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9/22/2017 12:11:25 AM
Posted: 8/5/2005 12:39:11 PM EDT
Has anyone installed the SIR 58M (now SM) on an Armalite M15A4 midlength carbine with ease ?

I'm now on my second and having the same problem - the yoke which fits under the barrel nut goes fully into place but the two screw holes on either side are not 'in' far enough to insert the screws.

After much difficulty with the first one, I did some research and see my 58SM rail is now different than pictured on the ARMS site (upper vent holes are different). I called Armalite who said they had some difficulty installing them and had to clamp the rail and yoke together to insert the screws.

The ARMS site and little manual say you have to remove the delta ring which I did, but not specifically the front triangular handguard cap. Mine definitely hits the SIR and the Armalite tech said he had to remove them on all he's installed ... the ARMS rep said you dont have to remove it.

The ARMS rep had me send the first SIR back, and when he tried it on an in house Armalite it too would not fit. He said maybe the yoke holes were drilled 'off'. He tried a second SIR and it fit the Armalite, and the yoke screws dropped into place. I was sent this second SIR this week and yet on my Armalite it has exactly the same in that the yoke holes do not line up at all. The yoke threads sit maybe .030" too far out from from lining up all the way with the holes in the rail.

Now this could be caused if the top of the Armalite rcvr rail was too far above the bore centerline. The top of the rcvr rail sits 0.568" above the top of the nut, the nut is 1.36" OD, so the upper rail is 1.248" above the bore centerline.

Is there that much variance in rcvr specs for this to be the cause of this SIR rail not fitting? The holes in the rail are a loose enough fit that the screws can move around to find their way provided the yoke is within some reasonable variance of where it should be. Yet on mine, the yoke screw threads will not line up at all with the SIR holes even when clamped with a soft faced quick-grip.

My only solution at this point is to cut the front handguard cap off and then ream the holes in the SIR rail until I can get the yokes screws to catch the yoke. So much for the guarantee of fitting an Armalite and the "No special tools are required to install. A coin is all that is needed."
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