I started coating bullets in 2014 when it was winter in ne Ohio. When spring came around, I did head-to-head testing with coated bullets vs traditionally lubed bullets. I used a beater 629 for the testing.
I had cast 5 different bullets using the same alloy (8/9bhn). 1/2 of each type of bullet got either powder coated or traditionally lubed with LBT soft blue lube. All bullets were sized in a Lyman 450 (.430") along with the lubed bullets being lubed in the 450.
I used mixed brass (blammo ammo) and ww lp primers and the same reloading dies to reload all the test loads. Made ladder tests using 7 different powders with those test bullets.
I tested the reloads @ 25ys using a rest and sitting at a bench. Decided to use 1 1/2" 6-shot groups (golf ball plinking loads) as an acceptable standard of accuracy.
At the end of the day the coated bullets had 13 different loads that would hold that 1 1/2" 6-shot standard (outside to outside holes in paper measurements). The traditionally lubed bullets had 3 different loads.
13 vs 3
Where the coated bullets more accurate??? No!!! It was simply easier to find accurate loads. Easier by a 4+ to 1 ratio.
The coated/cast bullets were more consistent than the traditionally cast/lubed counterparts. The mechanics of the lubed cast bullets are pressure dependent. Meaning:
You hit the loud button and the bullet is off to the races. Pressure is exerted on the bullets base (bottom drive band). This compresses the grease groove forcing (hydraulically) the lube outward and forward. The forward pressurized lube gets between and in front of the front drive band/bands sealing and lubing the bbl/bullet. This type of setup is dependent on a loads pressure. Different bullet alloys need different pressures to compress and do the difference in soft/hard lubes. Get the combo right and you have an accurate load. Otherwise, things like flame cutting or bullet distortion start to enter the vocabulary.
A picture that was sent to me, they are 44cal swc's cast out of different alloys and shot using different loads/pressures. You can clearly see the lube grooves at different states of compression.
A coated bullet does not have this alloy/compression/load pressure dependency. The coating is already where it has to be sealing/coating the bbl/bullet. That leaves bullet distortion to affect the accuracy of a cast/coated bullet.