I'm assuming you have the first edition from your description?
First edition:
Remington 700 ML was introduced in 1996 in both ML (blued) and MLS (stainless) in .50 and .54 caliber. Both came with a black synthetic stock. Based on the Model 700 short action receiver. The bolt has a round hammer in place of a firing pin and originally designed use a #11 cap for ignition. It ended production somewhere around 2004.
Google: "Remington 700 ML 209 conversion kit" if it has not already been upgraded to use 209 primers. Sample search results below.
The Remington 700ML 209 Conversion Conversion Kit gives you the ability to use hotter 209 primers and virtually eliminates misfires & delayed ignitions. Shop here!
www.muzzleloadingbullets.com
If you have a first edition it's most likely not what you're looking for and not designed to use the same power charges as the second edition. Personally, I would take back to the pawn shop. Sorry for the bad news.
Second edition:
Remington Model 700 Ultimate Muzzleloader (Remington 700UML) was introduced in 2014 and still in production. New ones are now showing up post Remington's bankruptcy and still cataloged as of 2023. Second edition also uses a Remington model 700 short action but with a conventional bolt with firing pin. The Ignition is basically from a cut down 308 Winchester case with a Remington 9 1/2 Magnum large rifle primer.
The Rem 700UML is designed for and will ignites up to four 50-grain pellets of 777 substitute for a muzzle velocity around 2400 fps with a Remington saboted .45-caliber bullet weighing 250 grains. Three-pellets of 777 delivers about 2200 fps (200 fps less) with the same Remington 250 grain bullet. The Remington bullets are rebranded Barnes Magnum MZ's that have a unique magnum sabot for the larger powder charge. So basically, the 700UML will beat most .45-70 Gov’t loads if that's what you're after.
Personally, if it's a 700UML take it to the range and test fire it with Remington's suggested load you may be surprised. I've had one since 2015 and believe it one best muzzleloader ever produced, only limited be state fish and game regulations. Mine will shoot 1 1/2 to 2 1/2-inch groups at 200 yards using 777, and always under 4-inch groups at 300 yards. I think it's more about me than the gun as far as group size goes. I have found no advantage in using up to 150 grains of Blackhorn 209 power as it's more expensive and produces the very same average group sizes in my gun. Plus, I like the advantage of the pellets over lose power while in the field. I have tried a multitude loads and keep coming back to Remington's original and recommended design load as it produces the best average results day after day in my option.
Some informational links for the 700UML:
Shopping for the Accutip Muzzleloader Bullet - Learn more about the latest Muzzleloading and other hunting or shooting gear at Remington.
www.remington.com
www.barnesbullets.com
Good luck and hope you have a Remington 700UML rather than the old 700ML.