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! I checked rings and bases and all fasteners and they are all tight. I Have never had the groups just drop off so abruptly before. I bought this rifle from my uncle and i know it only had 3 shots fired through it before i bought it (he bought it new). It is Sako 75 Finnlight in .300 mag. Has anyone else experienced this? I have 4 other Sakos and never had this happen. I've experienced fouling, but not quite like this. I have cleaned it up good, Just waiting on the weather to go back out, We got dumped on today.

Every new rifle needs some tlc. I would clean it after every 5 shots for the first 20-25 shots. Everyone has there own way but this would be the minimum to start with. The gun will tell you what it needs after that. my humble opinion... Shayne

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 .

Thats great to hear, I'm patiently waiting for time to get back out after a good cleaning.

This is not too unusual. If you are hunting where 10 to 15 shots is not enough, like a management hunt, get a bore snake. I find it brings it right back for 10 or so more shots and it is quick.....Tom

For 1000yd prone/sling long range matches the 300WinMag is a common caliber and drill is two sighters, then 20 rounds for record and time alloted is 30 minutes. There is no cleaning between shots. Very accurate round w/ good shooter at long range. Barrels naturally become too hot to touch w/ bare hands.
Don't know what the barrel profile is of your rifle, but believe the heat is causing more of your dispersion of shots than just cleaning. Not totally discounting the cleaning matter, but next time might want to give it more time between strings/groups and let it cool down. The match rifle barrels have a lot bigger profile than a sporting rifle and able to take the sustained firing without throwing the shots due to heat related stress issues.