Acuracy probably won't be that much affected at close range, but it will most likly suffer, depends on how consistand the balls are flattened and how straight you load them. Other than smaking with a hammer I don't know how you would flatten them.
I have a pair of 51' navies by Uberti that won't shave lead with .380 balls, but will securly seat, the chambers are sligtly cone shaped and have a chamfer at the mouth, just have to make sure I rame them good and they work fine. .375 ball I can shove in with my thumb.
I cast my own now, but I used to order .380 balls instead of .375, where do you get yours?