Thanks for the quick reply Dragon,
answering to your question is a complex matter:
in Italy we have all firearms ( imported or not) listed in a governament catalog and any single weapon has his own specifications ( fixed stock or collapsible, barrel lenght and son on) in theory you cannot modify the firearm in any way but there are some exeptions.
For example if the firearm is listed with the collapsible stock ( and our mp15 is ) you can also put the fixed one...but you cannot do the reverse.
If it has a flash hider, you cannot change it but if it has not...you can ad the one ylu like.
I know is funny but this is italy
this is the link to the "national catalog of firearms".
- Armi e Tiro
P.S.
I choose the collapsible stock and burd cage flash hiders.