Do you have a caliper? Measure the area in front of the m203 notch. I've converted several bear creek m4 barrels to dissapator, but they are very close to .750 where the FSB mounts. I've measured a dpms barrel and it was quite a bit smaller. You have a couple issues if it is to much smaller, for example if the barrel is .730, the difference is .020, so half of that is .010, that means your sight post will have to be that much higher. Some people have talked about cutting pieces of a feeler guage to make shims, that'd probably work if you need it. I install my fsb with the handguards so that I get the length right and I bore sight it to get it straight, but I live fire test it to verify zero before I locktight it on tight. In the past I found a roll pin that fit the gas tube hole, but it was a little big so I had to drill the hole out, but now I think it'd be better to just get a gas tube and cut it off so it sticks out about 1/2" to hold the handguard end cap. You don't want to ruin that arms sight in case you decide to repurpose it. I also peened the cap where it gos around the barrel to make it tighter.
If you need to make shims from a feeler guage the equation is: .750 - (your barrel diameter) = (the difference) ÷ 2 = (the size shim you need)