180 is an excellent weight to be stuck with. It has the right penetration for moose and does not blow up a lot of meat on deer. One thing you can do is buy some more of that same ammo that worked well and be done with it.
You want to know why it won't shoot 165s. First, twist has nothing to do with it. Twist is an obvious variable in barrels and therefore gets blamed for every anomaly. Unless you are trying to shoot a 215 gr VLD, twist has nothing to do with your rifle results.
As stated by someone else, the round nose bullet is closer to the rifling. This might be the reason, but I doubt it. Remington cuts chambers with very deep throats (for legal reasons, not ballistic reasons). Every bullet has a huge jump to the rifling in a Remington.
Some rifles will shoot many different loads fairly well. others will be very fussy. For the latter you have to try various brands and bullets to find what the rifle likes. If you rifle is fussy, a flat based bullet is usually the best choice.
A poor barrel will do best with a round nose bullet. Is your barrel in good shape? no rust or pitting? Has it been well cleaned? Put a wet patch through it and let it sit muzzle down over night and see if a new patch comes out blue in the morning. If it does, your barrel is not clean.
My 7600 30-06 A.K.A "The Meat Seeker" Don't drives tack with 165's like she does with 180's...Any you other fellers have this conundrum with yer 7600? My pa's friend had a old box of 165 grain core lokts, they shot real good. But I've tried some federals and remington 150's and 165's and she just don't fire em like the big old round nose 180 grain core lokts...Do elaborate! I realize twist is a big deal, from what I can figure it's a 1:9. She don't shoot federals worth a hooey anyway no matter the grain, blue box anyhow. On a side note, I'd like to reload but It kinda gives me the heeby jeebies. 
