The 505 will not fit on the 500 The 505 has a shortened magazine tube which means that the barrel lug (where attaches to magazine tube) is shorter (From rear of barrel) on the 505

Read below the pictures of the barrels They'll fit mossberg.com/category/series/maverick-88/

No, the 505 youth barrel does not fit standard 500 or Maverick 88 shotguns or vice versa John. Only exception is the Maverick 88 and 500 series interchange.

Maverick 88 pump-action models are fully-interchangeable with Mossberg 500 barrels (within gauge and capacity). The diverse selection of available accessory barrels allow shooters to adapt their Maverick 88 for multiple hunting or security purposes.

I'm pretty sure it will fit John