It's not in Mass Pike Tunnel West in my game. Is there another location where I could find it?

... Mod one yourself, if you have the ranks in Gunsmith? Or just shoot up every baddie who uses guns until you get one?

That's essentially a random drop. Aside from a handful of named legendaries "uniques" and the first two weapons you get in Vault 111, nothing is purposely placed, so I don't know why you think that it should always spawn in the Mass Pike Tunnel.

Hunting rifle + scope + long ported barrel + calibrated powerful receiver.

Uhhh... maybe I've been playing too much.

I had a Silenced Marksman's Sniper Rifle show up in the bell tower of the Old North Church the first time I ever went there. Now, I make it a point to check the bell tower every time I show up for the first time, but there's never been any weapon of any sort there. So just because you found it once doesn't necessarily mean you'll find it again.

Round about level 15, maybe even lower (possibly very low if you head far enough south), you'll start to see some rifles with sniper scopes. The .38 pipe rifles will be more common, but some of them will have additional addons (Marksman stock, ported barrels, various receivers) but eventually you may get a .308. Like Marss said, they are random. If I'm not playing a Gun Nut character (which eats up several perks and doesn't fully culminate until Level 39) I generally keep a look out for weapons that have mods I want to remove and apply to legendaries of the same chambering. Keep in mind, even if two weapons have the same caliber, there is a distinction between "pipe variants" and non-pipe. It should only matter for the .308 caliber as far as I've noticed.

I'm a bit of a gun nut in real life, and very glad that they included it. But I'm really looking forward to the mods that add a more believable variety of weapon types and additional mods. Totally lore breaking, but I absolutely adored the weapons mods for FO3 and FONV, being a particular fan of H&K shotguns, and AK series assault rifles.