Personal opinions based on experiences with my .177 cal HW95 and .177 Beeman R9! I believe that as long as the pellet head is large enough to be "sized" in the barrel leade there will be consistent accuracy because every pellet that passes through the smaller leade will be swaged to the exact same size and shape. That "consistent accuracy" will vary dependent on the geometry of the bore.
For example a couple of my .177 HW95 barrels there were a couple minor constrictions, one where the barrel was pressed into the barrel pivot block, one where the "numbers" were pressed into the barrel, and a tighter constriction at the "muzzle choke" These constrictions could be easily detected by carefully pushing a new CPL through the bore.
Anywhoo........I have a .177 Beeman R9 break barrel which has a rather loose leade that is very accurate when shooting 7.9 grain CPLs with their "largish" 4.54mm-4.55mm heads. With both the .177 R9 and the tight leade .177 HW95 the "fat headed CPLs" were/are very accurate, however loading a couple hundred CPLs into the HW95 during a shooting session WILL give me a "sore loading finger". I made up a pellet head sizer that gave me CPLs with 4.48mm heads and they were extremely accurate from the tight leade HW95, but not so much with the looser leade Beeman R9. I later found that when I opened up the pellet head sizer to give 4.50mm CPL heads the pellets were accurate with both the "looser leade" R9 and "tighter leade" HW95 without the sore finger syndrome. Playing with cheap Winchester Domes I bought at Tractor Supply (black & gold label) I found that the pellet heads were rather small.........
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When shot at only 18 yards those cheap pellets weren't very accurate from the .177 "larger leade R9", however they were pretty accurate at 18 yards from my "tighter leade HW95 (shot sitting on a "bucket" resting the gun on cross sticks).............
LOL....each group had one flier. Humm....bad pellet or bad shooter? I vote for the SHOOTER!
I recently bought a few tins of 4.52mm AirArms domes and found that most of the heads measured 4.51mm.........
so I shot a group "straight from the tin", then ran some AA domes through my 4.50mm sizer and shot another group, again at only 18 yards sitting on a bucket resting the gun on cross sticks. The sized AA dome group was a bit tighter than the "straight from the tin group", however that simply could have been "shooter related" rather than pellet head size related............
Anywhoo....as mentioned previously, I believe that as long as the selected pellet has a head large enough to be "swaged when loading", every pellet going down the bore will be the exact same size and shape. The issue is when the pellet heads vary so much that some are "swaged at loading" and some are so loose fitting in the lead that they rattle down the bore! This is the issue I had with the supposedly 4.52mm JSB Exacts I bought a couple years ago. All were a bit undersized, however some had heads so small and loose fitting that the pellet would flip out of the leade simply when relatching the R9 barrel! That episode years ago prompted me to measure a few thousand supposedly 4.52mm JSB Exact pellet heads and this was the result............
At that time I also had a tin of supposedly 4.50mm JSB Exacts and found pellets in the 4.50mm tin with heads larger than many from the 4.52mm tin and visa-versa!