I would argue that the notional capabilties where a lot less in years previous than today.
Years ago the nation of rifleman started shooting a long time before the Army, however my opinion is that while marksmanship may have been superior - combat marksmanship was often not.
Secondly advances in Night Vision, IR Lasers, Sound Suppressors have antiquated the majority of weapons systems, especially Sniper Weapon Systems that existed pre 9/11, or forced extensvie modernization efforts.
Our capabilities have increased exponentially since then, and the Militaries of at least Canada, the United States of America, Australia, and the United Kingdom, know a ton more about combat shooting than they did before.
We kill many more of the enemy in small arms engagements than they do of us, and we have adopted weapons systems that take into account the majority of the combat requirements.
However unlike SOF entities that are generally used for a variety of specific missions, and have equipment that is specialized for those missions, conventional forces are the jack of all trades, and they are equipped to do the broad spectrum of operations with pieces of equipement that compromise.
I will argue against part of my Arms Room concept for a bit:
10-11" 5.56mm guns do not do well with C77/M855 SS109 type ammuntion that relies of impact velocity, and one just has to ready MSG Paul Howe's account of what happend to CAG when they ran out of their specific 50gr BTHP loading for their CAR-15's and M4's in Somalia, and had to use M855 from the Rangers. The targets (malnourished Somalis) did not have enough tissue for the M855 to yaw and fragment, and they had multiple occasions of failure to neutralise on multiple CoM shots even in very close.
SOF units that can get specialized ammunitions for Short Barrel guns, can do very well with them, also on some missions it makes sence for them to carry a very lightweight relatively underpowered system (like the HkMP7), when they are either doing a specific hit that is expected at close range and they can get head shots, or when they are doing SR, and their backup is a radio and specific fire support from USAF or OGA aerial elements.
So unless an across the board ammunition change - like the USMC going to the ATK SOST round - short barrel gun systems can be a liability especially when needing to make longer range shots with effect.
Prior to Afghanistan, no one really expected the Squad or Section to need shoot beyond a 300m window, and a lot of the equipment for the convenstional Armies where tailored to the <300m fight.
Iraq for the US Military compounded that, with the majority of engagement being urban in nature. The same with the British with their NI experiences and Iraq.
Now in Afghanistan, the terrain in a lot of areas does not lend itself to Mechanized warfare, so the LAV/Stryker/Bradley/Warrior and its integral firepower cannot always support the troops, additionally the layout of the conflict does not always allow for the area to be pounded with 155mm/JDAM's etc.
Thus the Section/Squad Dm with a "Battle Rifle" type system can fill the need for precision fire to the 500-600m envelope and beyond.
Now 5.56mm can sometimes fill the bill, there was a R22eR CSM in Bosnia years ago that got a 873m kill with a C7A1 - however it did take a number of rounds, and from what I was told the CSM had spent most of his career in Recce as a sniper - so he had skill and experience on his side as well. 5.56mm does have problems with long range barrier penetration, and also unless using match ammuntion, does not have the best long range accuracy, and trajectory does not allow for range or wind errors like some 7.62mm loads (however in others it can be superior), but single loading 80gr SMK rounds (to long for magazine feeding) may be effective for the Long Range KD matches at Camp Perry etc., but does not really make any sense in Combat.
I will agree wholeheartedly with Illegio that adopting a SharpShooter system should mean that the same thought also goes into ammuntion selection as the platform and using C21 ball should be an emergency activity and not standard practice.
The UK Sharpshooter stayed with the Brit 155gr Ball round, as the British, unlike the US, Australian, and Canadians, beleive that OTM rounds should not be issued. I beleive however in talking with some UK pers, that that was more an interim method, until they can staff a specific round thru the system.
Personally I beleive that a 16" 7.62mm M16FOW based System fills the bill for this role, especially when the Country issues a M16based platform as its general issue system. One of the biggest drawbacks to this was until very recently none of the 7.62mm AR systems where reliable enough for this role.
Now you have our (KAC) SR-25 EM Carbine and to a lesser extent the LMT MWS, and Larue OSR. That are field reliable, and accurate enough to engage enemy targets and score kills to 800m.
SOF elements have been issuing shorter than rifle 7.62mm systems for some time, and the need has now been identified for conventional forces too.