5.50 diabolo pellet source ?

As far as I know 5.51 is as small as they come. And, for what it's worth, I think my Matador preferred the "small" pellets most in that video because it had a particularly aggressive choke. Ed alluded that the R2.0 Matadors had this idiosyncrasy, and that is was "corrected" in the R2.5 and later models. Most of my .22's do best with 5.51 and 5.52. I know you know this, but I want to reiterate that the sorting exercise was not to determine the "best" pellet head diameter. I only wanted to show that finding an optimal pellet size for your (LW) barrel was necessary for achieving maximum accuracy.

"Ted"As far as I know 5.51 is as small as they come. And, for what it's worth, I think my Matador preferred the "small" pellets most in that video because it had a particularly aggressive choke. Ed alluded that the R2.0 Matadors had this idiosyncrasy, and that is was "corrected" in the R2.5 and later models. Most of my .22's do best with 5.51 and 5.52. I know you know this, but I want to reiterate that the sorting exercise was not to determine the "best" pellet head diameter. I only wanted to show that finding an optimal pellet size for your (LW) barrel was necessary for achieving maximum accuracy.

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I do like the way you put the "corrected" in quote marks "" Ted I have one of the early production R3s and it has a very aggressive choke still.. It has to be my most pellet fussy PCP Ive ever owned but once I found a good supply of 5.50mm 18.1gr JSBs it has also remained my most accurate long range PCP I have ever owned.
I did loads of scope cam testing before I found the correct head size and there is really no better way.
Unfortunately JSB have many dies and it is nearly an impossible task to get a complete tin of any specific head size without the tedious task of measuring every one and sorting them into new tins.
I was lucky enough to be sent an entire box of 30 tins of a single batch production (Edgun labeled) JSBs amongst many boxes I imported from CZ that all had a very consistent 5.50mm head size.
I think once I have used them all up I will replace my barrel to something that will shoot the larger pellets as they are in much better supply. I think the 5.48-5.50mm pellets are actually an anomaly with a new or worn out die (I'm not sure which) and are not the norm for JSB.
I have however found the 18.1gr JSBs that are relabeled to Cometa tins are often these smaller head sized pellets too.. maybe these are worth checking out if indeed you find the 5.49-5.50mm are the correct size for YOUR bore and choke.

Heres a vid I made a while back to show the effects of using the correct v/s the incorrect head size pellet for your barrel. 
Please note; As Ted has said it is not saying that 5.50mm is the best pellet for all guns... 5.49-5.50mm is just what my super tight choked R3 prefers.
With the correct head size pellet matched to your bore you can also shoot them much faster than the norm and still keep them stable. I can shoot my 5.50mm well over 1000fps in my Matador without any noticeable change in accuracy.

Oh yes the choke fouling... Im glad you mentioned it.. my R3 is real bad for it to the point that I have to clean it with a bronze bore brush in the choked section just to get it clean.. 
I have tried re-sizing larger pellets down to the 5.49mm with both a head sizing die and also one that does the skirts as well, but both methods were unsuccessful for reasons I cant explain. The 5.52 pellets sizes down to 5.49mm spiraled just as wildly as a 5.52mm right out of tin. I can only put that down to the amount of lead that contacts the bore as it goes through the "crush" zone of the choke. 
Interestingly my .22cal P12 had no choke and by no means a "quality" made barrel but it was the least pellet fussy barrels Ive tested.. it shoots the Jsbs 16gr and 18gr of all head sizes (even mixed) into tiny little single hole groups at 50 meters. If it had the same twist rate as my R3 Id be willing to bet it would out shoot it all the way to 100m too.. however it does not so the pellets loose the needed rotational speed to perform to the best of the 16-18grs JSB pellets potential at 100 meters.

Until I owned this unchoked .22 P12 I would have said a PCP barrel has to have a choke to get the best accuracy but now Im not so sure. I think there are far more unmentioned variables that come into it and I am still on the fence as to which barrel has the most accuracy potential.
I see Lothar Walther offer both choked and non-choked airgun barrel blanks though so maybe there are different niche's for both? 

"Wingman"
With the correct head size pellet matched to your bore you can also shoot them much faster than the norm and still keep them stable. I can shoot my 5.50mm well over 1000fps in my Matador without any noticeable change in accuracy.

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Yep - I have seen this, too. I lack the skill or luck to have found such a combination for myself. But, I have personally shot such guns [belonging to others] who have. I have a few guns that got very close to doing this. But, the additional speed (950+ fps) caused an increase in fliers. The groups got tighter at 100 yards, but the fliers became more frequent and much more pronounced. If I cherry-picked my results, or kept my groups to 5 or fewer shots, it would have appeared that I owned laser gun. Some of those small groups were the best I have ever put on paper. But, 10-shot groups always brought me back down to earth, usually to the tune of 7 in one ragged hole, and 3 others barely on the 5" target. My search continues...

This is the first I've heard of choked air rifle barrels. I've used shotguns with chokes for pattern size, but why on an air rifle. Wouldn't it be better to have a uniform barrel the whole length that the pellet travels?

Just a word of caution to those using cleaning rods. I would not do that. Use a pull through setup like Otis or another coated cable system. It is all too easy to ruin the crown. I know because I did it to one of my Ruger 10/22s. That gave me the excuse to upgrade. I gave it to a buddy with a lathe and he got it going again.