Ah, I have been doing short-gas front ends. On L1A1s, I go 4" short on the gas system, and the barrel about 12" with a welded 5-slot FH for 16" overall. I'm trying to get Matt at Ironwood to make a special run of wood HGs for me for these, otherwise I just shorten whatever is handy. I have hundreds of the laminated wood Australian HGs and they are easy too.
The metrics I shorten by 3" since that is what DSA uses and has HGs for them.
What muzzle device do you plan to use? And will you finish at 16"?
If you change barrels, you will have to change locking shoulders and re-headspace. But if we shorten yours, no problem.
I built my first L1A1 as a 16" para, and I love it. I replaced the pivot pin on the stock with a stronger solid one, and now there is no release button to dig into my hand. Lots of little details. My latest is an L2A1 Heavy Barrel, but I need two small parts to finish it, and they are few and far between.
Email me if you want, and tell me what you want to do.