Looking for some help with my build.   It’s a Gibbs side charging upper with the CMMG RDB bolt and barrel,  The lower is New Frontier C-5 for H&K mags.  First the bolt was dragging on mag follower.  Then it failed to feed.  And then we noticed the bolt binding right at the start of engaging the lugs.  Is something not compatible?  Hoping somebody has been there and done this.

Try it on a standard lower?

The NFA C-5 lower is designed for straight blowback not the delayed blowback RDB action.  The CMMG RDB actually puts the feed height back to match the gas operated 556.  Back in the day when Colt invented the AR9, they raised the feed height of the mag to feed 9mm and now that is the standard that pretty much all straight blowback AR's match, even it is Glock mags or whatever mags.
In some cases, the lower works with both the RDB and straight blowback like some of the .45 lowers out there, I think I heard the cheap 9mm PSA Glock lower feed height works with both as well.
I have an NFA S-9 loaner marked XK9 under an ATF variance and it works with both straight blowback and the RDB system but you do have to remove some material off the top of the feed block as well as remove the ejector in the lower (which I assume you already figured out since the ejector in a CMMG RDB Banshee is in the bolt).
See this link as I copied what he did.  https://www.ar15.com/forums/ar-15/New-NFA-S-9-Lower--Scorpion-mags---Pics--review-and-build-in-progress-with-CMMG-RDB/15-754694/

However, that is obviously Scorpion mags not MP5 mags.

1.  With the gun being empty, and not buffer or buffer spring installed, see if the bolt can freely go forward with no mag in the gun and see if you can feel if it is rubbing on the feed block or not.
2.  I suspect the C-5 lower is presenting the mags too high as discussed above, depending on how much it is, you could simply remove a little bit of material off the top of the mag catch so the mags sit lower.  I know the catch is probably proprietary so I'd reach out to NFA and see if they will sell you a spare.

Pictured below is my CMMG Dissent RDB bolt (full auto hybrid Dissent build - documented on my site here: https://c3junkie.com/?page_id=1733 ) - using fixed ejector in the upper vs in the bolt face for better reliability than the CMMG Banshee.  Sounds like you may not have this issue and it just may be the mag is feeding too high.


Also bear in mind if that do remove material from the catch and you now push up on the mag while firing it will most likely induce a malfunction unless you make additional changes to prevent over-insertion and I am not familiar enough with the C-5 to know the best way to mitigate that.  

Are you using the G4 enhanced cam pin?

A blowback lower with an RDB upper?

As I posted above sometimes it works.    New Frontier S-5 Scorpion straight blowback lower works also works for the RDB after removing the ejector in the lower and modifying the feed ramp.  Friend also runs a cheap PSA blowback Glock lower with his RDB and he just removed the fixed ejector in the lower only.