[Rachen]

What was/is the longest serving small arm? I remember some of the M-2s were really old.
Or does anyone have a breakdown through the eras?
I’m sure that I could locate the information myself... but hey, there’s some knowledge here.

I believe, and correct me if I am wrong, that the LONGEST serving small arm in the US military was the Springfield Trapdoor Rifle and it's carbine variants. Right after the War Between The States ended in 1866 and especially after the Fetterman Massacre where it was brought up in a court of inquiry about how the troopers' rifled muskets were too slow loading against native warriors armed with Spencers and Henrys, the US Dept. Of Ordnance arranged for thousands of Springfield 1861 rifled muskets to go through the Allin conversion, enabling them to use the .50-70 cartridge. THAT served the forces until 1873, when the rifle was officially manufactured and chambered in the .45-70. And right into the Spanish-American War, US troops were still using the Trapdoor against the Spanish who were primarily using bolt action Mausers and Arisakas. I have heard that the clouds of smoke given off by the big BP cartridges literally lit up the US lines and the Spanish artillery took a heavy toll on these. And only because of BP's shortcomings that the US later had to adopt smokeless powder. And about pistols, I think the 1911 was the longest serving. From 1905 all the way until now, where it is still carried by special operations units.

But what make and model? "Muzzle loading long rifle" encompasses a lot

It started off with the Kentucky and Pennsylvania rifles, which really were hunting guns of extreme accuracy that could be found on the mantels of almost every frontier home and was the first thing that their owners grabbed when the call to arms came. It was sharpshooters like the ones led by Daniel "The Old Wagoner" Morgan that decimated the British at the battle of Saratoga. Muzzleloading smoothbores were primarily an infantry arm used by rank and file grunts doing the work of hammering blows and counterblows against the enemy in CONVENTIONAL battle. A great part of the Revolution, 1812, and the Indian conflicts were fought by irregulars, sharpshooters, private bands of warfighters and hunters who used home-built rifles and picked off the leaders of the enemy with long range aimed fire.

I'm not sure if it's considered a small arm - - but - -I was very surprised to learn that the Gatling gun was in service for 49 years.
From 1862 thru 1911.

Actually it is still used on a WIDE scale today The miniguns ranging from the rifle-caliber ones mounted on Humvees and choppers to the giant ship-smashing behemoths found on the destroyers. Actually, EVERY large navy in the world has a Gatling system of some kind for land, sea and air use.