Cans add back pressure.Sometimes too much pressure.Ask the 308 crowd or dpms sass crowd.They buy lots of my gas blocks.In your case not enough pressure to cycle without the can.Check gas port hole size.On shorties 1/8" is std.

ok, now I have a different but related question.

in my 5.56 AR15's if I load up a subsonic 5.56 round I know it will not cycle the next round.

If I load up a subsonic 300 Whisper round the same thing happens.

why is the Whisper round supposed to work but the 5.56 round will not?

is it only a function of a heavier bullet?:confused:

A 220 grain bullet requires four times the energy to drive to 1000 fps as a 55 grain bullet. Caliber doesn't matter much. It also carries four times the energy when it leaves the muzzle. E=1/2M*V^2 Note also that it only takes 1/9 the energy to drive any bullet to 1000 fps that it takes to drive it to 3000 fps which is why it's very difficult to make a gun which will shoot both sub and supersonic bullets without modifications. AR-15's were never designed to shoot multi-energy cartridges. . Not like a bolt action!.

So yes it's the heavier bullet mostly. You really have to look at the pressure curve vs bullet position in the barrel using an appropriate powder. Burn rate of the powder can also make a big difference in the gas impulse available at the gas port. Quickload computer simulations can give insight on the avaialble impulse energy, though it's not a direct output of the program.

perhaps I should look to a different powder.

currently I am using H110. for both subs and supers

where should I look for a little better pressures as a starting place?