6.5cm??? :-O so like over THREE TIMES the diameter of a 20mm cannon? Yeah I'd say that is a little bit overkill!!!
You could get away with a projectile one TENTH of that diameter and still be ok.
OK now seriously.
The old timers in Ontario where I grew-up thought nothing of moose hunting with a .30-.30. There was the rare .300WM and more common (but not COMMON common) were .300 savages. Typically back then though most people thought of a .308 as an huge and powerful big bore...at least compared to the .30-.30s
For the people who feel that a .308 is plenty for any NA big game...ummm...have you ever BEEN to the West coast?? You will NOT be doing yourself any favours if you shoot a pissed-off Alaskan Brown Bear with a .308. Like a doped-up Vietnamese sapper with a satchel charge running full speed towards the bridge you are guarding, shooting a grizz with a .308 is gonna be like shooting Charlie with a 5.56. They are often gonna keep coming.
Of COURSE shot placement is everything (with less powerful calibers) and I saw an OLD documentary where an Eskimo hunted Polar bears with a .22 and would pile them up with a single shot...at the bears' ASSES!!! Iirc he knew exactly where to hit the huge bear that would instantly immobilize him. RIGHT IN THE SPINE.
However, for the rest of us I would highly recommend AGAINST polar bear hunting with a .22. Even if it WAS legal.
I lived on the coast for just about 20 years and spent quite a bit of time in the bush. I've never really been a super fast, flat shooting, tiny projectile.
I've shot just about everything on 4 legs that walks West of the rockies. Deer (whitetails, mulies, blacktails AND the TINY Sitka blacktails on the QCI -HG?), moose, black bear, grizzly etc etc..no sheep or goats though and no dogs or cats and no pigs. I've spent time in the bush picking shrooms too (no no...PINE mushrooms..God).
I would not depend on a .308 for protection in those woods. Actually maybe a .308 WOULD be adequate because I've seen bears...even smaller blacks, walk or run right THROUGH a barrage of 12gauge slugs...which is what I usually carried in the woods. Well ok maybe not stroll right through a barrage of 12ga lead as if it was a drizzle of rain but...
In situations when there is a bear...heck even if there are just fresh brown bear TRACKS nearby...well for me anyway...shot placement might not be exactly what I want it to be. It might not present itself with a clear ass shot for my .22 to target.
For me in the BC bush it was always a gauge or my .444 Marlin.
Hey, a .308 might have similar energy as a gauge or .444Marlin...and a .308 really IS nothing to sneeze at.
BUT...
IF I was looking for a Canadian big game caliber (ESPECIALLY Western Canadian big game) a .308 wouldn't even be on my list..unless I wanted a dual purpose gun. One that would be OK for big game hunting but that also doubled as my SHTF 1000m sniper rifle. BUT..for a purely big game bolt action hunting rifle, since the price of the gun would be pretty much identical -but not ammunition of course but it isn't gonna be your fungun anyway- for me personally..and I live in AB now not the W.Coast..we still have some very mean very big bears here- I'd be looking for a .300 WinMag or a .338. I know the .338 is crazy but it will take ANYTHING that walks off it's feet even without perfectly relaxed and nicely placed shot...and...for that long range SHTF sniper rifle...
Idk. I'm probably just being silly. Projectiles have come a LONG way since the last time I was looking at .308s.
For me though, I think of a .308 as kind of a minimum acceptable Canadian big game caliber and a somewhat washed-up military sniper round.
I think there are better calibers that would get ALL those jobs done.
Now, if you live in Ontario (not Ontario Polar Bear country though) a .308 is probably all you're gonna need for the biggest ON moose and they will kill a black bear...eventually lol. Those things can take a lot. They are DEFINITELY at least part cat.
If I was being charged by even just a black bear and there was a rack of 4 or 5 loaded long guns right beside me and one was a .308, one was a .300winmag, one was a 30-06, one was a .270 and the other was a 12gauge...the .308 would be the LAST one I reached for.
If one was a .223 IT would be the one I would NOT grab.
A 6.5 mm (not CM) wildcat...like a 6.5 on a .300 Weatherby Mag...now THAT would do some SERIOUS damage. It's small-ish and FAST and flat and that isn't really my thing but it would probably or maybe? stop that charging grizz in it's tracks?
I still prefer the big heavy lobbers but that's just me.
If you are PURELY moose hunting for not world record Yukon monsters, are going to have plenty of time to place your shot and you aren't reaching-out much past 200 or 300metres then a .308 will be great. Cheap ammunition, not too crazy of recoil so you can re-acquire that target pretty quickly (but you placed that first one so well you wont need a 2nd) etc etc..
But that is a lot of IFs and iften times when hunting, all the stars and planets do NOT perfectly align like that so for me...a .308 is pretty much a no-go for me for big game hunting.
Plus I'm not a wicked shot so I can use all the help I can get ;-)