I am at level 26 and Gun Runners hasn't seemed to get weapons like the Hunting Shotgun. Is this a glitch? What level do these higher power weapons appear in the shops?

They definitely should be appearing at your level. Sometimes, though, the Gunner Runners' inventory takes a very long time to update.

They sell these guns at other places. But yeah they usually appear at around level 15...I'm 17 and I have bought it. :confused:

According to the G.E.C.K., the Hunting Shotgun is supposed to be available from the Gun Runners from L11 on. Now, at least one T4 weapon is guaranteed to spawn for each restock, however there are six of these in the Gun Runners' loot tables so it might take a goodly number of refreshes to get a particular one. Weapons such as the Anti-Materiel Rifle or Riot Gun are Tier 5 and as such start showing up at L16.

This also applies to weapon mods, and as there are a much larger number of these it can get really annoying trying to get a particular one for a particular weapon.

Have you waited 73 in game hours? The Vendors only reset on Wednesday's and Saturdays (it might be Sunday).

However when you load the game, it always starts on Sunday, even if you played 2.89 in game days, then saved and exited.

The Sotre has 5 inventory tiers. Something like these:

• Tier 1 (player level 1): Silenced .22 pistol, Varmint rifle, 9mm pistol, .357 magnum revolver, Single shotgun
  
• Tier 2 (player level 1): 10mm pistol, 9mm SMG, Cowboy repeater, Service rifle, Caravan shotgun
  
• Tier 3 (player level 6): .44 magnum revolver, Lever-action shotgun, Silenced .22 SMG, 10mm SMG, Hunting Rifle, Sawed-off shotgun
  
• Tier 4 (player level 11): Trail carbine, Hunting revolver, Assault carbine, Sniper rifle, Hunting shotgun
  
• Tier 5 (player level 16): Riot shotgun, 12.7mm submachine gun, Marksman carbine, Minigun, Anti-materiel rifle, Light machine gun, Brush gun

Or check this: http://fallout.wikia.com/wiki/Vendortron

Indeed. I have yet to find the 10mm SMG less spread mod. I grab the trail carbine scope first time I see it as it becomes very hard to find later on.