If you want to use your existing carbine extension, the VLTOR stock previously linked will work, but the A1 length will be A2 at shortest possible with a carbine extension - it has battery storage, but thats it.
A stock I really like with a huge storage compartment and good value for money is the Mags EFX-A1, which can be bought as a complete kit from stag, including the receiver extension:
http://www.stagarms.com/product_info...roducts_id=373
- solid, great cheekweld but somewhat weighty.

