I have a new RRA M-4 upper from Pete at Legal Transfers.  Its a great upper.  I wanted to put a rail system on to hang lights, disco balls, cheese graters and other gadgets from, so naturally I picked up a Knight's Armament KAC RAS drop in rail system.

I've installed many of these in the past without issue.  

With the RRA upper, it seems as if the outer diameter of the barrel is too large, therefore the KAC RAS won't fit under the A2 front sight plate.  
It fits just fine on my DPMS, and DTI uppers.  What to do?

Have you had problems like this?

There is a little tab that should fit under the handguard cap, shouldn't be any problem.

Yep, I'm familiar with the tab.  Press the tab under the endcap, slip the rest in and 'voila!  It just doesn't fit.

It's because RRA uses a HBAR profile which makes it impossible for the RAS to fit under the handguard cap. There is a tab on the underside of the top rail that has to have material to be removed in order to get it to fit.

Yep. The rail needs to be modified to fit the RRA barrel. Know a few people that have had to do this for rra barrels.

Yup the heavy profile of some RRA barrels prevents the rails from dropping in.

There is two little grey tabs on the front inside of the rails.
These need to be filed down to make the front of the rails fit under the handguard cap.

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