You did fine - -
"Personal collection" is as good an answer as any, and better than some.
"Recreational shooting" is another good one.
I've seen a number of Forms 4 filled out with the "All lawful purposes," and they went through just fine. I worry a little that one of these might be reviewed by some antagonistic clerk, who might ask, "Oh? And why would you feel compelled to mention
THAT? Do you sometimes use other guns for
ILLEGAL purposes?"
Honestly, though, in a fair amount of dealing with BATFE functionaries over the years, they've all been pretty helpful and courteous.
FWIW: Years back, my very pro-gun sheriff designated me to review and sign off the various ATF forms. I've handled several dozen. Some have gone through slowly, but I don't know of one that I've vetted ever being denied. Oh, and, I've never refused to sign one presented to my agency . . . .
Johnny