Do shotguns & rifles made prior to 1899 come under the same
ATF regulations as modern weapons. Such as barrel and the
overall length? And if so does this extend to Muzzel loading
firearms to also include cap & ball revolvers with detachable
stocks and less than 16" barrels & replicas of the same?

I would personally go on the ATF's website and see if you can research that. If not, I'd seek out a local office that if not too far, can be visited and asked face to face. If you do it over the phone, make sure you get the Agent's name and ID#.

Research is good, I agree. And I believe the correct year is "1898". Once a weapon is no longer classified as a modern firearm the rules definitely change. However, C&R firearms still have NFA issues:

Curios & Relics | Bureau of Alcohol, Tobacco, Firearms and Explosives

The National Firearms Act (NFA) is pertinent to your question:

Section 2.2 Antique firearm. Firearms defined by the NFA as “antique firearms” are not subject to any controls under the NFA. The NFA defines antique firearms based on their date of manufacture and the type of ignition system used to fire a projectile. Any firearm manufactured in or before 1898 that is not designed or redesigned for using rimfire or conventional center fire ignition with fixed ammunition is an antique firearm. Additionally, any firearm using a matchlock, flintlock, percussion cap or similar type ignition system, irrespective of the actual date of manufacture of the firearm, is also an antique firearm. NFA firearms using fixed ammunition are antique firearms only if the weapon was actually manufactured in or before 1898 and the ammunition for the firearm is no longer manufactured in the United States and is not readily available in the ordinary channels of commercial trade. To qualify as an antique firearm, a fixed cartridge firing NFA weapon must meet both the age and ammunition availability standards of the definition.

Which means that if you have a sawed-off 12 ga double gun using shotgun shells made in 1886, it IS an NFA weapon. If it is a muzzleloader, it is not.

That makes it clear to me. I wasn't up on the Ammo angle. That
is the reason a Pre 1898 12g shotgun cannot have barrels less
than 18".