I have noticed that same problem. Sometimes standing in front of him (basically as long as you are in the same unit/square/location - whatever),
then waiting the 3-4 days seems to fix that issue. Same goes for other merchants (occasionally indoor vendors have the same problem).
BTW Alexander is excellent for .44 MAG, and .45-70, just not so much .50BMG.
True, he is, but the Khans' Armorer or Quartermaster Brandon are even better for them. The latter will not sell to you unless you have a 'Liked' or better NCR rep and either pass a 50 speech check or have done
For the Republic, Pt. II to the point where Col. Moore has authorized it, but once he
does start selling to you you're pretty much set as a Guns or EW user.
Sergeant Contreras is a good one-time bulk supplier of .44s and 12.7mms (often has 1200+(!) of the latter), however his inventory has issues even after you upgrade it and rarely, if ever, refreshes.
Got annoyed on my latest character as the Arms Merchant at 188 Trading Post had her stock 'lock up' on me, which hosed one of the best suppliers of weapon mods and rare ammo.