with the screw.  This can be seen in CARifles photo.

Here's the interface

what DOE said above. there are no plastic threads.
i also heard that it was an oversight on FN's part, as described above.