shape of pellets

Most countries restrict the power of airguns to the point that they can't really push slugs that well. In the US more conventional bullet shapes are starting to catch on though. The Diablo pellet shape allows for a larger cross-sectional area and stable flight characteristics while not being too heavy for the guns. 

Diabolo pellets were made to shoot in smooth bore barrels originally. The work well in rifled barrels and are very forging unlike bullets. Almost every pellet will fire decently in your gun, but bullets you may try 10 different ones before you find one your gun will shoot, and you may find your gun will not shoot slugs.

Also there is an advantage to pellets over slugs in some situations. When you are shooting in a backyard or populated area, pellets are much safer to shoot. They slow down quickly and do not go nearly as far if you miss your target or it goes through it. Also when you shoot in the air at a bird on a wire or tree branch the pellet will not travel as far if you miss.

I make slugs for all kinds of air rifles and they are hard to match up to every rifle. Diabolo pellet shape are universal to all the pellet rifles.

The slugs shoot a much flatter trajectory than a diabolo pellet. For example, when I shoot my Cricket I zero the rifle for 50 yards with JSB 25 grain. At 50 yards my 37 grain slugs shoot a little lower than my pellet because it is 12 grains heavier. However, at 100 yards the pellet and the heavier slug have the same POI. This means at 100 yards the slug is traveling faster than than pellet, plus it's 12 grains heavier weight which means the FPE is much higher for the slug.

My Cricket is tuned to about 980 fps for JSB 25 and the slugs shoot about 850 fps, muzzle velocity. Even with these speed differences at the muzzle, the slug surpasses the pellet performance at range.

Past 100 yards and the pellet falls fast where as the slug is hitting with less drop and way more energy. In a lot of situations this is an advantage, but as I described above, sometime the slower pellet is the better choice. I own a pest business / animal trapping, and we choose different ammunition based on the job and safety factors for the job with these things in mind.

I can not put the link to nielsen specialty ammo but the 25 caliber slugs I make for .250 barrels are very accurate and shoot flat. They work in Cricket, marauders, and other .250 barrels. They do not work in Korean barrels because they are larger bore barrels.