They are recoil operated so 11-12" is possible but, very short A5s require a bit (OK, a lot) of work to balance the spring rates and lengths, etc. If you do choose to shorten the buttstock, remember to be aware of just how far into the stock the bolt recoil mechanism goes.
Also, do not forget that when you shorten the barrel, you will still have some latitude to adjust things with the barrel recoil spring, and the bronze friction piece. Also a magnum recoil spring may be needed.
When you shorten the barrel of a Browning long recoil operated shotgun, you will gain much acceleration initially since you have much less mass to get moving. However, that same lack of mass turns into a lack of momentum, which may be needed as part of the energy to operate the whole cycle. If it turns out you do not need the extra momentum, then you need to be aware that your now faster moving lighter pieces may beat up your receiver. Here is where the stronger barrel recoil spring may help. Lastly, a stronger bolt recoil spring (mainspring IIRC) may help slow down a too energetic barrel, but it will also launch the bolt back into battery with too much force as well.
Oh, and recoil
can be rather stout.