It's hard to judge experience from the opposite side of a computer screen. Post count often means #### (example: sunray) so we're left to fill in the gaps ourselves. There were a couple of warning flags...
1) You mentioned putting a +20 MOA rail on a run of the mill hunting rifle. One could be forgiven for wondering why you went to that trouble when for $50 you could have had a set of wildly superior Talley Lightweights. When guys show up in the hunting forum talking about a tapered (not canted!) rail on a hunting rifle it often follows that the OP may be a kid who got hoodwinked by counter staff.
2) You spoke of "$3 per round" ammunition as though this was a thing. At $3 per round you're not even shooting premium factory ammo. $3 per round is peanuts, talk to us when you hit $10-$25 per round for factory ammo. Nor are you handloading. These are often signs that one is dealing with a youngster with little experience.
3) The bullet and weight you're running is light for moose and especially for bison. This also speaks to lack of experience.
4) You speak of the "hoopla" surrounding magnum rounds and then tell "new shooters to magnum rounds" that they're no big deal and make the statement (and a bold one at that) that they don't feel different than your Enfield, 30-06 or 308. Unless you have a 5 lb .308 or your '06 is a Remington Ti (First Gen) then there is simply no way that they are comparable. If they are then the experienced shooters in the crowd would tend to think that you are probably flinching the hell out of all of them.
5) You're happy with a 5 MOA group off a bipod. A real GunNut would claim that his targets were eaten by a sow black bear and her cubs while you were in the outhouse rather than admit they shot groups like that.
Now, as harsh as the above might seem, I'll not cast judgement. I've seen rifles take a schwack of elevation to get to zero. In fact I had a very expensive custom rifle that zeroed at +80 MOA on the scope off a flat (non-tapered base). I sorted that out by having a custom +40 base done up by my rifle builder. Now the rifle is a dream. So your rifle might just be shooting low. As much as I loathe a one-piece base on a hunting rifle (OK, a Jerk-O-Matic can wear one or a BAR but THAT'S IT!!!), you might just be in the space where you need that. Ken Farrell offers his bases in configurations up to +40MOA. Before you go down that road I would mount the scope up again in a set of steel weaver rings on your base and see if you can get it to zero. If you can then crank up until you get to the end of the adjustment and count the clicks it took to get there. See where that leaves you in terms of internal adjustment. Even if you zero it would be good to know whether or not you're at the top end of the adjustment or you have some wiggle room. You want to be near the middle of the adjustment range when you zero. If you can't zero then start looking at 1) scopes with more internal adjustment, or 2) get another base with more slope. Example, your scope has 52 MOA of internal adjustment. If you run out of elevation and you're still 6" low at 100 then you definitely need a +40 base. If you can zero and have 6 MOA still remaining you won't regret going to a +30 MOA or +40 MOA rail. If you need more than that you're looking at a bill from a good gunsmith but you can pretty much request whatever wild taper you want. You'll pay for it but if that's what it takes to get the rifle hunting under glass then that's what it takes.
I forgot to ask...is this a new scope or a used one? Could it's guts be hinky? Do you have a scope that you know is bulletproof that you could try on it?
Good luck.
