folks,

just got a KAC ambi-single-point-sling-mount... you can see an example of this mount here:
ar15.com/forums/topic.html?b=7&f=25&t=165705&w=myTopicPop

i have a problem with attaching my new mount, however, and i was wondering if anyone has seen the same thing.  it seems that the protrusion on the KAC ambi-mount is too large to mate with my bushmaster lower receiver.  let me be a bit more exact; out come my digital calipers...

the protrusion on the KAC ambi-mount measures 0.505" in dia. but the mating hole on the rear face of the receiver measures only 0.495".  so there is 0.010" (10 thousandths of an inch) interference.  i don't know what the design dimensions were supposed to be on either the mount or the receiver but it appears somebody is off one way or the other.

losdos.dyndns.org:8080/public/verymisc/KAC-oversize-boss.jpg


has anyone else had this problem?  note that my lower is a flat-top purchased from bushmaster in the last few months.  

ps
be advised there was another problem with the KAC ambi-mount; the roll pin which prevents rotation of the buffer tube was pressed too far into the mount.  thus, initially it was not possible to slip the ambi-mount over my buffer tube.  i had to use a hammer and a punch to drive the pin back into the mount by some 0.015" or so -- until the mount could accept the buffer tube.  driving the pin back in is, of course, very cumbersome because you can't hit it straight on; instead, you have to hit it an angle and hope it doesn't distort too much. i tried using a C-clamp but one small enough to fit into the buffer opening wasn't strong enough to budge the roll pin. anyway the folks at KAC should know of this problem as well as the more critical one described above.

jim aka ar-jedi

I just put one on my BM.  I had o use a dremel and a nedle file to get it to fit.

I dont know crap about this particular situation but is it one of those COLT(etc.) only deals where it may not fit a BM and other makers specs?  Just a thought since it seems KAC has always been right on in my experience.

The Colt receiver print calls out for that hole on the receiver to be .499, +.004, -.000. So in theory, that hole should be no larger then .503 and no smaller then .499 (before plating). Plating will add .001 of build-up to surface dimension.   So, both the reciever and the mount are out of spec. That's what they call "stacking tolerances".

Who made the receiver?

LS

LordStoner asks:
> Who made the receiver?

from my original post:
> note that my lower is a flat-top purchased from bushmaster in the last few months.

jim aka ar-jedi

folks,
i carefully worked the protrusion on the mount with a file, and after 20 minutes or so and a couple of test fits i got the mount into the receiver.  it's a very tight line-to-line fit now, which is goodness.  

thanks for the insight above.  all's well.

jim aka ar-jedi

I had the same problem with a RRA lower.

                  Scott

I mounted one on a RR Colt M16.I didnt have a a problem with the mount  but the role pin was did need to be adjusted.Before I mounted this I had a single point sling that was a stamped plate like the GG&G.I had to file down the pin on that also.SJR556.