Academic question about an antique single shot pistol

[Haywire1]

So remington made a series of rolling block pistols which are just cool( to me anyways), and while i am fully aware of the calibre list that somehow makes an antique pistol not an antique, my question is:

Hypothetically speaking, assuming one had said rolling block pistol in a .45 caliber( not 45lc), could one rechamber it to .410( the shotgun round, not .41 colt) and it remain an antique?
1) .410 is not on the list of calibres that would move a pistol from antique to restricted
2) would chambering it in .410 turn it from an antique pistol to a shotgun, which would then be prohibited by oal?
3) or should one simply chamber it into .460 s&w with a long throat, mark it as such, and just never fire .460s&w out of it? (460 s&w is 45lc based but with oal of 2.3”, fired length .410 would be 2.5”

[M1917 Enfield]

I had one that a skilled gunsmith had previously converted to .357 mag.

He replaced the original .50 cal barrel with a new custom 8" .357 bull barrel, new manufacture small firing pin breechblock and new hammer (both 4140 steel), new extractor, new receiver pins (also 4140) and re-case hardened (proper colour case treatment to match original) the receiver.

Basically everything on it was new manufacture except for the original antique dated receiver, the walnut fore grip and the pistol grip.

I shot .357 mag 180gr. lead bullet handloads (up to 28,000 psi) with no issue at all, including excellent extraction, low pressure signs on the fired brass and near perfect not stretched or bulged brass. The barrel and chamber area was very thick at about almost an inch total.

It was very accurate to boot and would make a excellent bush survival gun.

I asked my gunsmith how much he would have charged to do the same and he told me just the new custom made parts alone would be about $1000, with another $850 for the colour case hardening treatment and another $700 or so to put it altogether with a re-blue of the other new parts and test fire it.

So it was not a cheap gun to either buy or build, but was a beautiful and truly custom one of a kind. Just the originals in .50 blackpowder cost about $3000 without the rebuild work.

I sold it to a good friend with first option to buy it back if he should want to ever get rid of it.