MP5 Flexing between receiver and barrel/gas tube
 

I purchased a G&G TGM A3 ETU MP5 with the retractable stock. The battery goes in the front handguard where there is limited space. I was frustrated with how difficult it is to reinstall the handguard so I put a PTS EPEF2 Vertical Grip on it to store the battery and drilled a hole in the handguard to run the wires.

The vertical grip creates extra leverage and if I pull on the grip the barrel flexes downward and if I push on the grip slightly upward. With the factory handguard the flex was not noticeable. With the vertical grip the muzzle moves about 1/3 of an inch.

It was recommended to JB Weld the joint between the gas tube and the receiver which would likely work as there is a small gap between them when the barrel flexes down.

Are there any other methods to stiffen up the joint between the barrel / gas tube and receiver that less permenant solutions?