First off, THANK YOU for taking the time to write out a decently worded post. It's so much easier to read through and digest.
I don't really mean to nit-pick...but you're dancing around the question posted by the guy who started the thread (at least how I interpret it).
Paraphrasing..."All things being equal...is one better at longer range than the other?" I suppose I'm kind of zeroing in on the accuracy difference aspect of that.
All things being equal...Their barrels are equally firmly attached to the receivers, they're barrels are either both one-piece or two piece, the hopups are the same, the rubbers are the same, the bbs are the same, etc... I'd agree though...yank any front end sideways and it's not going to be good for hitting what you're aiming at. You've got more leverage to pull on a long gun...but only if you're pulling at the front bit of it.
I suppose I'm thinking of the question as BNIB, is one better than the other at long range? Not..."after nocking it around woods, this one holds up better than the other". Not..."how do I take one of the two and make it shoot more accurately"?
Your VSR vs Mk23 example is flawed in this context...they are different guns, barrels, hopups and even propellant sources, etc...
I have cut and recrowned AEG barrels...my CQBR is much less accurate than my M4 (velocities and stuff being "equal"...impossible test, but it's noticeably evident to me at least). Another comparative example would be a TM SOCOM M14 vs a TM M14.
NO argument from me that you can make
a short gun shoot well (and shoot better than a lot of long guns on the field) and that there are more effective "fixes" than simply adding a longer barrel, and that simply adding a longer barrel on it's own might yield debatable benefits...absolutely true.
But a longer barrel is still more accurate than a short barrel....all things being equal.
*********really, the real steel internal ballistics are a different topic and forum*********
IIRC, The longer barrel lengths and it's association with "long range accuracy" for real steel comes from the tweaking the pressure curve of the gas build up behind the bullet. Some of the most accurate rounds in short range are so over-bored that they're not very "efficient". But the power curve that they generate is particularly balanced for the bullet shape/size/etc... used for that purpose. As you shoot with bigger rounds, you need to keep that pressure building behind the bullet...and one of the ways to do that is through lengthening the bore (extending the Dwell time I suppose).
The 6mm PPC might be the most accurate round in the world...up to 300-400yrds. At 600yrds it's really just hanging in there. At 800 it's a mess (and the hold over is nuts). Caveate that some rounds are made for punching paper...and others are made for punching meatier things.