The key word here is consistantly. When I read that it I hear "the vast majority of the time".
I use to maintain the webpage and do all the newsletters and stats for the Hawks Ridge 1000yd BR club here in NC for several years. I've fired in several states and ranges over many years of BR competition to where the name is the game is to shoot the smallest group or highest score humanly possible.
And one of the best days for conditions for shooting 1000yds I think I have ever seen over an extended period of time was in 2005 at this club. Light Gun class fired first and for the next several hours it was a dream to shoot.
There were 75-80 LG shooters that day firing in relays of 12 shooters each. If you average each winning group per relay (5 shots at 1000yds) on that day for 8 total relays you get an average winning group of 4.257". The largest group fired that day to win a relay was 5.146" and the lowest score out of 50 pts was (1) target with a 48 score. And there were (4) perfect 50s shot that day for winning score targets. Obsolutely a trigger pulling contest. Days like this tell you how good your rifle is working and how good you gun handling really is. There is no holding off or nothing. It's simply shooting smooth and steady and not making the stupid mistake.
To give you a better perspective if you take all 2nd place groups out of each relay and avergae them.. they average 5.148" for (8) 2nd place groups. Only (1) of those 2nd place groups was in the 6" range . All the rest were 3" to 5" groups. Unreal conditions.
I consider the above results to be about as true representation of what is capable of rifles built with today components under the best conditions. Firing for the Hawks Ridge club starts at 9am and with 8 relays probably finished around 11am time frame. So over a 2 hour time frame with some of the best shooters, equipment, and conditions available, they averaged 4.257" and almost a perfect 50 score at 1000yds to win. This is best case scenario. I've seen several of these "winning" guns from this day get blown off paper at other matches when mother nature decided to flex her muscles a little bit. Been there done that.
So, when someone says they can consistantly shoot under 3", the BS flag goes up in a hurry. And as Kirby already pointed out comparing a 50 against std center fire cartridges in 6.5 through 338 caliber isn't a true apples to apples comparison.
So no I'm not buying it.
Now if we are talking 3 shot groups or groups within a group, I've witnessed some amazing 3 and 5 shot strings while working the pits and spotting the shots on target as they come through. I've personally fired a 5/8" (5) shot group in Heavy Gun (HG) class one time at Range #4 at Quantico. That was the first 5 shots, can't remember what all 10 measured but it was in the 5 or 6" range when it was all said and done. Doug ? fired 9 out of 10 shots into 1.9" up at the PA club in 1999? I think it was. The last shot opened up his group to 3.???" overal ctc. Still a great group. But 9 shots in 1.9" at 1000yds.. that is a pretty group. You can see a picture of it on the The Original Pennsylvania 1000 Yard Benchrest Club, Inc. The Birthplace of Long Range Shooting.
website. But all of these "groups" are ... woulda, shoulda, coulda and oh well, better luck next time.