Yeah,thats one issue with the cheap gun,the tips for the different size studs are exactly the same other than the hole in it.The studs have a different thickness head,so that when you use the largest ones,the ring does not make good contact on a flat surface with the thick headed studs{the center piece does not depress enuff},you have to kinda lean it a little.The smaller studs allow the ring to make good contact all the way around.
I guess they figure on people using the big studs for heavily damaged areas where the surface is completely irregular and the smaller studs on the more flat panels where its minor damage????
There may be adjustments for this on a more exspensive gun??I am going to surface the tip of the larger diameter ones soo there is enuff clearance to fully depress the gun on a flat panel.The way it is now with the large studs you cannot make contact on a flat panel with the ring without tilting the gun.