Here's an updated pic of my AR15 and 15-22 twins:
On my setup, the handguard conversion measures 25.3cm from the tip of the upper receiver to the side edge of the magpul handguard compared to the factory rail which measures 25.4cm , however that doesn't include the front sight. If you include the front sight it measures 29.75cm.
On my 15-22 I measured from the tip of the bayonet lug to the shoulder of the barrel after the threads and it measures at 7.35cm.
On all these measurements, I would like to point out that I have NOT ground of the positioning tabs on the upper receiver that index with the factory rail so my AR15 barrel nut is 6mm away from the edge of the receiver on the side and top and 4mm on the bottom.
It doesn't look like that barrel shroud will work if you install Magpul MOE Midlength AR15 drop in handguards using the 15-22 handguard conversion parts and true shot tech A2 front sight. These guys are right, you have to have something on the front end to hold the handguard cap for the handguards to clip into. On an AR15 there is an extra shoulder machined onto the barrel that the handguard cap rests on and then the front sight or gas block pushes up against it on the other side. Since the 15-22 barrel doesn't have that shoulder, I had to tap the gas hole on the A2 sight and put a screw through the handguard cap and into the sight to hold it on there. If not, then when you remove the handguards the handguard cap would rattle loose and slide up and down the barrel.
It looks like you've already decided on getting another freefloat handguard and have the shroud fit inside. The shroud websites says it measures 1.5" OD so you'll want a handguard that is at least that measurement ID. Here is a link to a spreadsheet with handguard dimensions and weights that should prove helpful.
https://docs.google.com/spreadsheet/ccc?key=0Ajl2UPK0UHPscHdzd2hlZ2hOUXNGblVaTWtaTkYyRFE#gid=0