450 bushmaster is short stroking

[Bmt85]

You can get a 20" rifle gas to run, you might have to play around with it. I have a 20" rifle gas build that will just barely run factory 250gr FTX ammo with a XH buffer. A standard buffer won't be an issue.

If you don't want to mess with the rifle gas, then the midlength system is the preferred choice for 16" to 20" barrels, imho. You can go to a carbine gas, but they are usually over gassed with barrels longer then 16', but should still function just fine. You might still have to play with it to get it to run correctly (add weight to bcg, heavier buffers, or adjustable gas block). I also have a Bushmaster 16" carbine gas upper that ran fine with factory ammo and an H buffer. Ended up reloading (wanting to push the limits) and needed to run an XH buffer. Factory 250gr FTX ammo still runs just fine with the XH buffer.

As far as rail space and scopes, you want to run a forward angled/cantilever type mount for a scope on an AR. Mounting the scope on both the upper receiver and handguard can cause accuracy issues.