I just received my S510 XS Ultimate Sporter Xtra FAC, Black Soft Touch yesterday from Pyramydair. It was a refurb gun (came in a soft case with the repair tag still attached to the case), in .177, and I picked it up when they offered an additional 20% off refurbs last week.
Man what a pleasant and accurate shooter! I just scoped it up (didn't even clean the barrel) and went out and shot. Best group so far is 0.14" at 25 yards, with multiple other groups at 0.2" or less, and I'm pretty new to air rifle benchrest shooting. Very nice predictable trigger (I haven't even tried to adjust it) which I found to be (surprisingly) at 1#11oz after I finished shooting. Very back yard friendly without any additional supressor. Plus on medium (3/5) power it looks like I can stretch it to about 70 shots before it falls off the reg.
I too was under the impression that the 'XS' guns were the regulated ones, as stated above.
My first experience with Air Arms guns came a few weeks ago when I got my Galahad, used from Baker Airguns. My initial impressions regarding that gun were 'Wow, these guys really know what they are doing!' This impression continues with my new-to-me S510.
It is not quite as accurate in my hands (yet) as my Red Wolf, but heck I've only shot 70 rounds through it so far.
So my personal thoughts, if you are thinking about one, is get a regulated (XS) S510 and you won't be disappointed. Watch the air tank size in the model you are looking at - I wanted and bought one which had the 288cc tank, whereas others have 186cc, or even as small as 115cc tanks (carbine and TDR). 70 regulated shots out of a non-bottle type gun is excellent to me.