Does anyone know what this could be?  Thanks!

 Banshee

Depends on barrel length. Colt lists them in the Armorers Manual as follows:

Barrel Length           Gas Port
20"                      .092
16"                      .062
14.5" M4                 .062
11.5" commercial         .074
11.5" XM177              .067
11.5" carbine            .072
LMG                      .104
M16A2 20"                .092+ .001
     14"                .062+ .001
Commando 10.5"           .081+ .001
LMG                      .106+ .001
R6511 (222) 20"          .0995+ .001
7.62x39 20"              .125+.001
FYI we had 2 16" FA Bushmasters that just wouldn't run. Both would hit about 3 rounds, and then fail to extract. All the usual remedys didn't work, so I tried my Colt M4, and M16A2 barrels on the Bushy lowers. Both worked like a champ. We had to send them back to BM to fix the gas port diameter-16" is an odd length for a full auto.    

Thanks much!

 Banshee