I would like to think , that blowing air through the gas system would tell you something about the Volume of gas running through the system , but IMO it only will show that some air is getting through , not how much . If the Port is half way covered , you wouldn't know it from being open all the way . The only way is to take measurements of the two ports ( Gas Block & Barrel Port ), to line them up .
I agree , you absolutely need to make sure there is no gas restriction . As said , I find it odd that it functioned fine the first few rounds . Something changed & the Gas system would be the first thing to make sure is working to spec's .
The reason I asked if the BCG is an AR 15 or M 16, is the weight difference between the two . Using a lighter Buffer , if system is indeed short cycling , would be less expensive & less trouble , but a light weight BCG with a Pistol Gas system , to me would be the way I would assemble .
I would like to see the bottom of the fired casings & a side view also ,of the cases .Looking for scratches on the side of the case .
I had an XM177 ( 10 1/2 bbl . ) that came to me because it would not reliably function in full auto , I had to lightly ream the Barrel's gas Port till it functioned reliably . Its not something that's normally done , but last resort , if all else fails . This is the only AR 15/M16 Barrel I had to do this to & that's over thirty years of working on them .
As said , I find it odd that it functioned fine the first few rounds . Something changed