Your definition of a Brush "Rifle"
is spot on. It usually just means and short, light, heavy caliber rifle used in heavy brush with a MV slow enough that deflection is not an issue. But no matter the length all big caliber front stuffers fit the heavy caliber bill. Before this whole ML addiction I got myself into, I too was a 30-30 fan for thick brush, which is where I hunt latter in the season when the old wise buck is leary of stepping into a field or big clearing. For early season Bean Field hunting I used my .303 for shots to 200 yards, more than that it was 300 Win Mag time. I only mentioned the removable breech plug on a sidelock barrel for ease of cleaning. I always worry that I don't get the breech clean enough. But then again a breech scraper would do the trick too.