We've been using CandleSnuffer's 22" X 28" rectangle target for the 200yard match. It is not difficult to keep 5 consecutive shots on this target, offhand.
To shoot accurately, I firmly believe one must use as heavy a gun as can be mustered, along with as large a calibre as can be handled. That includes the powder charge needed for long range shooting.
Merely look at the BR guys and the loads they shoot at 100 and 200yards to achieve their BEST accuracy - then look at the calibres and twist rates to see what works- in both veins.
To attempt to shoot accurately offhand at 100 or 200yards, one must use an accurate load to start with, or you merely double or triple your potential group size.
A hate to say it here, but judging from some of the load combinations some of the guys here use, I am not at all surprised that people cannot put 5 or 10 shots on a 3foot circle at 200yards offhand.
Like Taylor said, you've got to make the rifle shoot as accurately as possible FIRST at the longer ranges, then practise offhand at 50, 100 and 200yards.
As to barrel length, it's a tossup - or contest between the longer sight radius helping and the extra length hindering due to follow through having to be longer. My 31" Sporting rifle has made some nice groups at 200 - round 2" to 2 1/2", and one nice 6 shot group, lost count, of 1 1/4" wide by 3" high.
Yes - those are bench groups at 200yards, but that accuracy helps immensely in the final talley. That was when Taylor and Candle Snuffer both used 50's, I think they were, neither of which had the 'bench' accuracy my little 14 bore was capable of (iirc).
If I was to make a rifle for shooting 200- it would be a 10 to 11 pound 16 bore and of English design, or maybe a Jaeger- and it would be a moose and elk rifle as well.
Taylor's got his .62 Hawken now shooting in the 3" arena at 200. It now has an opportunity to beat my .69 - albeit a slim one - HA!