As far as I know the only firearm that has to be 50 caliber is handguns. Rifles have to be 44 caliber and up.
https://www.pacode.com/secure/data/058/chapter141/s141.43.htmlb) Flintlock muzzleloading deer season.
(1) Permitted devices. It is lawful to hunt deer during the flintlock muzzleloading deer season with a flintlock muzzleloading firearm. The firearm must be an original or similar reproduction of muzzleloading firearm manufactured prior to 1800. The firearm’s ignition mechanism must consist of a hammer containing a naturally occurring stone that is spring propelled onto an iron or steel frizzen which, in turn, creates sparks to ignite a priming powder. The firearm must have open or aperture sights and be a .44 caliber or larger single-barrel long gun or a .50 caliber or larger single-barrel handgun that propels single-projectile ammunition.
(2) Prohibitions. While hunting deer during the flintlock muzzleloading deer season, it is unlawful to:
(i) Use manmade materials attached to the hammer or frizzen to create sparks.
(ii) Use telescopic sights.
(iii) Use or possess multiple projectile ammunition or ammunition other than required under section 2322(a)(4) of the act (relating to prohibited devices and methods), except as authorized under section 2525 of the act.
(iv) Use or possess a device or ammunition not provided for in the act or in this subsection, except as authorized under section 2525 of the act.
(c) Muzzleloading deer season.
(1) Permitted devices. It is lawful to hunt deer during the muzzleloading deer season with a muzzleloading firearm. The firearm must be a .44 caliber or larger single-barrel long gun or a .50 caliber or larger single-barrel handgun that propels single-projectile ammunition.
(2) Prohibitions. While hunting deer during the muzzleloading deer season, it is unlawful to:
(i) Use or possess multiple projectile ammunition or ammunition other than required under section 2322(a)(4) of the act, except as authorized under section 2525 of the act.
(ii) Use or possess a device or ammunition not provided for in the act or in this subsection, except as authorized under section 2525 of the act.