So if you say your gun likes X size pellets, exactly what head size are those? Do you know for sure?
In all due respect, Ross' comment makes some sense. There are some tins / brands of pellets that I have measured that mostly have a pretty tight range of +-1 head size so if the shooter happens upon one of those tins then it makes sense to (unknowingly) say that my gun likes X brand, weight and head size (printed on the can) of pellets. They continue to feed those particular "brand, weight and printed head size" pellets to their rifle and a lot or two down the road their accuracy goes haywire. Why, because those +-1 tins are pretty rare to find and the manufacturer's QC isn't consistent enough to meet the specs on the can. Consistently providing the shooter what's printed on the can costs money, time, resources and equipment maintenance all of which drive costs up and can't account for residual wear in the dies and variation in the process.
Even though one large manufacturer (JSB) makes the majority of pellets that many of us tend to shoot and re-brands those pellets, all are not created equal. Air Arms for instance has JSB make their pellets on proprietary dies owned by AA so their pellets are NOT the same as the average JSB pellets. The range of head sizes are generally much tighter than JSBs. I have also found that certain models of FX (re-branded JSB pellets) have had much tighter head size range than JSB. Therefore, it seems logical that FX may have theirs made on proprietary dies too but that is not fact only a non-SWAG.
The Wally World CPHP's have the widest range of head sizes that I have measured. Some of us plink with those pellets and they work fine for rolling tin cans but try to shoot tiny groups at 25Y-50Y with out of the tin pellets and you will see many fliers and groups that are not on par with other higher quality mass produced pellets. I have sorted many tins of CPHPs to cull out the head sizes that my guns prefer and it makes a huge difference when shooting for precision and accuracy.
I tried to measure head sizes with precision mics and it simply won't work. Yes, you may have the best mic on the planet but its useless because pellet heads are not round and you will chase your tail and give up on head sizing because your measurement system can't work like a PelletGage can. Once you find the head size that your gun likes you can rapidly sort pellets by purchasing a pair of Go-No-Go PelletGage Rs.
Here's a thought to consider ... I have often wondered if the choke in the end of pellet barrels was there to account for a wide range of mass produced pellet head sizes because a smaller choke effectively re-sizes those wide ranges down to a smaller size? I am no gun smith or firearms engineer but its crossed my mind more than once that chokes have a specific purpose to do that vary thing. But what about the unchoked barrels? Well they shoot just fine if you find the head size that they like. What do you think the slug manufacturers are offering so many different head sizes? Try shooting a tin of pellets that has a WIDE variation of head sizes like CPHPs through one of those unchoked barrels and watch your groups go crazy, LOL. You will have much improved accuracy with those pellets in a choked barrel.
I'm certainly no expert but I have learned quite a bit over the last few years shooting air guns. Some through reading internet resources like this one and some through seat of the pants experience. Respectfully I put more faith in the latter.
Hope all of my rambling makes some sense?
