I have a EARLY pre64 Winchester rifle, barrel is marked 300 magnum. I have not shot this rifle it sits in the back of the safe. I want to cast the chamber to double check size what is a good product to use for this?

Cerrosafe.

If it’s an early gun, and by early as in pre war early, you have a 300H&H

it has a post war type safety but you are correct I believe chambers 300hh just like butter

A .300 H&H would not chamber a .300 Win magnum rifle, nor would a .300 Win Mag chamber a .300 H&H rifle. If it chambers a .300 H&H round then you're good. Of course, some clever prior owner could have rechambered to .300 WBY without marking it, but you're still good shooting .300 H&H in it which will simply blow out to conform to the WBY chamber.

I have a pre64 that was rechambered to 300 Weatherby Mag, which was not uncommon. Mine is correctly marked.

If it bothers you and you are unsure a cast is an easy solution. If you don't like the idea of cerrosafe you can use sulphur. Some mix in graphite with the sulphur. Google how to do it.

You have a 300 H&H, Only the 300 H&H was marked 300 Magnum, all 300 Win mags were marked 300 win mag. but if you have doubts run it by a gunsmith and he can advise you. BTW at one time the 300 H&Hs were marked 300 magnum for a sales pitch to the then public as I recall. then later on marked 30 H&H...You stipulate early, is your gun pre war or pre 64 or has it been rechambered is your question to be?

It does not have the pre war safety however the serial number looks pretty low. also the stock is low comb for iron sites

According to Rule Winchester changed the caliber stamping in 1950. 1950's production started at 131730.

I would trade you my high comb stock in ecellent condition for your low comb stock? I shoot irons a lot and I shoot scopes with low comb stocks quite well..

I'm going to keep this one as is. I do have a low comb stock (well used) from another pre64 rifle it however was a non magnum action. It also had a bull barrel but its uncut with the metal butt plate. I would sell. I can send you pictures. Like I said well used. It was from Alaska goat gun lots of miles. I ended up pulling the action and rebarreling/restocking it.

Thanks for the offer but I need a near new trade.