Is bedding a replacement stock an automatic thing, or no? Anyone out there slap on a McMillan, Manners, Mesa, etc. stock without bedding and have no issues?

I’d shoot it first before worrying about bedding. I have a mcm edge and its not bedded and its quite accurate.

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Why bother if its a shooter. Todays rifles are almost all good shooters right out of the box

It would be worth a try and certainly possible.
But, I am never surprised at the need for mostly minor fitting and bedding on drop-in replacement stocks.

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With variations in receivers and stocks it is not uncommon to have to bed any stock.Simple way to see if it needs bedding; tighten front screw then tighten rear screw. If the rear screw doesn't go from loose to tight in about 1/2 turn or less, something is not fitting correctly. Now do the same procedure but tighten the rear screw first. Again if the front screw doesn't go from loose to tight in less than 1/2 turn it means that the action or stock are being put in tension.
Usually takes only about a tablespoon or less to skim bed so it's a very minor job.

I usually bed everything. Yes you can shoot it first just to see.

All mine are bedded, but I purchased all my McMillan stocks secondhand and they were bedded already or needed some modifications anyway so I had them bedded. I also had larger diameter pillars installed than McMillan uses.

First one was already bedded when I bought it, so I just decided to re-bed.

Second one was actually a Montana rifle inlet, but I built a M70 .338-06 and dropped in there. Full custom so why not bed?

Third was a stock inlet for a two piece M70 trigger guard, and I had it changed to a one piece for my EW.

And your barrel lug may not fit as it should. The stock may be inletted with a perfectly square recess but your rifle may have a barrel lug that is not machined perfectly square. I would at the least bed the lug recess, but since you are at it, just bed the stock under the receiver and tang. No need to bed any of the barrel channel.

It’s only a sample of one, I have a Grayboe McMillan clone that claimed to not need bedding. I dropped a 6.5 CM Remington SPS barreled action in it with a good fit. It holds sub .5 MOA to several hundred yards with factory Hornady American Gunner ammo. Bedding it couldn’t hurt but I’d think any benefits would be measured in 10’s if not 100’s of an inch.