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Discussion Starter · #1 ·
So, I've just recently realized that my upper with a Geissele SMR installed is not compatible with my lower, which is a non-416 lower. Unfortunately, I learned this the hard way and I think I ruined my locking bolt because I'm an idiot. Praying I didn't ruin the threads on the handguard itself, but I'll know tomorrow when I get a new screw.


Anyway, I've seen some posts and pictures of people who have had to modify their SMR to fit their lower by shaving off a rear portion of the handguard, near where it touches the front takedown pin. Does anyone have any experience or guidance with this? As blasphemous as it feels, I think I'm left with no choice but to modify one of my SMRs and I'm hoping I can go into this with a battle plan, as opposed to just hackin away at it.

When I was attempting to put my upper and lower together, it seemed as if the handguard was only a hair too close to the front of the lower. Is a cut as drastic as the one in the picture above needed, or can I just sand the corner of the handguard down a tad?
 

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Discussion Starter · #3 ·
didn't realize this was a problem for others too. I dremeled it down a bit. Esthetics look fine. Funny, the HK hand guards fit just fine.
After you shaved yours down did you cover/coat the exposed metal in anything? I never coated my gas block after I cut the sling loops off and it’s been fine for the past few months.
 

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After you shaved yours down did you cover/coat the exposed metal in anything? I never coated my gas block after I cut the sling loops off and it’s been fine for the past few months.
No, I can’t really tell when it’s installed unless I give it a close examination. It’s aluminum, I can’t imagine not being coated will hurt it. YMMV
 

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Discussion Starter · #8 ·
Any pics? I have a SMR on a MR556 upper sitting on a Colt lower without any issues.
Visually, it looks like it should fit and is only ever-so-slightly off, so pictures really wouldn't do much justice, unfortunately. I also ruined my locking bolt trying to lock it like an idiot, so I can't really put it back together at the moment lol.
 

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Discussion Starter · #10 ·
Ended up not needing to do any modifications. It appears that it only didn't fit on my Anderson lower, but was fine on my other lower, which is perhaps a good thing 😂.

The only issue I've noticed is when pivoting the upper on the front takedown pin, the handguard eventually strikes the front of the lower. Not sure if this is normal, but not a huge deal anyway -- just have to be careful, as it's chewed up the bottom of the handguard a little bit.
 
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