It is just a story.
Here is a list of ALL US sniper rifles. Notice none of them are called a Bravo 501
1. U.S. Army & USMC M1903-A4 (also: USMC M1903-A1/Unertl)
2. U.S. Army & USMC M1C & M1D
3. U.S. Army M21
4. U.S. Army M24 SWS
5. U.S. Army & U.S. Navy M25/XM25
6. U.S. Marine Corps M40A1
7. U.S. Marine Corps M40A3
8. Barrett M82A1 (XM107)
9. Savage 110FP & 10FP
10. Remington Model 700P & 700LTR
11. Remington SR8
12. Remington 700 SPS Varmint
13. Remington 700 SPS Tactical
14. Remington 700 VTR New!! (04/24/08)
15. Winchester Model 70 Custom Sharpshooter
16. Harris M86
17. Robar SR-60 & SR-90
18. Stoner SR-25
19. Brown Precision Tactical Elite
20. HS Precision Pro 2000 HTR
21. Dakota T-76 Longbow
22. Tac-Ops Tango-51 ... [Purchase]
23. Tac-Ops Bravo-51 ... [Purchase]
24. Tac-Ops Green Hornet
25. Tac-Ops 700P Tune-up Package
26. Ruger M77 MkII VLE
27. Winchester Model 70 Stealth
28. Weatherby TRR
29. Patriot Arms Genesis
30. Weatherby Vanguard Varmint Special
31. Rock River Arms Varmint A4
32. DPMS LR-308B
33. KMW Long Range Solutions Custom Tactical Rifle (.260 rem)
34. Savage 10 FCP McMillan
There was a Baretta 501 sniper rifle made in 1985 and used by Italians. I doubt a US Marine would be referring to that gun.