It is true that light travels with a constant speed, but with different lenses do you have different focuses after all, and with different colours do you have different wave patterns and so on with some warmer than the other. That is at least what I think that the long barrel is forth, it is not a barrel in normal sense, but one giant lens.
Hmm, depends on how you interpret the barrel's purpose - the latest IG Codex simply describes it as "reinforced", with the range and power of the weapon actually coming from the higher energy discharge (the hot-shot chargepack), which leads to reduced beam dissipation.
This would still mean that you cannot reduce the barrel size, mind you, else (even if one were to go by my interpretation) you'd probably melt your own gun with the first couple shots.
My idea was based on keeping the barrel sizes whilst simply switching what makes a lasgun a compact lasgun. I was assuming it's not the barrel, but rather the alignment of the various parts ... like a bullpup rifle is shorter than one with a magazine in front of the trigger, even though it may have the exact same barrel size - simply because it shifts part of the machinery into the stock.
That being said, I am also relying entirely on visual impressions here - according to which the "inner" barrel of a long las seems exactly as long as that of a lasgun, the difference between both actually being the silencer-like muzzle. Under these conditions, any tinkering with the pattern that would reduce the length of a lasgun would also reduce the size of a long las by exactly the same length.
One could even assume that the vaunted long las is actually just a modified lasgun with a different chargepack and barrel - just like the Stalker sniper boltgun of the Space Marines has, in GW fluff, been described as a bog-standard boltgun where someone just stuck a targeter on top and attached a silencer. But that's merely a wild guess, of course.
You lose half the range when you Compact a weapon, which kinda removed the point of a sniper-rifle.
If he gets a telescopic sight (which any good sniper should have), this would only become an issue if he wants to assassinate people rather than being the "designated marksman" of his cell... What makes the long las interesting is probably the Accurate trait moreso than the 50 meters of difference it has to the lasgun - or at least this is what I'm assuming doomande is after?
My group's sniper had a similar question: can sniper rifles be broken down to fit in a foam-lined briefcase, like you see in movies? My ruling was that 'breakdown' would be an upgrade to an existing weapon, costing 50% of the weapon's base cost. If that helps...
Technically, every rifle should be able to be broken down into parts. They're not made from one piece, after all, and proper cleaning would even require disassembly.
However, given that this is 40k, I might well consider that this is actually forbidden because only properly anointed personnel is allowed to open the blessed iron skin of a machine to tend to the spirit beneath or some such Mechanicus nonsense! Not that the owner actually has to abide by such teachings, though, if he or she is from a place more accustomed to technology yet mostly untouched by AdMech presence (such as an Underhive) - but this would depend on the individual player character. Might make for some interesting religious conflict RP potential within the group, though.