G&P gearbox into Madbull receiver = won't go

[mimesis]

Trying to fit a regular G&P gearbox into a Madbull mbk, specifically the Daniel Defense model.

 

But the upper and lower receivers won't mate properly, 1 mm gap at the front, so front pin holes don't align, and pin won't go in.

 

I've tried everything.  If I take out the charging handle and fake bolt cover, it seems to fit OK.

 

I've tried two hop-up units.  If I take out the hop-up unit, and leave the charging handle, etc., it also fits OK.

 

But not both at once.

 

I've tried it without just the fake bolt cover, but that doesn't improve things.  I tried removing the plastic extension to the charging handle, no improvement.

 

I'm stuck.

 

Any suggestions for fixes?

 

Also: experience with gearboxes from other companies that actually fit MB bodies??

[QQexDERA]

Which MB bodies? Earlier and later ones have notable differences.

[mimesis]

Later.  The one with the two small pin locators at the rear, that fit into the lower receiver.

[NeoVeNoM]

that's most likely a CA copy.

Try fitting a Classic army gearbox.

In the past they clearly stated that their bodies were for Classic Army only.

[mimesis]

There are two versions of MB metal receivers--one is for the older CA design, the other for more "modern" mechboxes.

 

Anyway: here is the answer to my question.  It's not an easy fit.  In particular, the raised ridge that runs right along the top of the G&P gearbox hits the spring up in the "roof" of the upper receiver, that enables the charging handle to spring back into position.  Currently, I have a rubber band in there, and the two halves will mate and the front pin will go through.  When the charging handle return spring is in there, they won't.