I put a new VARI-X III scope on my .300 Win Mag yesterday and went out to shoot. I lapped the rings and bore sighted it and was 4 inches high and 3 inches right on first shot. This is a new rifle with about 5 rounds through it. After 5 shots i am in the bull, I change my target at 100 yds and fire a 5 shot group that is sub 1" with 150grain Remington Core-Lokts factory ammo. I walk down and change target and shoot another 5 for 15 of the box and it is tighter than the first. I'm thinking spectacular for factory ammo! I go change my target again and shoot the last 5 and the group opened up to 4" plus softball size and is a round circle pattern! I did not clean barrel during this 20 rounds and consisted of approx 30-40 min. Barrel was pretty warm after last 5. Could this be fouling? When i mounted scope i put a fine pencil mark at front and back of rings on scope and on dovetails front and back of mounts so i could see at a glance if anything moved and that is fine. Also after last group i slid a dollar bill along barrel to see if i had pressure anywhere and that was fine as well. This drove me crazy last night!

Could be fouling. Especially in a new rifle. Clean it up good and try again.
FYI, it ain't gonna take many boxes of that store bought ammo to equal a nice entry level reloading kit.

I have all the stuff and do reload, I have (er had) 2 boxes of core-lokts(now 1 box left) i was shooting up for the brass.

scope mount is loose.. likely bases

Nope, checked em. Rings and bases are tight.

Bum scope?

Check with a collimator.

Maybe loose bedding screw?

I suppose thats possible. Just because it's new don't mean it's good i suppose. Whats the best way to check for that if it gets knocked off every shot? Given that the first 15 shots were spot on and the last 5 went all over i sure won't rule that out. Didn't think of that. If i put my boresighter back in and it's way off i should be able to see if somthing's a miss shouldn't i?

I would clean it and try again.

Yeah, you are going to have to get a base-line with the collimator, shoot it until you get wide shots, then check it again.

Did you let the barrel cool completely between strings?

Don’t worry about it...the same happens to my Sako 300wm...nothing unusual...It only need good gleaning. After every 10-15 shots you must clean the barrel well to keep the group sub 1”.

My buddy shots a Sako TRG in 300wm and he is shooting some incredible groups at up 500 metres and he cleans the barrel every 10 shots to keep a good accuracy .

Still trying to get back out, It snowed like the dickens all day today. I'm hoping to shoot a box tomarrow.

off topic but I'll tell you I often, with standard cartridges, bought loaded ammo and fired it to save the brass. if it wasn't a big difference in cost over buying the brass itself. nice with a new rifle to know you're shooting ammo that overall is going to give at least average accuracy versus me developing a load at the same time I'm sighting in.

of course now I couldn't afford that, load everything!!!!

Red