hand guard and receiver rail height mismatch

[ianD]

I got an Aero Precision upper receiver which is the high profile design and a Troy alpha rail in high profile as well.  Problem is when assembled there is maybe a .010"-.02" step down from the upper rail to the hand guard rail.  I've triple checked and they're both the high profile design.  I called Troy and the guy said they usually match up with the Aero Precision uppers.  I emailed Aero Precision and haven't heard back.  Is anybody out there using this setup, and if so is everything flush?      

[slugger43]

Yeah, me too.  Troy Alpha, Aero Precision.  I don't really give a rip.  Plan is to put BUIS on at some point, but the height difference is minimal, easily allowed for with the front sight adjustment.  Would look better if it was flush, but I can put up with a little ugliness, since my "ghost gun" runs superbly and shoots better than I am capable of.  Hope yours comes out super smooth and super accurate!

But no, they don't match up.

[Robocop1051]

For 308AR rifles, even the low vs high rails aren't set in stone measurements. Do not expect cosmetic uniformity unless you buy all your parts from the same manufacturer. Once you start mixing and matching there's always chance for some misalignment. 

[ianD]

Thanks for the replies, guess I'll just live with it. I'm sure once I squeeze off a few rounds I'll quickly forget about it. Slugger, where did you get your lower from? Got my 80% lower from Colfax Tactical and I have some slight fitting issues with the takedown pins and the aero precision upper. Did yours go together smoothly?

[slugger43]

Got mine from 80percentarms.com.  Fit was interesting.  Had a problem with router chatter on the first blank, did some minor damage, got a 50%-of-cost replacement (and a new router) and did #2.  JB-welded the first one and re-routed, so ended up with two lowers.  The first one was a little too snug at the pivot pin, could use a very minor filing job.  The second one was a super fit at both pins, and everywhere else.  

Robo is dead right about fit and alignment with different brands in the wonderful world of 308AR.  And it's almost certainly more pronounced in the 80% part of that world.  That's probably why God gave us small files, polishing compound and Dremel tools.  Reliability and accuracy are important and achievable.  Cosmetic perfection is, in my opinion, over-rated.

 

[82HALO]

I know about the fit and finish! Have an Aero upper with a Fortis switch 308 handguard, and there is a good 1/8" to 1/4" gap between the two! The height is perfect though! Have a thread about it over in the tool area.

[ianD]

The pivot pin goes in good on mine but the two won't actually pivot the full range they're supposed to, only opens up maybe 45 degrees so I'v got interference somewhere. The receiver's mate well, only a slight gap that you can just barely see light through, but the take down pin won't go through all the way.  Nothing too difficult to make work though, got spoiled after how smooth the 80% ar15 went.

[Slugger31]

S&W M&P 10 upper with Aero Precision free-float handguard. 
More than 0.050” offset. 
I wanted to install Magpul MBUS pro 45° offset iron sights, but I don’t think that will work. 

[shooterrex]

Is the rail higher than the receiver or lower?

[DNP]

From the picture it looks like the hand guard rail is lower than the receiver. 

[98Z5V]

10 hours ago, Slugger31 said:

S&W M&P 10 upper with Aero Precision free-float handguard. 
More than 0.050” offset. 
I wanted to install Magpul MBUS pro 45° offset iron sights, but I don’t think that will work. 

0.060" is the offset difference between DPMS High and DPMS Low.  That's exactly what this is, here.  No pics needed.  Aero Precision is all DPMS High pattern, so it looks like the S&W M&P-10 is a DPMS Low pattern.  You need a handguard that's based on DPMS Low profile - and the Aero isn't.

Check it:

 

[DNP]

But the receiver is higher than the rail...?

[98Z5V]

39 minutes ago, DNP said:

But the receiver is higher than the rail...?

In the original post yes - all this new stuff is for the guy that just reawakened this thread 11 hours ago... 

[98Z5V]

11 hours ago, Slugger31 said:

S&W M&P 10 upper with Aero Precision free-float handguard. 
More than 0.050” offset. 
I wanted to install Magpul MBUS pro 45° offset iron sights, but I don’t think that will work. 

This is the Necro-post, 11 hours ago, that brought all this back up... 

Nothing to do with the original poster, except for rail mis-match.

[DNP]

Aha. I gotta read the whole thing. Short on time. I been skimming.