I am running a standard can.
I understand, assembling the parts is only half of the process. Getting everything to play together properly is the other half.
I have some aluminum mags I am going to try.
Man, I really hope that I wasn't coming across as an a$$. It's just a peeve of mine. I totally understand that there are some guys that really do learn by doing and experience is their teacher. I kind of fall into this category, but once I realized how much I didn't even know what I didn't know, I got a book and read it from cover to cover. Once you can visualize all of it in your head, that's when it becomes tinker toys.
Look, for a 300BO rig, assuming that you don't want to use an adjustable gas block, I've found that an ideal place to be with a standard can is to tune the rig to run just right with supers, but just slightly undergassed with subs, meaning that it will cycle, but the bolt won't always lock back on empty. This way, it'll run good with supers, but when you add the can, the added pressure from the can will have it running right suppressed with subs.
Let's just assume that bolt carrier group and gas block are right. That would leave the size of your gas port in question. If you have a selection of drill bits, just move the gas block to the side, or whatever, and find a drill bit that fits snugly in the gas port hole. Now, measure that with a caliper or micrometer. What's your measurement? If memory serves, it should be around .095 or so; UNLESS you got a barrel ported for subs to be run suppressed, in which case, it's gonna do what you described when shooting subs unsuppressed.
Honestly, the sweetest and most versatile running rig I've put together was a 10" with the gas port at .1 and running a RifleSpeed gas block to tame the gas.
In any case, before you go drilling anything, try running one round in a true 300bo mag and see what happens. Actually, when running the one round in the mag, it shouldn't matter what mag you use. We just need to confirm undergassing. BTW, do this with the standard buffer. Or and H1. The H3 is getting a little heavy.