I thought ammo refills worked fine in RDR Multiplayer but I wouldn't be surprised if Rockstar went the same route as GTA online with RDR2 by making all clothing and gun stores in Singleplayer accessible in Online.
I do hope they keep every player's ammo and explosives capacity low, there's no danger of running out of ammo or explosives in GTA Online because you can carry a ridiculous 9,999 rounds on most guns and if your far from a gun store you can buy your ammo and explosives on the Interaction Menu.
You could carry 100 rounds in RDR Multiplayer and you got given ammo upon respawning unlike GTA Online, even if they made a ammo system similar to GTA Online in RDR2 I personally wouldn't change the ammo and explosive capacity from what players had in RDR Multiplayer as it kept people from camping for too long and stopped people spamming explosives unlike GTA Online.