Dillon 550? not mine.
I use an RCBS "RC" Press with a Longer Handle to both Reload .25ACP and to Reform 5.7x28mm or .22 Hornet cases into Lengthened .25ACP/.25 Stevens parent cases.
The Main Problem I found with the .25ACP is that Development seems to have Stopped with the Cartridge with the use in short Barrel "Pocket Pistols". I have only found an early (About WW1) mention of any commercial attempts to market a Rifle in this Caliber.
John Browning designed it as an easier Feeding Cartridge than the the .22LR of the same era (1904-1908) with equivalent Ballistic performance. The .22LR has improved greatly since, but not the .25ACP. It is even Tested only in a 6" barrel! The .22LR Is tested using a 24" barrel.
Now I KNOW the case will Perform to higher Pressure loading in a SUITABLE Firearm (Don't try it in a typical Blow-Back Action Pistol) as I have fired successfully several rounds of .25ACP loaded with 3.0 to 3.1 Grains of BE-86 and a 63 grain Lead FP bullet out of a converted Ruger 'Single Eight' Revolver.
This is a compressed Load of Propellant that has a somewhat slower Burn Rate than the Typical Bullseye charges recommended.
My Revolver also has a 10-5/8" barrel on it, over 5 times the length of the typical 'Pocket Pistol" ones.
Factory .25ACP fired in the same revolver do feel 'weak' by comparison, naturally.
Anyway, my main problem reloading .25ACP is ACCURATE measurement of the Small Propellant charges (Bullseye = 1.3 to 1.4 Grains) even with a Balance Beam Scale; as its resolution is only 0.1 grain as Marked.
Doing Load Development trying to hold 0.01 grain accuracy repeatably is a problem.
At least in the Lengthened cases the total charge weight gets up to where 0.1 grain accuracy is 'good enough'.
Best Regards,
Chev. William